Simple Thermostat with two transistors. Tony van Roon, www.uoguelph.ca/~antoon

Parts List: 

IC1 = LM3911
 R1 = 15K
 R2 = trimmer potentiometer, any type
 R3 = 24K
 R4 = 7.5K
 R5 = 10K
 C1 = 0.1uF, ceramic
 C2 = 22uF, electrolytic

Notes:
The LM3911 requires only a few resistors and capacitors to turn it into a full-function temperature controller. The schematic diagram illustrates the simplicity of the overal circuit. The resistor network consisting of R1, R2, and R3 are used to provide the set point voltage for the feedback input of the LM3911.

Resistor R4 limits the current through the internal voltage refernc3 of the LM3911 and can be selected for a wide range of values. 7K5 (7500 ohms) is specified in the application notes of the device.

Resistor R5 pulls the output of the IC high when the temperature is below the set point. The internal resistor is in series with a diode that allows a switching voltage of up to 35V to be used with the device. Electrolytic capacitor C2 will absorb any spikes at the output (like a fan).

Newark is 1-800-463-9275.
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Allied Electronics is 1-800-433-5700.
Sayal Electronics has no 1-800 number