help selecting General purpose PNP transistor


Joined Apr 16, 2010
2N3906 should work fine.

I think I bought the parts for that thermostat once, but I never built it. I wonder if I still have the TL431.

ETA: The 2N3906 does not have the ECB output; it is EBC, so you will have to adjust the PCB layout.
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Joined Sep 9, 2010
It'll be more accurate, I think, than a 7805 for instance. But that works, too. You could use a 7812 for the power supply and the reference voltage.

I've seen mixed opinions on this, but I believe the LM35 needs a load on its output to stabilize it. I believe the datasheets recommend a 200Ω resistor.


Joined Aug 21, 2008
The TL431 is set up as a 1.62 volt fixed reference. They probably used it instead of a fixed regulator because it is very stable (50 ppm/°C) and can also be very accurate (1%).