Sep 272015
 

Well, lately I’ve been doing a bit of work hacking the firmware on the Rowetel SM1000 digital microphone.  For those who don’t know it, this is a hardware (microcontroller) implementation of the FreeDV digital voice mode: it’s a modem that plugs into the microphone/headphone ports of any SSB-capable transceiver and converts FreeDV modem tones to analogue voice.

I plan to set this unit of mine up on the bicycle, but there’s a few nits that I had.

  • There’s no time-out timer
  • The unit is half-duplex

If there’s no timeout timer, I really need to hear the tones coming from the radio to tell me it has timed out.  Others might find a VOX feature useful, and there’s active experimentation in the FreeDV 700B mode (the SM1000 currently only supports FreeDV 1600) which has been very promising to date.

Long story short, the unit needed a more capable UI, and importantly, it also needed to be able to remember settings across power cycles.  There’s no EEPROM chip on these things, and while the STM32F405VG has a pin for providing backup-battery power, there’s no battery or supercapacitor, so the SM1000 forgets everything on shut down.

ST do have an application note on their website on precisely this topic.  AN3969 (and its software sources) discuss a method for using a portion of the STM32’s flash for this task.  However, I found their “license” confusing.  So I decided to have a crack myself.  How hard can it be, right?

There’s 5 things that a virtual EEPROM driver needs to bear in mind:

  • The flash is organised into sectors.
  • These sectors when erased contain nothing but ones.
  • We store data by programming zeros.
  • The only way to change a zero back to a one is to do an erase of the entire sector.
  • The sector may be erased a limited number of times.

So on this note, a virtual EEPROM should aim to do the following:

  • It should keep tabs on what parts of the sector are in use.  For simplicity, we’ll divide this into fixed-size blocks.
  • When a block of data is to be changed, if the change can’t be done by changing ones to zeros, a copy of the entire block should be written to a new location, and a flag set (by writing zeros) on the old block to mark it as obsolete.
  • When a sector is full of obsolete blocks, we may erase it.
  • We try to put off doing the erase until such time as the space is needed.

Step 1: making room

The first step is to make room for the flash variables.  They will be directly accessible in the same manner as variables in RAM, however from the application point of view, they will be constant.  In many microcontroller projects, there’ll be several regions of memory, defined by memory address.  This comes from the datasheet of your MCU.

An example, taken from the SM1000 firmware, prior to my hacking (stm32_flash.ld at r2389):

/* Specify the memory areas */
MEMORY
{
  FLASH (rx)      : ORIGIN = 0x08000000, LENGTH = 1024K
  RAM (rwx)       : ORIGIN = 0x20000000, LENGTH = 128K
  CCM (rwx)       : ORIGIN = 0x10000000, LENGTH = 64K
}

The MCU here is the STM32F405VG, which has 1MB of flash starting at address 0x08000000. This 1MB is divided into (in order):

  • Sectors 0…3: 16kB starting at 0x08000000
  • Sector 4: 64kB starting at 0x0800c000
  • Sector 5 onwards: 128kB starting at 0x08010000

We need at least two sectors, as when one fills up, we will swap over to the other. Now it would have been nice if the arrangement were reversed, with the smaller sectors at the end of the device.

The Cortex M4 CPU is basically hard-wired to boot from address 0, the BOOT pins on the STM32F4 decide how that gets mapped. The very first few instructions are the interrupt vector table, and it MUST be the thing the CPU sees first. Unless told to boot from external memory, or system memory, then address 0 is aliased to 0x08000000. i.e. flash sector 0, thus if you are booting from internal flash, you have no choice, the vector table MUST reside in sector 0.

Normally code and interrupt vector table live together as one happy family. We could use a couple of 128k sectors, but 256k is rather a lot for just an EEPROM storing maybe 1kB of data tops. Two 16kB sectors is just dandy, in fact, we’ll throw in the third one for free since we’ve got plenty to go around.

However, the first one will have to be reserved for the interrupt vector table that will have the space to itself.

So here’s what my new memory regions look like (stm32_flash.ld at 2390):

/* Specify the memory areas */
MEMORY
{
  /* ISR vectors *must* be placed here as they get mapped to address 0 */
  VECTOR (rx)     : ORIGIN = 0x08000000, LENGTH = 16K
  /* Virtual EEPROM area, we use the remaining 16kB blocks for this. */
  EEPROM (rx)     : ORIGIN = 0x08004000, LENGTH = 48K
  /* The rest of flash is used for program data */
  FLASH (rx)      : ORIGIN = 0x08010000, LENGTH = 960K
  /* Memory area */
  RAM (rwx)       : ORIGIN = 0x20000000, LENGTH = 128K
  /* Core Coupled Memory */
  CCM (rwx)       : ORIGIN = 0x10000000, LENGTH = 64K
}

This is only half the story, we also need to create the section that will be emitted in the ELF binary:

SECTIONS
{
  .isr_vector :
  {
    . = ALIGN(4);
    KEEP(*(.isr_vector))
    . = ALIGN(4);
  } >FLASH

  .text :
  {
    . = ALIGN(4);
    *(.text)           /* .text sections (code) */
    *(.text*)          /* .text* sections (code) */
    *(.rodata)         /* .rodata sections (constants, strings, etc.) */
    *(.rodata*)        /* .rodata* sections (constants, strings, etc.) */
    *(.glue_7)         /* glue arm to thumb code */
    *(.glue_7t)        /* glue thumb to arm code */
	*(.eh_frame)

    KEEP (*(.init))
    KEEP (*(.fini))

    . = ALIGN(4);
    _etext = .;        /* define a global symbols at end of code */
    _exit = .;
  } >FLASH…

There’s rather a lot here, and so I haven’t reproduced all of it, but this is the same file as before at revision 2389, but a little further down. You’ll note the .isr_vector is pointed at the region called FLASH which is most definitely NOT what we want. The image will not boot with the vectors down there. We need to change it to put the vectors in the VECTOR region.

Whilst we’re here, we’ll create a small region for the EEPROM.

SECTIONS
{
  .isr_vector :
  {
    . = ALIGN(4);
    KEEP(*(.isr_vector))
    . = ALIGN(4);
  } >VECTOR


  .eeprom :
  {
    . = ALIGN(4);
    *(.eeprom)         /* special section for persistent data */
    . = ALIGN(4);
  } >EEPROM


  .text :
  {
    . = ALIGN(4);
    *(.text)           /* .text sections (code) */
    *(.text*)          /* .text* sections (code) */

THAT’s better! Things will boot now. However, there is still a subtle problem that initially caught me out here. Sure, the shiny new .eeprom section is unpopulated, BUT the linker has helpfully filled it with zeros. We cannot program zeroes back into ones! Either we have to erase it in the program, or we tell the linker to fill it with ones for us. Thankfully, the latter is easy (stm32_flash.ld at 2395):

  .eeprom :
  {
    . = ALIGN(4);
    KEEP(*(.eeprom))   /* special section for persistent data */
    . = ORIGIN(EEPROM) + LENGTH(EEPROM) - 1;
    BYTE(0xFF)
    . = ALIGN(4);
  } >EEPROM = 0xff

Credit: Erich Styger

We have to do two things. One, is we need to tell it that we want the region filled with the pattern 0xff. Two, we need to make sure it gets filled with ones by telling the linker to write one as the very last byte. Otherwise, it’ll think, “Huh? There’s nothing here, I won’t bother!” and leave it as a string of zeros.

