I am starting my first pj build and was looking around for a thermostat switch. I ended up building my own and decided to share it on LL. I am using a primary power switch to turn the fans, lamp, and lcd on/off. This circuit is used in parallel with that to keep the fans running after I power down the pj until it cools off.

D1 is the temp sensor and can be placed anywhere if you put long enough leads on it. 1n914 diodes are rated for use in up to 340 degrees F. I don't recommend you keep the entire circuit in the lamp chamber as not all of the parts are rated for the heat.

The switch is a dpdt rated for 5A at 240VAC/24VDC. That is serious overkill but it was the only suitable switch RadioShack had in stock.

Decide how many degrees above (your) room temp you want the circuit to active at. That number is T.
The following formula will give you the number of millivolts representing that temp increase.
(5/9)*(T-32)*2.5

Set your multimeter/voltmeter to mV, clip the red lead to pin 5 and the black lead to pin 4 of the LM339.
Adjust R6 until you see the mV you got from the formula.
Replace LED1 with your fan(s) and R7 with a wire.

This circuit does run off of 12V but it is locally noisy so don't use the same 12V brick for your controller card.
I have tested this design on a breadboard and it works but I don't want anyone saying I fried their pj so....
Use this circuit at your own risk.



I will try to answer any questions but sometimes I confuse myself...