Mr. Murphy strikes again in the new year…
A post a while back, mentioned that the NPN transistor footprint on the Thermo Plug was wrong. I should have tracked this down earlier, but due to all the supply issues I only found some time just now (with thanks to Lennart for helping figure this one out).
Here’s the board footprint:
It’s not hard to see that from left to right, the pinout is emitter-base-collector.
Guess what… NPN TO-92 transistor footprints are not standardized!
Here’s the one from the BC549 I’ve been shipping in a few recent units:
Bzzzt… you have to mount it the other way around to make it work!
To give you an example of the mess w.r.t. orientation, here’s a diagram I found on Solarbotics:
And here’s the one from the 2N4401 I’m going to use for the Thermo Plug from now on:
Well, at least it’s just mirrored, with the base always in the middle. Not quite as bad as with voltage regulators (such as with MCP1702 vs LM2940, where the order is completely different).
If you’ve got a kit with the BC549, please make sure to mount that transistor 180° rotated w.r.t. what’s shown on the pcb. All units sent out from now on will have an NPN transistor which matches what’s on the board.