Step 2: Organising the space

Having made room, we now need to decide how to break this data up.  We know the following:

  • We have 3 sectors, each 16kB
  • The sectors have an endurance of 10000 program-erase cycles

Give some thought as to what data you’ll be storing.  This will decide how big to make the blocks.  If you’re storing only tiny bits of data, more blocks makes more sense.  If however you’ve got some fairly big lumps of data, you might want bigger blocks to reduce overheads.

I ended up dividing the sectors into 256-byte blocks.  I figured that was a nice round (binary sense) figure to work with.  At the moment, we have 16 bytes of configuration data, so I can do with a lot less, but I expect this to grow.  The blocks will need a header to tell you whether or not the block is being used.  Some checksumming is usually not a bad idea either, since that will clue you in to when the sector has worn out prematurely.  So some data in each block will be header data for our virtual EEPROM.

If we don’t care about erase cycles, this is fine, we can just make all blocks data blocks, however it’d be wise to track this, and avoid erasing and attempting to use a depleted sector, so we need somewhere to track this.  256 bytes gives us enough space to stash an erase counter and a map of what blocks are in use within that sector.

So we’ll reserve the first block in the sector to act as this index for the entire sector.  This gives us enough room to have 16-bits worth of flags for each block stored in the index.  That gives us 63 blocks per sector for data use.

It’d be handy to be able to use this flash region for a few virtual EEPROMs, so we’ll allocate some space to give us a virtual ROM ID.  It is prudent to do some checksumming, and the STM32F4 has a CRC32 module, so in that goes, and we might choose to not use all of a block, so we should throw in a size field (8 bits, since the size can’t be bigger than 255).  If we pad this out a bit to give us a byte for reserved data, we get a header with the following structure:

15 14 13 12 11 10 19 8 7 6 5 4 3 2 1 0
+0 CRC32 Checksum
+2
+4 ROM ID Block Index
+6 Block Size Reserved

So that subtracts 8 bytes from the 256 bytes, leaving us 248 for actual program data. If we want to store 320 bytes, we use two blocks, block index 0 stores bytes 0…247 and has a size of 248, and block index 1 stores bytes 248…319 and has a size of 72.

I mentioned there being a sector header, it looks like this:

15 14 13 12 11 10 19 8 7 6 5 4 3 2 1 0
+0 Program Cycles Remaining
+2
+4
+6
+8 Block 0 flags
+10 Block 1 flags
+12 Block 2 flags

No checksums here, because it’s constantly changing.  We can’t re-write a CRC without erasing the entire sector, we don’t want to do that unless we have to.  The flags for each block are currently allocated accordingly:

15 14 13 12 11 10 19 8 7 6 5 4 3 2 1 0
+0 Reserved In use

When the sector is erased, all blocks show up as having all flags set as ones, so the flags is considered “inverted”.  When we come to use a block, we mark the “in use” bit with a zero, leaving the rest as ones.  When we erase, we mark the entire flags block as zeros.  We can set other bits here as we need for accounting purposes.

Thus we have now a format for our flash sector header, and for our block headers.  We can move onto the algorithm.

Step 3: The Code

This is the implementation of the above ideas.  Our code needs to worry about 3 basic operations:

  • reading
  • writing
  • erasing

This is good enough if the size of a ROM image doesn’t change (normal case).  For flexibility, I made my code so that it works crudely like a file, you can seek to any point in the ROM image and start reading/writing, or you can blow the whole thing away.

Constants

It is bad taste to leave magic numbers everywhere, so constants should be used to represent some quantities:

  • VROM_SECT_SZ=16384:
    The virtual ROM sector size in bytes.  (Those watching Codec2 Subversion will note I cocked this one up at first.)
  • VROM_SECT_CNT=3:
    The number of sectors.
  • VROM_BLOCK_SZ=256:
    The size of a block
  • VROM_START_ADDR=0x08004000:
    The address where the virtual ROM starts in Flash
  • VROM_START_SECT=1:
    The base sector number where our ROM starts
  • VROM_MAX_CYCLES=10000:
    Our maximum number of program-erase cycles

Our programming environment may also define some, for example UINTx_MAX.

Derived constants

From the above, we can determine:

  • VROM_DATA_SZ = VROM_BLOCK_SZ – sizeof(block_header):
    The amount of data per block.
  • VROM_BLOCK_CNT = VROM_SECT_SZ / VROM_BLOCK_SZ:
    The number of blocks per sector, including the index block
  • VROM_SECT_APP_BLOCK_CNT = VROM_BLOCK_CNT – 1
    The number of application blocks per sector (i.e. total minus the index block)

CRC32 computation

I decided to use the STM32’s CRC module for this, which takes its data in 32-bit words.  There’s also the complexity of checking the contents of a structure that includes its own CRC.  I played around with Python’s crcmod module, but couldn’t find some arithmetic that would allow it to remain there.

So I copy the entire block, headers and all to a temporary copy (on the stack), set the CRC field to zero in the header, then compute the CRC. Since I need to read it in 32-bit words, I pack 4 bytes into a word, big-endian style. In cases where I have less than 4 bytes, the least-significant bits are left at zero.

Locating blocks

We identify each block in an image by the ROM ID and the block index.  We need to search for these when requested, as they can be located literally anywhere in flash.  There are probably cleverer ways to do this, but I chose the brute force method.  We cycle through each sector and block, see if the block is allocated (in the index), see if the checksum is correct, see if it belongs to the ROM we’re looking for, then look and see if it’s the right index.

Reading data

To read from the above scheme, having been told a ROM ID (rom), start offset and a size, the latter two being in byte sand given a buffer we’ll call out, we first need to translate the start offset to a sector and block index and block offset.  This is simple integer division and modulus.

The first and last blocks of our read, we’ll probably only read part of.  The rest, we’ll read entire blocks in.  The block offset is only relevant for this first block.

So we start at the block we calculate to have the start of our data range.  If we can’t find it, or it’s too small, then we stop there, otherwise, we proceed to read out the data.  Until we run out of data to read, we increment the block index, try to locate the block, and if found, copy its data out.

Writing and Erasing

Writing is a similar affair.  We look for each block, if we find one, we overwrite it by copying the old data to a temporary buffer, copy our new data in over the top then mark the old block as obsolete before writing the new one out with a new checksum.

