Tuesday, August 7, 2012

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Thermistor Temperature Warning Alarm

A simple 7555 buzzer circuit that will activate when a preset temperature is reached. Please note that there is no hysteresis in this circuit, so that if the temperature changes rapidly, then the buzzer alarm may activate rapidly
Thermistor Temperature Alarm CircuitThermistor Temperature Warning Alarm Circuit

7555 Pinout

The Astable multivibrator using the low power CMOS timer IC 7555 which is the low power version of the popular 555 IC. The reset pin 4 of IC1 is used to activate the alarm. The astable will work only if the reset pin 4 becomes high. The reset pin is connected to the positive rail through the 10 K NTC thermister. The thermister offers high resistance in cold and its resistance becomes low to few ohms when the temperature in its vicinity increases. So when the temperature is low reset pin of IC1 remains low and astable is in off position and buzzer remains silent. When the temperature near the thermister increases, its resistance decreases and provides voltage to the reset pin of IC1 and the astable starts working.

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