Automatic Fan Switch

Parts List:

 R1 = 1K2        C1 = 22uF, 25V, electrolytic or tantalum
 R2 = 1K2        D1 = 1N4001, regular silicon diode
 R3 = 15K       Re1 = Relay, 9-12V
 R4 = 1K2        Q1 = 2N2222, 2N3904, PN100, etc. NPN transistor
 R5 = 270       IC1 = LM339, quad op-amp
 R6 = 2K2        P1 = 47K, 10-turn trimmer potentiometer
Th1 = 1.7K type available at Radio Shack

How It Works:
The op-amp used is a readily available LM339. This IC contains four op-amps but we use only two of them. Use the others for spare or do something else with it.

For the thermistor I used a small, very expensive ($52), glass-encapsulated type because I had it laying around but almost any type will do. I experimented with a couple different types and they all worked.

Copyright and Credits:
Tony van Roon, VA3AVR, 2002. Re-posting or taking graphics in any way or form of this project is expressily prohibited by international copyright © laws.