Trickery is in invoking the wear levelling algorithm on an as-needed basis.  We mark a block obsolete by setting its header fields to zero, but when we run out of free blocks, then we go looking for sectors that are full of obsolete blocks waiting to be erased.  When we encounter a sector that has been erased, we write a new header at the start and proceed to use its first data block.

In the case of erasing, we don’t bother writing anything out, we just mark the blocks as obsolete.

Implementation

The full C code is in the Codec2 Subversion repository.  For those who prefer Git, I have a git-svn mirror (yes, I really should move it off that domain).  The code is available under the Lesser GNU General Public License v2.1 and may be ported to run on any CPU you like, not just ST’s.

May 032015
 

The Problem

I’ve been running a station from the bicycle for some time now and I suppose I’ve tried a few different battery types on the station.

Originally I ran 9Ah 12V gel cells, which work fine for about 6 months, then the load of the radio gets a bit much and I find myself taking two with me on a journey to work because one no longer lasts the day.  I replaced this with a 40Ah Thundersky LiFePO4 pack which I bought from EVWorks, which while good, weighed 8kg!  This is a lot lighter than an equivalent lead acid, gel cell or AGM battery, but it’s still a hefty load for a bicycle.

At the time that was the smallest I could get.  Eventually I found a mob that sold 10Ah packs. These particular cells were made by LiFeBatt, and while pricey, I’ve pretty much recouped my costs. (I’d have bought and disposed of about 16 gel cell batteries in this time at $50 each, versus $400 for one of these.)   These are what I’ve been running now since about mid 2011, and they’ve been pretty good for my needs.  They handle the load of the FT-857 okay on 2m FM which is what I use most of the time.

A week or two back though, I was using one of these packs outside with the home base in a “portable” set-up with my FT-897D.  Tuned up on the 40m WICEN net on 7075kHz, a few stations reported that I had scratchy audio.  Odd, the radio was known to be good, I’ve operated from the back deck before and not had problems, what changed?

The one and only thing different is I was using one of these 10Ah packs.  I’ve had fun with RF problems on the bicycle too.  On transmit, the battery was hovering around the 10.2V mark, perhaps a bit low.  Could it be the radio is distorting on voice peaks due to input current starvation?  I tried after the net swapping it for my 40Ah pack, which improved things.  Not totally cleared up, but it was better, and the pack hadn’t been charged in a while so it was probably a little low too.

The idea

I thought about the problem for a bit.  SSB requires full power on voice peaks.  For a 100W radio, that’s a 20A load right now.  Batteries don’t like this.  Perhaps there was a bit of internal resistance from age and the nature of the cells?  Could I do something to give it a little hand?

Supercapacitors are basically very high capacity electrolytic capacitors with a low breakdown voltage, normally in the order of a few volts and capacitances of over a farad.  They are good for temporarily storing charge that needs to be dumped into a load in a hurry.  Could this help?

My cells are in a series bank of 4: ~3.3V/cell with 4 cells gives me 13.2V.  There’s a battery balancer already present.  If a cell gets above 4V, that cell is toast, so the balancer is present to try to prevent that from happening.  I could buy these 1F 5.5V capacitors for only a few dollars each, so I thought, “what the hell, give it a try”.  I don’t have much information on them other that Elna Japan made them.  The plan was to make some capacitor “modules” that would hook in parallel to each cell.

My 13.2V battery pack, out of case

My 13.2V battery pack, out of its case

Supercapacitors

Supercapacitors

For my modules, the construction was simple, two reasonably heavy gauge wires tacked onto the terminals, the whole capacitor then encased in heatshrink tubing and ring lugs crimped to the leads. I was wondering whether I should solder a resistor and diode in parallel and put that in series with the supercap to prevent high in-rush current, but so far that hasn’t been necessary.

The re-assembled pack

I’ve put the pack back together and so far, it has charged up and is ready to face its first post-retrofit challenge.  I guess I’ll be trying out the HF station tomorrow to see how it goes.

Assembled pack

Assembled pack

The Verdict

Not a complete solution to the RF feedback, it seems to help in other ways. I did a quick test on the drive way first with the standard Yaesu handmic and with the headset. Headset still faces interference problems on HF, but I can wind it up to about 30W~40W now instead of 20.

More pondering to come but we’ll see what the other impacts are.

Mar 022015
 

Well, it’s been a year and a half since I last posted details about the bicycle mobile station.  Shortly after getting the Talon on the road, setting it up with the top box and lighting, and having gotten the bugs worked out of that set-up, I decided to get a second mounting plate and set my daily commuter up the same way, doing away with the flimsy rear basket in place of a mounting plate for the top box.

VK4MSL/BM today after the trip home from work.

VK4MSL/BM today after the trip home from work.

That particular bike people might recognise from earlier posts, it’s my first real serious commuter bike. Now in her 5th year, has done over 10200km since 2012.  (The Talon has done 5643km in that time.) You can add to that probably another 5000km or so done between 2010 and 2012. It’s had a new rear wheel (a custom one, after having a spate of spoke breakages) and the drive chain has been upgraded to 9-speed. The latter upgrade alone gave it a new lease on life.

Both upgrades being money well spent, as was the upgrade to hydraulic brakes early in the bike’s lifetime. I suppose I’ve spent in those upgrades alone close to $1400, but worth it as it has given me quite good service so far.

As for my time with the top box. Some look at it and quiz me about how much weight it adds. The box itself isn’t that heavy, it just looks it. I can carry a fair bit of luggage, and at present, with my gear and tools in it it weighs 12kg. Heavy, but not too heavy for me to manage.

Initially when I first got it, it was great, but then as things flexed over time, I found I was intermittently getting problems with the top box coming off.  This cost me one HF antenna and today, the top box sports a number of battle-scars as a result.

The fix to this?  Pick a spot inside the top box that’s clear of the pannier rack rails and the rear tyre, and drill a 5mm hole through.  Then, when you mount the top box, insert an M5 bolt through the mounting plate and into the bottom of the top box and tighten with a 5mm wing nut.  The box now will not come loose.

vk4msl-bm-2713

Still lit up like a Christmas tree from this morning’s ride.

The lights still work, and now there’s a small rear-view camera.  On the TODO list is to rig up a 5V USB socket to power that camera with so that it isn’t draining the rather small internal battery (good for 4 hours apparently).

The station has had an upgrade, I bought a new LDG Z-100Plus automatic tuner which I’m yet to fully try out.  This replaces the aging Yaesu FC-700 manual tuner which, although very good, is fiddly to use in a mobile set-up and doesn’t tune 6m easily.

One on-going problem now not so much on the Boulder but on the Talon is an issue with pannier racks failing.  This started happening when I bought the pannier bags, essentially on the side I carry my battery pack (2kg), I repeatedly get the pannier rack failing.  The racks I use are made by Topeak and have a built-in spacer to accomodate disc brake calipers.  This seems to be a weak spot, I’ve now had two racks fail at about the same point.

