The monitor is intended for use in relatively small spaces ( like living rooms). The sensor is a readily available LM335 from a National Semiconductor. This gives an output voltage of 10 mV °C-1. This voltage is compared to IC1a and IC1b with two reference voltages. One of these is preset with P1 and the other with P2. The output of the comparators is used to switch D2, D3, and D5.
When the voltage produced by D1 is smaller than either of the reference potentials, the output of IC1a and IC1b are low: D2 will then light.
When the ambient temperature rises, the output of D1 rises proportionally. When the level of the sensor output lies between the two reference levels, the output of IC1a is high and that of IC1b is low. Diode D3 will then light, showing that the critical temperature has been reached.
At even higher, the output of IC1b will also go high and D5 lights, while the other two LEDs go out. At the same time, the relay will be energized via T1. The relay contact may then actuate an external load (e.g.. a buzzer).
Zener diode D4 ensures that T1 does not come on when D3 lights (since the output voltage of IC1b then rises slightly owing to the current through this IC).
The temperature at which the LEDs should light may be set with P1 and P2. Bear in mind, however, that the monitor is intended for normal temperatures between 25 °C and 100 °C.
The monitor draws a current of about 20 mA: when the relay is energized, this rises to about 50 mA.