Dec 252017
 

So some spare time today… I decide to construct a new I/O module to fix up the mistakes made with the previous iteration.  Mainly:

  • the TVS diodes… going for one with a higher clamp voltage so it doesn’t smoke when 12V is applied
  • switching to a 4-pin connector on the output side, with pins for 0V, GPIO, DRAIN and +12V
  • fixing the pin-out on the input side so it matches the PCB.
  • rather than having jumper leads to make the boards separable, we’ll make one monolithic board that plugs into all 8 channels simultaneously with one long connector.

For the TVS diodes, I ordered some TPD2E007 in SOT23… thinking those would be a reasonable size for hand-soldering.

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Now… how the bloody hell am I going to solder these little tiddlers?  I had thought SOT-23 was about twice that size.  Never mind, can’t un-buy them.

The circuit is pretty much identical to what came before.  My MOSFETs and 4.7nF capacitors seem to have gone walkies, not sure where.  No doubt the arrival of replacements will summon them back.  I decided to use SMT for many parts on this build.

0805 resistors and veroboard aren’t a bad combo really, just have a sharp blade handy to cut the track where needed, and the resistor can straddle the gap made.

For the TVS diodes, the common pin is to ground, so I made a bus bar running vertically down the PCB and scoring the tracks either side.  The common pins could be soldered to that, and the two I/O pins would straddle the division between each track.  Aside from me getting some parts off-by-one at first, this went well.

The zener and schottky diodes of course, being through-hole, went on the other side of the PCB.

I still have to locate where my MOSFETs have gone, and I think I found some 12 ohm resistors (through-hole).  I can use some 0805 1k resistors for the MOSFET gates.  So that’s some MOSFETs and 4.7nF, probably 0805 size capacitors that need ordering in the new year.

Jul 022017
 

So… earlier in the week I received some 74HC574s (the right chip) to replace the 74HC573s I tried to use.

My removal technique was not pretty, wound up just cutting the legs off the hapless 74HC573 (my earlier hack had busted a pin on it anyway) and removing it that way. Since the holes were full of solder, it was easier to just bend the pins on the ‘574 and surface-mount it.

This afternoon, I gave it a try, and ‘lo and behold, it worked. I even tried hooking up one of the LED strings and driving that with the MOSFET… no problems at all.

I had only built up one of the modules at this stage, so I built another 5 on the same piece of strip board.

The requirement is for 5 channels… this meets that and adds an extra one (the board was wide enough). For the full accompaniment and to have ICSP/networking via external connections, a second board like this could be made, omitting the MOSFETs on four of the channels to handle the ICSP control lines and reducing the capacitances/resistances to suit.

Somewhere I have some TVS diodes for this board, but of course, they have legs, upon which they got up and ran away. Haven’t resurfaced yet. I’m sure they will if I buy more though. The spare footprint on the top-left of the main board is where one TVS diode goes, the others go on the I/O board, two for each channel.