Interestingly on the Boulder which also has disc brakes, I use the standard version of the same rack, and do not get the same failures.  Both are supposedly rated to 25kg, and my total load would be under 16kg. Something is amiss.

A recurring flaw with the Topeak racks

I’m on the look-out for a more rugged rack that will tolerate this kind of usage, although having seen this, I am inspired to try a DIY solution.  Then if a part fails, I can probably get replacement parts in any typical hardware store.  A hack saw and small drill are not heavy items to carry, and I can therefore get myself out of trouble if problems arise.

Jan 032015
 

I’ve been riding on the road now for some years, and while I normally try to avoid it, I do sometimes find myself riding on the road itself rather than on the footpath or bicycle path.

Most of the time, the traffic is fine.  I’m mindful of where everyone is, and there aren’t any problems, but I have had a couple of close calls from time to time.  Close calls that have me saying “ode for a horn”.

By law we’re required to have a bell on our bikes.  No problem there, I have a mechanical one which is there purely for legal purposes.  If I get pulled over by police, and they ask, I can point it out and demonstrate it.  Requirement met?  Tick to that.

It’s of minimal use with pedestrians, and utterly useless in traffic.

Early on with my riding I developed a lighting system which included indicators.  Initially this was silent, I figured I’d see the lights flashing, but after a few occasions forgetting to turn indicators off, I fitted a piezo buzzer.  This was an idea inspired by the motorcycles ridden by Australia Post contractors, which have a very audible buzzer.  Jaycar sell a 85dB buzzer that’s waterproof, overkill in the audio department but fit for purpose.  It lets me know I have indicators on and alerts people to my presence.

That is, if they equate the loud beep to a bicycle.  Some do not.  And of course, it’s still utterly useless on the road.

I figured a louder alert system was in order.  Something that I could adjust the volume on, but loud enough to give a pedestrian a good 30 seconds warning.  That way they’ve got plenty of time to take evasive action while I also start reducing speed.  It’s not that I’m impatient, I’ll happily give way, but I don’t want to surprise people either.  Drivers on the other hand, if they do something stupid it’d be nice to let them know you’re there!

My workplace looks after a number of defence bases in South-East Queensland, one of which has a railway crossing for driver training.  This particular boom gate assembly copped a whack from a lightning strike, which damaged several items of equipment, including the electronic “bells” on the boom gate itself.  These “bells” consisted of a horn speaker with a small potted PCB mounted to the back which contained an amplifier and bell sound generator.  Apply +12V and the units would make a very loud dinging noise.  That’s in theory; in practise, all that happened was a TO-220 transistor got hot.  Either the board or the speaker (or both) was faulty.

It was decided these were a write-off, and after disassembly I soon discovered why: the voice coils in the horn speakers had been burnt out.  A little investigation, and I figured I could replace the blown out compression drivers and get the speakers themselves working again, building my own horn.

A concept formed: the horn would have two modes, a “bell” mode with a sound similar to a bicycle bell, and a “horn” mode for use in traffic.  I’d build the circuit in parts, the first being the power amplifier then interface to it the sound effect generator.

To make life easier testing, I also decided to add a line-in/microphone-in feature which would serve to debug construction issues in the power amplifier and add a megaphone function.  (Who knows, might be handy with WICEN events.)

Replacing the compression drivers

Obviously it’d be ideal to replace it with the correct part, but looking around, I couldn’t see anything that would fit the housing.  That, and what I did see, was more expensive than buying a whole new horn speaker.

There was a small aperture in the back about 40mm in diameter.  The original drivers were 8ohms, and probably rated at 30W and had a convex diaphragm which matched the concave geometry in the back of the horn assembly.

Looking around, I saw these 2W mylar cone speakers.  Not as good as a compression driver, but maybe good enough?  It was cheap enough to experiment.  I bought two to try it out.

I got them home, tacked some wires onto one of them and plugged it into a radio.  On its own, not very loud, but when I held it against the back of a horn assembly, the amplification was quite apparent.  Good enough to go further.  I did some experiments with how to mount the speakers to the assembly, which required some modifications to be made.

I soon settled on mounting the assembly to an aluminium case with some tapped holes for clamping the speaker in place.  There was ample room for a small amplifier which would be housed inside the new case, which would also serve as a means of mounting the whole lot to the bike.

Bell generator

I wasn’t sure what to use for this, I had two options: build an analogue circuit to make the effect, or program a microcontroller.  I did some experiments with an ATMega8L, did manage to get some sound out of it using the PWM output, but 8kB of flash just wasn’t enough for decent audio.

A Freetronics LeoStick proved to be the ticket.  32kB flash, USB device support, small form factor, what’s not to like?  I ignored the Arduino-compatible aspect and programmed the device directly.  Behind the novice-friendly pin names, they’re an ATMega32U4 with a 16MHz crystal.  I knocked up a quick prototype that just played a sound repeatedly.  It sounded a bit like a crowbar being dropped, but who cares, it was sufficient.

Experimenting with low-pass filters I soon discovered that a buffer-amp would be needed, as any significant load on the filter would render it useless.

A 2W power amplifier

Initially I was thinking along the lines of a LM386, but after reading the datasheet I soon learned that this would not cut it.  They are okay for 500mW, but not 2W.  I didn’t have any transistors on hand that would do it and still fit in the case, then I stumbled on the TDA 1905.  These ICs are actually capable of 5W into 4 ohms if you feed them with a 14V supply.  With 9V they produce 2.5W, which is about what I’m after.

I bought a couple then set to work with the breadboard.  A little tinkering and I soon had one of the horn speakers working with this new amplifier.  Plugged into my laptop, I found the audio output to be quite acceptable, in fact turned up half-way, it was uncomfortable to sit in front of.

I re-built the circuit to try and make use of the muting feature.  For whatever reason, I couldn’t get this to work, but the alternate circuit provided a volume control which was useful in itself.

The pre-amplifier

For the line-level audio, there’s no need for anything more fancy than a couple of resistors to act as a passive summation of the left and right channels, however for a microphone and for the LeoStick, I’d need a preamp.  I grabbed a LM358 and plugged that into my breadboard alongside the TDA1905.

Before long, I had a working microphone preamp working using one half of the LM358, based on a circuit I found.  I experimented with some resistor values and found I got reasonable amplification if I upped some of the resistor values to dial the gain back a little.  Otherwise I got feedback.

For the LeoStick, it already puts out 5V TTL, so a unity-gain voltage follower was all that was needed.  The second half of the LM358 provided this.  A passive summation network consisting of two resistors and DC-blocking capacitor allowed me to combine these outputs for the TDA1905.

One thing I found necessary, the TDA1905 and LM358 misbehave badly unless there’s a decent size capacitor on the 9V rail.  I found a 330uF electrolytic helped in addition to the datasheet-prescribed 100nF ceramics.

