This temperature monitor alarm circuit continuosly monitors the room temperature and beeps when the temperature drops below 20 degrees centigrade. This capability to constantly monitor the room temperature can help lower your air-conditioning costs by reminding you to turn off your airconditioner. An optical display like a thermometer has a limited effectivity since it can be easily overlooked. A tone signal on the other hand is not easy to ignore.
Let us look at the temperature alarm schematic. R4 is the temperature sensor KTY10. Other temperature dependent resistor types can also be used in tis place. This sensor is connected to a resistance-bridge circuit and supplied with 5 volts. The IC1 CA3130 works as a bridge amplifier. As long as the temperature is higher than the value preset by P1 and P2, the output of IC1 is 0 volt.
Once the temperature drops below the preset value, the plus input of IC1 receives more current and its output level swings to 5 volts, thus switching the oscillator gate U1 on. This gate creates a pulse of aroung 0.2 seconds. This pulse in turn switches on the oscillator gate U3. The tone signal is generated by a piezo-electric crystal which is driven by gate U4.
Thre frequency of the tone signal is aroung 5 kHz. The current consumption is 2 mA which allows the circuit to be powered by a simple power supply. The sensor must be installed outside the housing so that is will not be falsely triggered by the heat of the transformer.
To align the circuit, points A and B must be shorten temporarily so that the alarm beeps. Potentiometer P3 controls the volume of the beep. The chosen room temperature can be roughly tuned by P1. P2 is used as a fine tuner. In normal operation, remove the short between points A and B.