|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|
The Th1 thermistor is a regular NTC (Negative Temperature Coeficient) of about 1000 ohms.But other types will worktoo.Adjust the sensitivity with the 20K trim-pot P1.
When the thermistor is at operating temperature, the resistance is highThe 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.