Power supply

Since I’m running on batteries with no means of generating power, it’s important that the circuit does not draw power when idle.  Ideally, the circuit should power on when either I:

  • plug the USB cable in (for firmware update/USB audio)
  • toggle the external source switch
  • press the bell button

We also need two power rails: a 9V one for the analogue electronics, and a 5V one for the LeoStick.  A LM7809 and LM7805 proved to be the easiest way to achieve this.

To allow software control of the power, a IRF9540N MOSFET was connected to the 12V input and supplies the LM7809.  The gate pin is connected to a wired-OR bus.  The bell button and external source switch connect to this bus with signal diodes that pull down on the gate.

Two BC547s also have collectors wired up to this bus, one driven from the USB +5V supply, and the other from a pin on the LeoStick.  Pressing the Bell button would power the entire circuit up, at which point the LeoStick would assert its power on signal (turning on one of the BC547s) then sample the state of the bell button and start playing sound.  When it detects the button has been released, it finishes its playback and turns itself off by releasing the power on signal.

Sound effect generator

Earlier I had prototyped a bell generator, however it wasn’t much use as it just repeatedly made a bell noise regardless of the inputs.  To add insult to injury, I had lost the source code I used.  I had a closer look at the MCU datasheet, deciding to start from a clean slate.

The LeoStick provides its audio on pin D11, which is wired up to Port B Pin 7.  Within the chip, two possible timers hook up: Timer 0, which is an 8-bit timer, and Timer 1, which is 16-bits.  Both are fed from the 16MHz system clock.  The bit depth affects the PWM carrier frequency we can generate, the higher the resolution, the slower the PWM runs.  You want the PWM frequency as high as possible, ideally well above 20kHz so that it’s not audible in the audio output, and obviously well above the audio sampling rate.

At 16MHz, a 16-bit timer would barely exceed 240Hz, which is utterly useless for audio.  A 10-bit timer fares better, with 15kHz, older people may not hear it but I certainly can hear 15kHz.  That leaves us with 8-bits which gets us up to 62kHz.  So no point in using Timer 1 if we’re only going to be using 8-bits of it, we might as well use Timer 0.

Some of you familiar with this chip may know of Timer 4, which is a high-speed 10-bit timer fed by a separate 64MHz PLL.  It’s possible to do better quality audio from here, either running at 10-bits with a 62kHz carrier, or dropping to 8-bits and ramping the frequency to 250kHz.  Obviously it’d have been nice, but I had already wired things up by this stage, so it was too late to choose another pin.

Producing the output voltage is only half the equation though: once started, the PWM pin will just output a steady stream of pulses, which when low-passed, produces a DC offset.  In order to play sound, we need to continually update the timer’s Capture Compare register with each new sample at a steady rate.

The most accurate way to do this, is to use another timer.  Timer 3 is another 16-bit timer unit, with just one capture compare output available on Port C pin 3.  It is an ideal candidate for a timer that has no external influence, so it gets the job of updating the PWM capture compare value with new samples.

Timer 1 is connected to pins that drive two of the three LEDs on the LeoStick, with Timer 4 driving the remaining one, so if I wanted, I could have LEDs fade in and out with it instead of just blinking.  However, my needs are basic, and I need something to debounce switches and visibly blink LEDs.  So I use that with a nice long period to give me a 10Hz timer.

Here is the source code.  I’ll add schematics and other notes to it with time, but the particular bits of interest for those wanting to incorporate PWM-generated sound in their AVR projects are the interrupt routine and the sound control functions.

To permit gapless playback, I define two buffers which I alternate between, so while one is being played back, the other can be filled up with samples.  I define these on line 139 with the functions starting at line 190.  The interrupt routine that orchestrates the playback is at line 469.

When sound is to be played, the first thing that needs to happen is for the initial buffer to be loaded with samples using the write_audio function.  This can either read from a separate buffer in RAM (e.g. from USB) or from program memory.  One of the options permits looping of audio.  Having loaded some initial data in, we can then call start_audio to set up the two timers and get audio playback rolling.  start_audio needs the sample rate to configure the sample rate timer, and can accept any sample rate that is a factor of 16MHz (so 8kHz, 16kHz up to 32kHz).

The audio in this application is statically compiled in, taking the form of an array of uint8_t‘s in PROGMEM.

Creating the sounds

I initially had a look around to see if I could get a suitable sound effect.  This proved futile, I was ideally looking around for a simple openly-licensed audio file.  Lots of places offered something, but then wanted you to sign up or pay money.  Fine, I can understand the need to make a quid, and if I were doing this a lot, I’d pay up, but this is a once-off.

Eventually, I found some recordings which were sort of what I was after, but not quite.  So I downloaded these then fired up Audacity to have a closer look.

The bicycle bell

Bicycle bells have a very distinctive sound to them, and are surprisingly complicated.  I initially tried to model it as an exponentially decaying sinusoid of different frequencies, but nothing sounded quite right.

The recording I had told me that the fundamental frequency was just over 2kHz.  Moreover though, the envelope was amplitude-modulated by a second sinusoid: this one about 15Hz.  Soon as I plugged this second term in, things sounded better.  This script, was the end result.  The resulting bell sounds like this:

So somewhat bell-like.  To reduce the space, I use a sample rate of 6.4kHz.  I did try a 4kHz sample rate but was momentarily miffed at the result until I realised what was going on: the bell was above the Nyquist frequency at 4kHz, 6.4kHz is the minimum practical rate that reproduces the audio.

I used Audacity to pick a point in the waveform for looping purposes, to make it sound like a bell being repeatedly struck.

The horn

I wanted something that sounded a little gutsy.  Like an air-horn on a truck.  Once again, I hit the web, and found a recording of a train horn.  Close enough, but not long enough, and a bit noisy.  However opening it up in Audacity and doing a spectrum analysis, I saw there were about 5 tones involved.  I plugged these straight into a Python script and decided to generate those directly.  Using a raised cosine filter to shape the envelope at the start and end, and I soon had my horn effect.  This script generates the horn.  The audio sounds like this:

Using other sound files

If you really wanted, you could use your own sound recordings.  Just keep in mind the constraints of the ATMega32U4, namely, 32kB of flash has to hold both code and recordings.  An ATMega64 would do better.  The audio should be mono, 8-bits and unsigned with as lower sample rate as you can get away with.  (6.4kHz proved to be sufficient for my needs.)

Your easiest bet would be to either figure out how to read WAV files (in Python: wave module), or alternatively, convert to raw headerless audio files, then code up a script that reads the file one byte at a time. The Python scripts I’ve provided might be a useful starting point for generating the C files.

Alternatively, you can try interfacing a SDCard and embedding a filesystem driver and audio file parser (not sure about WAVE but Sun Audio is easily parsed), this is left as an exercise for the adventurous.

Finishing up

I’ll put schematics and pictures up soonish.  I’m yet to try mounting the whole set up, but so far the amplifier is performing fine on the bench.

