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.

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

---

Back to Circuits Page
Copyright ©2003 - Tony van Roon