##
A common Electonics project Heat Sensor

Here is a simple circuit which can be
used as a heat sensor. In the following circuit diagram thermistor and
100 Ohms resistance is connected in series and makes a potential divider
circuit . If thermistor is of N.T.C (Negative temperature Coefficient )
type then after heating the thermistor its resistance decreases so more
current flows through the thermistor and 100 Ohms resistance and we get
more voltage at junction of thermistor and resistance. Suppose after
heating 110 ohms thermistor its resistance value become 90 Ohms.then
according to potential divider circuit the voltage across one resistor
equals the ratio of that resistor’s value and the sum of resistances
times the voltage across the series combination. This concept is so
pervasive it has a name: voltage divider. The input-output relationship
for this system, found in this particular case by voltage divider, takes
the form of a ratio of the output voltage to the input voltage.

This output voltage is applied to a NPN transistor through a resistance. Emitter voltage is maintain at 4.7 volt with a help of Zener diode.This voltage we will use as compare voltage. Transistor conducts when base voltage is greater than emitter voltage. Transistor conducts as it gets more than 4.7 base Voltage and circuit is completed through buzzer and it gives Sound.

This output voltage is applied to a NPN transistor through a resistance. Emitter voltage is maintain at 4.7 volt with a help of Zener diode.This voltage we will use as compare voltage. Transistor conducts when base voltage is greater than emitter voltage. Transistor conducts as it gets more than 4.7 base Voltage and circuit is completed through buzzer and it gives Sound.

*Circuit Diagram of Heat Sensor*

You may also be interested in Project using Precision Temperature Sensor .