Aug 032014
 

Well, for a few weeks now I’ve been (metaphorically) tearing my hair out, trying to figure out why I had such bad antenna problems on VHF.  HF, I still have work to do as right now, the RF just induces currents where it pleases, including in my microphone cabling, but that’s a different matter.  VHF until recently had been rock solid.

I tried replacing coax, re-terminating leads, checking solder joints, replacing antennas.  Yesterday, I re-wired the entire antenna system, doing away with the BNC connectors in the top box and hard-wiring the antenna mounts to the coax inside.  Rode up to Ashgrove today thinking I had fixed the problem.

Nope.

Each bump in the road, I watched the S-meter graph move from S9 to S4 and back again.

What could it be?  Why is it that it only occurs when I’m mobile, and not stationary?  There’s a bad link somewhere, but where?  No amount of jiggling cables was locating the problem.  Finally today, I decided to take a peek inside the FT-857D.

Ohh, well that would do it!

Ohh, well that would do it!

I looked closely at the point where the N connector solders to the PCB. I noticed a small line around the wire where it met the solder blob. So I picked at it with pliers, and pulled it away from the blob: it was a dry joint!

Tomorrow, I shall know if this was the final problem.  At least I don’t run full power on FM, my license only affords me 30W continuous, so the only time I do 50W is when I’m doing SSB at which point it’s only on voice peaks.

Update: It’s been a few days, the difference is like the difference between chalk and cheese.  Prior to the fix my set was deaf as a post, and it’s not hard to see why!

Aug 052013
 

This last week the local repeaters here in Brisbane have been rather quiet.

One repeater I used to use a lot has been acting up, and so rather than potentially try to exacerbate a possibly worsening issue, I figured I’d leave it well alone until it was fixed.

Another I lurk on, has been working fine, but many of the people I’d talk to, are away on holidays.

So, I figured I’d dust off my trusty HF whip and give the lower frequencies another crack.  This time last week I was getting nowhere on 15m.  Maybe wrong place at the wrong time (Aside: is there ever a right time to be in the wrong place?) and so didn’t get anywhere.

40m I knew worked on this particular antenna, so I’ve been lurking there… calling in on the Coral Coast net on 7060kHz in the mornings, and tuning up and down the band on the way home.  I make note of my listening frequency via APRS-IS, see my tracker or look for VK4MSL-10 on aprs.fi.

I knew the antenna worked there, not perfect, but it did work.  It works particularly well when the other station is equipped to pick up weak stations.  Earlier this evening, I set out from my workplace listening on 7060kHz where I was this morning.  I noticed it was chock-a-bloc full of stations north of us.  Indonesia and surrounds by the sounds of things.

Couldn’t make a head or tail of what they were saying, so I moved up the band, stumbled across a couple of “local” stations chatting around 7175kHz.  Turns out one was portable in Barcaldine, didn’t catch the name, but the callsign was VK2DQC, I think.  (I didn’t write it down.)  We chatted for a short while, but apparently my signal was up and down like a yo-yo.

No surprise, I started the QSO walking up Greer Street, Bardon, continued my next over riding down The Drive, Cecil Road and Bowman Parade then up through Sunset Park.  Anyone who knows that stretch knows it goes up then down then up again then down.  I finished the chat as I came down Monoplane Street, Ashgrove.

Tuning around, I found another pair talking on 7158kHz.  Bob VK6KJ and Bruce VK2??.  As they were talking a third station, Joe, W5?? called in from Florida USA.  To say I was impressed would be an understatement, all three were coming in Q5, and signal strengths in excess of S6 in most cases.  Bob was peaking S9.

Joe mentioned is misfortune of having some equipment destroyed in a storm, and this necessitating the replacement of a computer along with its OS.  Apparently he’s not a fan of “Window 8” (as we call it at work).  I did try to call Joe but must’ve doubled and wasn’t heard.

I did though, manage to make contact with Bob.  He was located in Albany, about 400km south-southeast of Perth, and running 400W into a two-element beam pointed at the US.  With my measly 100W and stubby home-made antenna, I apparently was registering a Q5S5 signal with the odd drop-out.

Clearly Bob’s end was doing all the work, but impressive nonetheless.  Seeing as the evenings can be particularly quiet, I think I’ve found a new past-time to while away the hour-long trip home, stirring up HF on the deadly-treadly-mobile!

Jun 092013
 

Over the last year or so, I’ve done a number of improvements to the bicycle mobile station.  I’ve kept meaning to document what’s happened, as a number of people have asked about the station, and not everyone gets to see it up close.

A big move was when the FT-290RII 25W PA died, I was using the FT-897D a lot, and that thing is a heavy lump of a radio to lug around.  So I bought its smaller sister, the FT-857D with its remote head kit.

A second move was from the heavy 40Ah battery pack to a much lighter 10Ah pack.  Then, in July last year, I bought myself a new pair of wheels.  The ’09 model Boulder pictured earlier still gets regular use and is good on the road, but longer trips and on hills, it’s a drag, and the tyres are not good on dirt.

Thus I bought a Talon 29 ER 0… in contrast to the Boulder, this bike is designed with mountain-biking sports in mind, so a little heavier duty, better gearing and suspension.  Sadly not dual-suspension … they don’t seem to make one that will take a pannier rack on the back like I require.  Nonetheless, this one has been going well.

VK4MSL/BM Mk3: New and improved

VK4MSL/BM Mk3: New and improved

Rather than buying an open basket like I did on the other, I went one step further, I bought a motorcycle hard top-box and mounted that on the back.  Thus the FT-857D could live in there, sheltered from the weather.  I later also bought pannier bags: my battery, some tools, spare tubes, visors for the helmet, etc, live in one bag, my clothes live in the other.

The station is otherwise, not much different to how it was in concept.  The antennas now mount on opposite sides of the top box with right-angle aluminium.  I still have to work on grounding for the HF side but even then, the station still delivers respectable performance on 40m.

On my way to BARCfest this year, I was being heard S9+40dB in Newcastle with 60W PEP.  I’d have ran 100W, but due to the earthing problems, I found I was getting a bit too much RF feedback.

The 2m antenna is similar to previous ventures, just a 51cm length of RG-213 with the jacket and braid stripped off and a PL-259 plug soldered onto one end.  It’s a simple design that’s easy to make, easy to fix, cheap and can be constructed from readily available parts.  If you can make your own patch leads, you can make one of these.

VK4MSL/BM: 2m antenna.  Just some RG-213 and a PL-259 connector is all you need

VK4MSL/BM: 2m antenna. Just some RG-213 and a PL-259 connector is all you need

70cm remains a work in progress.  In theory, a ¼? antenna resonant at 144MHz should also resonate at 432MHz, as this is the ¾? frequency.  In practice, this has been a pain to tune.  I basically just stick to 2m and leave it at that.

