Heat Sensor, Automatic temperature control

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.