|| Radio Shack
|| Digi Key
|| LM741 Op-Amp
|| NE741,µA741, etc.
|| 2N2222A transistor
|| See text
|| 1N4148 Diode
|| 1N4001, or others
|| 10K Thermistor
|| KC003T in prototype
|| 12V Relay
|| RS is 1A (min)
|| 47K, 5% resistor
|| 5%, Resistor
|| see text
|| 1K8, 5% resistor
|| 2K2, 5% resistor
|| 50K Trimmer Pot
The Thermistor, or NTC (Negative Temperature Coefficient) of 10K, is a standard type. Most types will work. The one
in the diagram is a 10K model made by Fenwal (#197-103LAG-A01).
The resistance lowers as the surrounding temperature increases which affects the output (pin 6) and energizes the
small relay and Led1(optional, just cosmetic and can be left out).
P1 is a regular Bourns trimmer potentiometer and adjusts a certain range of temperatures. I used a 50K, 10-turn
type for a bit finer adjustment but any type will work.
R2 is optional in case your relays tends to 'chatter' a bit. It provides a bit of hysteresis when the set temperature
of the thermistor reaches its threshold point. This value may need to be adjusted anywhere between 120K and 470K
(although I indicated different values on the schematic).
Transistor Q1 can be a 2N2222(A), 2N3904, NTE123A, ECG123A, etc. Not critical at all. It acts only as a switch for
the relay so almost any type will work, as long as it can provide the current needed to activate the relay's coil.
D1, the 1N4148, acts as a spark arrestor when the contacts of the relay open and eliminates false triggering. Feel
free to use any other type, like a 1N4001 or something. Solder directly onto the '+' and '-' relay terminals.
If you need a 'Frost' sensor, just swap positions of the R1 and Th1 positions.
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Copyright © 2003 - Tony van Roon