As for coupling the radio to the head unit… I could use the leads that Yaesu supplied.  One distinct disadvantage with this is that it ties me into using only compatible equipment.  The other is that the connectors are just not designed for constant plugging/unplugging, and the 6P6C and 8P8C connectors become unreliable very quickly if you do this.  A solution was to make up a patch lead to go onto each end, and to use some standard cable in the middle.

Initially I did this with a 25-pin printer cable, but found the RF problems were terrible!  Three lengths of CAT5e however, did the job nicely.  Yes, I sacrifice one pin, right in the middle.  24 pins is more than enough.  I allocate six pins on one end for the head unit cable; choosing the wires so that the connections are consistent at each end.

The other end, I have a standard convention for microphone/control cabling.  The balanced nature of the CAT5e works well for microphone cabling on a radio like the FT-857D which was designed with dynamic microphones in mind.

The only other connectors I need then are for power, and for lights.  Power I just use Anderson PowerPole type connectors, the 30A variety… and for lighting, I use ruggedised 6-pin automotive connectors.

VK4MSL/BM Mk3: Rear connections onto top box

VK4MSL/BM Mk3: Rear connections onto top box

At the handlebars, things have been refined a little… the switches and push buttons are in plastic boxes now.  Here I still have to work on the front basket mount, this compromise of a former broomstick handle hose-clamped to the handlebars is a workaround for the basket bracket’s inability to clamp around the rather thick handlebars.  This arrangement is fine until one of the hose clamps slips (which happens from time to time).

For now I put up with it.  The controls from the radio are now mostly on the left side… Since the rear gear shift and front brake are on the right-hand side, I do far more with my right hand than with my left.  Thus doing this, I free up my right hand to actually operate the bike and use my less-busy left hand to operate the radio.

VK4MSL/BM: Front handlebar controls

VK4MSL/BM: Front handlebar controls

I mentioned earlier about HF… the HF antenna should look familiar.  It’s actually the same one I’ve been using for a while now.  Most distant contact so far has been into the Cook Islands on 20m.  I’ve had successful contacts on 80m, 40m, 20m and 15m with this antenna.  10m and 6m are the two that elude me just now.

VK4MSL/BM Mk3: With the HF antenna

VK4MSL/BM Mk3: With the HF antenna

It is a little difficult to see the entire antenna.  I did try to pick the angle to show it best… but if you look above the tree, you’ll see the tip of it immediately above the top box.  Below is a close-up shot to give you an idea where to look.

VK4MSL/BM Mk3: Base of HF antenna

VK4MSL/BM Mk3: Base of HF antenna

One big advantage of the new set up, is that night-time visibility is much better than before.  On the front I have a LED strip which lights up the path maybe 2m ahead of the front wheel.  Not a strong light, but ticks a box… my main headlight is on the helmet — people frequently assume they’re being filmed by it.  On the rear however, is a different story:

VK4MSL/BM Mk3: All lit up

VK4MSL/BM Mk3: All lit up

It doesn’t look like much in the day time, but it is quite bright at night.  The back uses two LED strips mounted in behind the red plastic on the top box, and one can easily read a book in the light produced.  Looking in the rear vision mirrors at night, the red glow can be seen reflecting off objects for a good 100m or so.

On my TO-DO list, is to mount switches to operate the brake light (just above the callsign).  Options include reed switches, hydraulic switches in the brake lines, or strategic placement of micro-switches.  I’ll have to experiment.  The other electronics is in place.

As to the other bike?  It’s still around, in fact if you look at the photo of the VHF antenna, you can see it in the background… along side the trailer I use when I do my grocery shopping.

I’ve done away with the basket on it, and gotten a second mounting plate, so the same top box fits on the back of the other bike, along with the same pannier bags, and same front basket.  It has done about 2800km since I bought the Talon (mid July, 2012), the Talon itself has done 2617km.

Thus I’d estimate the Boulder is well and truly past the 10000km mark, probably closer to 11000km now.  It’s still the primary means of getting around, averaging close to 100km a week and with a heavy load.  Not bad for a bike that’s designed for a little recreational riding.

May 012011
 

Well, this afternoon I decided to fix a couple of problems with the bicycle mobile… firstly, the mounting of one noisy headlamp.  I’ve re-done the mount using a more solid piece of plastic this time, so we shall see how it goes.  No noise on HF so things are looking up there.  I also fixed the headset connection which was causing speakers to drop out… the problem turned out to be in the headset connection on the bike, rather than in the helmet.

It was approaching 2PM and thus nearly time for the daily Travellers net on 21.185MHz USB.  I’ve never made a contact on 15m before, but knew the antenna did tune up there, so I gave it a shot.  Ross VK5KMH popped up with a 58 signal out of Adelaide listening… after a prolonged silence, I decided to throw a call out.  Ross responded, reporting my signal into his station was also a 58 signal.

This was from the driveway at my home location, using 100W transmit power (not far from where I reached VK100WIA on 20m with a CB whip).  So evidently this home brew whip works quite well on 15m.  I have since brought the FT-897D inside and plugged it into my G5RV-like antenna, and after moving off frequency to tune, I notice Ross is still a 58 signal, so evidently my HF antenna doesn’t do much better than the whip does.

Apr 292011
 

Well, the antenna I tuned up in my last post, I can say, while it doesn’t work that great on 80m, it did get a contact into Victoria this evening on the AWNOI net.  Terry VK2TEZ near Coffs Harbour gave me a 4-3 signal report, so still lots of room for improvement… part of that was due to static crashes from storms in NSW, but I think with a better tuned antenna, we should be able to get towards having a workable antenna.  At the moment the autotransformer I use has ~95 turns, with output taps at 0, 25, 50 and 75 turns.  I think one somewhere between 0 and 25, and/or some extra turns might help… so I might wind a new one and see where that gets us.

The headlight still continues to give me grief.  An interesting discovery though this evening.  Since the battery is no good, I’ve permanently mounted it to the bicycle frame.  This was achieved by removing the plastic bracket which is used to mount the headlight on the handlebars or on the helmet mount (using a rubber O-ring), and replacing this with a bracket bent out of a short piece of aluminium.  It fastens to the bicycle frame at the front right above the front wheel, using a bolt hole normally used for mounting rim brakes (my bike has disc brakes).

The upshot is that the headlight’s casing has a pretty good electrical connection to the bicycle frame.  Turns out this is a big no no with these lights.  Kiss goodbye HF if you do… you’ll get crap everywhere from 400kHz right up into the VHF.  I’ll have to do some further investigation, but I found that if I insulated the case from the frame, it helped on the 400kHz and HF emissions.  I think something parasitic is causing the 2m grief as this continues (that, or it’s less critical on the case being earthed).

For a while I thought it might’ve been something lurking around 415kHz… the standard IF frequency of most superhetrodyne receivers, but alas, can’t see anything there.  Otherwise it’d explain why it appears to be everywhere.  I definitely suspect it’s not supposed to be oscillating there though, so I think parasitic oscillations are the cause here.  I’m slowly researching my own power supply for the LED in this headlamp, so its days are numbered.

