Table 1
Part Description Radio Shack Digi Key Newark Notes
IC1  LM741 Op-Amp  276-007  LM741CN-ND  LM741CN  NE741,µA741, etc.
Q1  2N2222A transistor  276-2009  2N2222A-ND  2N3904  See text
D1  1N4148 Diode  276-1122  1N4148GICT-ND  1N4001  1N4001, or others
Th1  10K Thermistor  271-110  KC003T-ND  96F3309  KC003T in prototype
Re1  12V Relay  275-249  Z753-ND  83F8057  RS is 1A (min)
R1  47K, 5% resistor        yellow-purple-orange
R2   5%, Resistor        see text
R3   1K8, 5% resistor     Optional    brown-gray-red
R4,R5  2K2, 5% resistor        red-red-red
P1  50K Trimmer Pot        Multi-turn

Couple Notes:
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