Home > Circuit Bricks > Tutorial
UPDATED:.20:56 21 October 2013

A freezer alarm is needed to ensure the temperature stays below freezing. Sometimes doors and lids are not closed properly causing the contents to defrost and become potentially dangerous to eat.


  • Can detect when a temperature rises above freezing.
  • The temperature can be altered within a limited range.
  • The circuit must run from a 9 volt battery.
  • The alarm must sound a siren loud enough to be heard at some distance.

The sensor is a thermistor in a potential divider. The resistor is a fixed type as the adjustment will be in the voltage reference . A variable reference is used since the resistors at either end of the potential divider can set a narrow range of temperature. The comparator is a basic one with no input resistors since the inputs already will have resistors connected to them. Because the output from a comparator is 0.9v when low, a Darlington Pair driver is used at it needs 1.4v to switch on. The siren is easy. It will be connected with the negative to the previous output since the Darlington pair sinks currrent.

These parts can then be connected as per the block diagram above. All +V connections must connect to the positive power terminal. All 0V must connect to the negative terminal.

The final circuit will then look like the one below. There are a few alterations to the circuit to improve its functioning.

  1. R1 has been changed to 10K since this is about the resistance of the thermistor at zero degrees, so this will produce approximately half the supply voltage at its output.
  2. R2 and R3 have been changed to 4.7K. This effectively narrows the voltage range of the potentiometer and makes setting the correct temperature much easier.
  3. With all circuits, it is advisable to add a decoupling capacitor to the power supply, in particular near ICs. A value of 100uF should suit most applications.
Try this circuit out on breadboard or a simulation package. It works really well.
Written by Phil Townshend - 2008
www.edutek.ltd.uk - Working Electronics For Students & Teachers