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.