Description.
This simple circuit will produce an
alarm whenever the temperature falls below zero degree. A thermistor is
used here to sense temperature. The op-amp LM7215 is used to compare the
reference voltage and voltage from the thermistor network. Reference
voltage is given to the non inverting input (pin3) of the IC and voltage
from thermistor network is given to the inverting input (pin4).When
temperature becomes less than zero degree the voltage at the non
inverting input becomes larger than the voltage at the inverting input
and the output of the op-amp becomes high. This makes the transistor Q1
ON and drives the piezo buzzer to make the alarm. In the power supply
section, IC 7805 is used to derive 5V from the 9V battery.
Circuit diagram with Parts list.
