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.