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.
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Copyright © Tony van Roon, VA3AVR