The insulation was achieved by breaking a cheap plastic picnic knife, drilling a couple of mounting holes, and mounting the headlight on that.  That quelled the HF interference quite a bit, and I was able to listen to the HF bands on my way into Brisbane.  At least it was nice to listen to something other than that sodding wedding in the UK.  (C’mon fellas, yes, great and all but can’t we just confine it to one station?)

I was concerned about the longevity of this arrangement however.  And as it turned out, I was right to be concerned.  It broke as I approached the Normanby Fiveways.  I went over a bump, heard a crack, and noticed the headlight dangling by the power lead.  I pulled over, threw it in the basket and grabbed the backup headlight.  At least there was one on the helmet, a 1W LED, so I still complied with local laws for night riding.  I didn’t have a mounting for the backup light, I just pointed it forward sitting in the bottom of the front basket, with it on flash as a warning to drivers.

Once at the destination, I reverted the headlight back to being directly mounted on the bicycle frame.  Interference was intermittent, but when it was acting up, it did wipe out 80m with S6 noise.  Not good when most stations are barely making S6 as it is.  I wound up turning off the main headlamp as for the most part I could see where I was going, and I knew the route.  As I got out of town this was less of an issue due to the lack of traffic, and of course I was on bicycle paths or the footpath for 90% of it.  That at least allowed me to hear what was going on with the net.

The other flaw I had was that the helmet’s speaker connections were acting up… wound up unplugging the earpiece side of the headset adaptor and using the internal speaker.  Thankfully I could still use the helmet’s microphone and the rest of the wiring harness… just not the speakers in the helmet.  I noticed this as I pulled out of my street, in fact I was aware there was a problem, but now I know where the problem is now.  I’ll get onto it tomorrow.  And I’ll look at a better way to mount this headlamp in an insulated fashion as an interim solution to a power supply replacement.

Apr 172011
 

If you ever decide to put any kind of sensitive receiver on a bicycle, you’ll want to avoid this ugly duckling of the bicycle lighting world:

Nitelights Illuminator 900

Nitelights Illuminator 900

These lights are great from the illumination point of view, and they’re not badly priced either.  However, from an EMC viewpoint, they stink.  I was given one as a present some time ago.  The first night I got it, I mounted it on the helmet, charged up its battery, then went to work the next day.  That evening, tried using the radio on the bicycle as I rode home.  The interference rendered the radio totally useless.

At first I couldn’t figure out why the signals sounded so bad on receive.  I was stuggling to hear repeaters that were normally quite strong.  The only thing that was new was the headlight.  I got home, switched on the set in my room and tuned to 2m sideband, then put the headlight on flash.  The tell-tale static from the radio gave away the headlight as being the culprit.  Worst of all, the emissions weren’t conducted, they were radiated.

Since then I’ve spent the last few months trying to figure out ways to make this headlight less noisy.  The following page serves as a notepad, and I’ll keep adding to this as time goes on.

Initially when I used it I kept the battery in my shirt pocket.  This proved to be a fatal mistake, since the roughly ¼? power cable proved to be a very effective radiator of this interference.  I found winding the power lead up into a tight coil and moving the battery to the top of the helmet right behind the headlight helped things a bit.  Adding a 2.2mF capacitor in parallel to the headlight further reduced emissions to make it barely tollerable.

Then the battery pack died.  After a few weeks of non-usage, the cells collapsed.  So for a while the problem solved itself, I could no longer use the headlight.  The headlight’s battery pack runs at a nominal 7.2V (typical 8V).  Since I run a 12V supply on the bicycle, I set about making a step-down power supply that could drop my 12V supply down to 8V approximate to run the headlight.

Initially I tried using a LM7808 linear reg.  This worked, with one major drawback: the linear reg got bloody hot.  Turns out this headlight draws about 1A of current when at full brightness.  That means in order to drop 4V, the reg was dissipating 4W of power.  Ooops!  Poor thing.

I designed a crude switchmode power supply to do the work.  Using a voltage divider to provide a voltage reference, I used a LM311 comparator to detect when we were under voltage.  The output signal from this would pull down on the gate of a IRF9540N MOSFET which acted as the main switch.  Hysteresis was used to fine-tune the switching behaviour.  Capacitors at both sides would smooth the waveform.  470µF was used on the input side, 330µF on the output (we still have that 2.2mF capacitor not far away).  I also made liberal use of 100nF decoupling capacitors to try and control the rate of switching.

On the breadboard with a dummy load, my circuit performed pretty well.  A fairly smooth output with a bit of ripple at high load.  I mounted it in a box and tested it with the headlight, and presto, the headlight was back in service.  I mounted both on the bicycle directly, so as to minimise cable length and therefore radiation.

Since now the radio shared a power rail with the light, I knew there was potential for conducted emissions to cause problems as well as radiated.  The next evening I tried it out… 2m was lousy with the headlight turned on.  The good news is that it wasn’t much worse than before, but it still rendered the radio useless at times, particularly if the signal was weak from the repeater.

Recently, I added some 470µH inductors in series with the headlight and my switchmode power supply.  I also tried common-mode chokes to no avail.  A 1mF capacitor has been added in parallel to the 470µF capacitor in my power supply to further try and reduce the noise.

Low-pass filtering on headlight

Low-pass filtering on headlight: 2.2mF capacitor in parallel, 470µH inductor in series

Part of the problem was that I was shooting blind to try and find the interference.  Today, whilst tuning up the HF antenna, I decided to experiment and see what bands this headlight had an influence on.  To my (unpleasant) surprise, interference was severe right down to 80m.  I used the spectrum analyser built into the FT-897D to hunt for the culprit, and found it lurking at around 400kHz.  When the power supply initially starts up, it’ll be up around there, then it sinks down to 392kHz as the case warms up:

Noise from the headlamp detected around 400kHz

Noise from the headlamp detected around 400kHz

This appears to be the fundamental frequency for the switchmode power supply built into the headlamp.  It would appear to have a fairly sharp square-wave type pulse, as it contains very strong odd-order harmonics.  The same interference can be observed at around 1260kHz (3rd harmonic).

The fact that this frequency is so low, probably suggests it is being intermodulated with a parasitic oscillation at some higher frequency.  This I have not yet found, the CRO showed some other hash over the signal, but I will need to do some further investigation, probably with a more sophisticated spectrum analyser than the primitive one found in my transceiver.

I’d be interested to hear from others who have had issues with these headlamps.  In particular, interference to AM/FM radio reception or transceiver operation would be quite useful if the need to persue this with the ACMA ever comes up.  The manufacturer of the headlights has so far been unresponsive to my queries, so in the meantime I can only recommend that people avoid using these headlights if they intend to use any kind of radio receiver whilst riding at night.

In the meantime, I’ll be chucking further notes here as I find more on this issue.