(C) Tony van Roon

Parts List:
Part Description Radio Shack # DigiKey # Notes
R1  1K resistor      brown black red gold
R2  1K, NTC      
R3  10K resistor      black brown orange gold
P1  10K trim pot      bourns, multi-turn
D1  Led, red      Light Emitting Diode
D2  1N4148 diode  276-1101  1N4148DICT-ND  Signal Diode
Q1  BC547B, transistor      General purpose, NPN
Q2  BC547B, transistor      General purpose, NPN

Couple Notes:
The R2 thermistor is a regular NTC (Negative Temperature Coeficient) of about 1000 ohms. But other types will work too. Adjust the sensitivity with the 10K trim-pot P1.
When the thermistor is at operating temperature, the resistance is high.
The phone number for DigiKey is 1-800-Digi-Key.

The original circuit was published in the Dutch "Elektuur Magazine" in 1999.
The modified circuit above is © Copyright 1996, Tony van Roon.

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Copyright ©2003 - Tony van Roon