(C) Tony van Roon

Parts List:
Part Description Radio Shack # DigiKey # Notes
R1  10K resistor      brown black orange gold
R2  470 Ohm resistor     470QBK-ND  yellow purple black gold
R3  330K resistor      orange orange yellow gold
P1  20K trim pot      bourns, multi-turn
D1  1N4001 diode  276-1101  1N4001DICT-ND  General purpose, rectifier
D2  1N4001 diode  276-1101  1N4001DICT-ND  General purpose, rectifier
Q1  2N2222, transistor      General purpose, NPN
Q2  2N3904, transistor      General purpose, NPN
Re1  9 volt relay      1 Amp

Couple Notes:
The Th1 thermistor is a regular NTC (Negative Temperature Coeficient) of about 1000 ohms. But other types will work too. Adjust the sensitivity with the 20K trim-pot P1.
When the thermistor is at operating temperature, the resistance is high The 330K resistor (R3) is to introduce a little bit hysteresis back on the basis of Q2 to prevent relay chatter.

Final Notes and Adjustments:
Adjust the trim-pot R3 to match the voltage of you battery pack.

The phone number for DigiKey is 1-800-Digi-Key.

This is an older circuit published in "Hands-On Electronics" in 1989. The modified circuit above is © Copyright 1996 by Tony van Roon.

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Copyright ©2003 - Tony van Roon