This temperature switching circuit is designed to monitor temperature using a sensor manufactured by National Semiconductor LM335, which provides an output voltage of 10 mV / ° C. This voltage is compared to IC1a and IC1b two reference voltages. One of them is fixed and the other with P2 to P1. Comparator output is used for switching of D2, D3 and D5.
Note that this temperature switching circuit is designed for temperature monitoring "normal" between 25 and 100 ° C.
When the voltage produced by D1 is smaller than both reference voltages, outputs of IC1a and IC1b are in state "L" D2 will light.
When the ambient temperature increases, the voltage of D1 increases proportionally. When the sensor output is between two reference levels, out of IC1a is "H" and that of IC1b is "L". Lights up when the diode D3, indicating that the critical temperature has been reached.
At higher temperatures output IC1b's gone within the "H" state and turns D5, while the other LED will turn off at the same time the relay will be operated through T1. Relay can activate an external load.
Diode D4 prevent opening when T1 is on D3.
Temperature which should turn LEDs can be set with P1 and P2.
Circuit absorbs a current of approx. 20 mA when relay is actuated, it grows to approx. 50 mA.