Thermostat Alarm, Tony van Roon,

(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 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.

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