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This circuit is
configured as a
comparator.
R1 and R2 provide a
fixed reference voltage
at the non inverting
input.
The inverting input
voltage is set by the
other two resistors.
If the voltage at the
inverting input rises
above the reference
voltage, then the the
output goes to minus 12
volts and the buzzer is
energised.
The behaviour of the
circuit can be changed
by swapping the preset
and temperature
dependent resistors.
Light dependent
resistors etc can
replace the temperature
dependent one.
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