Mar 112015

Refrigerator door alarm circuit with two 555 timers

This refrigerator door alarm alerts you when the door has been left open. Very important, because the power consumption increases significantly when this happends.

How the Refrigerator door alarm circuit works?

This circuit uses an LDR or photoresistor as door sensor. When the sensor is illuminated by the light within the refrigerator, the circuit will emit an intermittent sound to attract attention. When the door is closed, the light goes off and the circuit will stop emitting the sound.

The refrigerator door alarm circuit uses two 555 timers, connected as shown on the diagram.

Refrigerator Door Alarm Circuit

  • When the LDR is not illuminated (the door is closed), the voltage on pin 2 (trigger) of the first 555 timer, is high and the voltage of the output (pin 3) is low. This causes that the second 555 timer remains disabled (low voltage level on pin 4) and no alarm sound.
  • When the LDR is illuminated (the door is open), the voltage level on pin 2 of the first 555 timer goes low and its output (pin 3) begins to oscillate (square wave).

When the voltage level of the first 555 output (pin 3) is on high level, it enable the second 555 timer, which oscillate like the first 555 timer, but at a higher frequency. The output of the second 555 timer is connected to a buzzer that will beep.

The circuit uses a 9 volt battery, and should be placed as close as posible to the inner light of the refrigerator. The circuit must be sealed in a waterproof box to prevent moisture from affecting its operation.

List of components for the Refrigerator door alarm circuit

  • 2  555 timers (IC1, IC2)
  • 1  1uF, 25V electrolytic capacitor (C1)
  • 1  100nF capacitor (C2)
  • 1  10K, 1/4W resistor (R1)
  • 1  LDR (R2)
  • 1  2.2M, 1/4W resistor (R3)
  • 1  1M, 1/4W resistor (R4)
  • 1  1N4148, semiconductor diode
  • 1  9 volts Buzzer


  2 Responses to “Refrigerator door alarm circuit with two 555 timers”

  1. There is no battery on this diagram.

    • Hello Vladimir

      Thank you for showing us the error in the diagram. The 9 volts battery is connected as follows:

      – The positive battery terminal is connected to the top of the R1 resistor
      – The negative battery terminal is connected to ground


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