Sunday, April 7, 2013
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Heat Sensor Circuit
Heat Sensor Circuit
This is the simple heat sensor circuit. It can be used to control any device using heat sensor. In this circuit a thermistor and a resistance is connected in series. This arrangement makes a potential divider circuit.
Here the thermistor is Negative Temperature Coefficient type.
So when the room temperature is increased its resistance decreases
simultaneously and more current flows through the resistor and the
thermistor. We find more voltage at the junction of the resistor and the
thermistor.
So when the room temperature is increased its resistance decreases
simultaneously and more current flows through the resistor and the
thermistor. We find more voltage at the junction of the resistor and the
thermistor.
Our
thermistor resistance value is 110 ohms. Suppose the resistance value
becomes 90 ohms after heating the 110 ohms thermistor. Then the voltage
across one resistor of the voltage divider
circuit equals the ratio of that resistor’s value and the sum of
resistances of the voltage across the series combination. This is the
concept of voltage divider.
thermistor resistance value is 110 ohms. Suppose the resistance value
becomes 90 ohms after heating the 110 ohms thermistor. Then the voltage
across one resistor of the voltage divider
circuit equals the ratio of that resistor’s value and the sum of
resistances of the voltage across the series combination. This is the
concept of voltage divider.
The final output voltage of the voltage divider circuit is now applied to the npn transistor (BC548) through the base resistor (3.3K ohms). Here the emitter resistor is replaced with a zener diode.
Emitter voltage is maintained at 4.7volt with the help of zener diode.
This voltage is used to compare voltage. Transistor conducts when base
voltage is greater than the emitter voltage. Transistor conducts if it
gets more than 4.7volt of base voltage. Then the circuit is completed
through buzzer and it gives sound.
Emitter voltage is maintained at 4.7volt with the help of zener diode.
This voltage is used to compare voltage. Transistor conducts when base
voltage is greater than the emitter voltage. Transistor conducts if it
gets more than 4.7volt of base voltage. Then the circuit is completed
through buzzer and it gives sound.
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