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fridge door alarm circuit diagram

Fridge Door Alarm Circuit

This fridge door alarm is using a 3V battery supply should be placed (in a small box) in the fridge near the lamp or close to the opening. With the door closed the photo resistor R2 presents a high resistance (>200K) thus clamping IC1 by holding C1 fully charged across R1 and D1.

fridgeWhen a beam of light enters from the opening, or the fridge lamp lights, the photo resistor lowers its resistance (<2K) stopping C1 charging current. Therefore IC1, wired as an astable multivibrator, starts oscillating at a very low frequency and after a period of about 24 sec. its output pin (#3) goes high, enabling IC2. This chip is also wired as an astable multivibrator, driving the Piezo sounder intermittently at about 5 times per second. The alarm is activated for about 17 sec. then stopped for the same time period and the cycle repeats until the fridge door closes. Some notes for the fridge door alarm:

  • Delay time can be varied changing C1 and/or R3 values.
  • Beeper repetition rate can be varied changing C2 and/or R4 values.
  • Stand-by current drawing: 150µA.
  • Place the circuit near the lamp and take it away when defrosting, to avoid circuit damage due to excessive moisture.
  • Do not put this device in the freezer.

Fridge door alarm circuit diagram

fridge door alarm circuit diagram

Components Values
R1 = 10K 1/4W Resistor
R2 = Photoresistor (any type)
R3 = 2M2 1/4W Resistor
R4 = 1M 1/4W Resistor
C1= 10µF 25V Electrolytic Capacitor
C2 = 100nF 63V Polyester Capacitor
D1 = 1N4148 75V 150mA Diode
IC1, IC2 = 7555 or TS555CN CMos Timer ICs
BZ1 = Piezo sounder (incorporating 3KHz oscillator)
B1 = 3V Battery (2 x 1.5V AA, AAA or smaller type Cells in series)

Sent by Jan Pelovici.
Source: http://www.redcircuits.com/Page99.htm

20 Comments

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  • martingarcia9931gmail-com

    i have a question. What kind of solder should i use? I am fairly new to this. thanks

  • sushmita pawar

    I constructed this circuit on breadboard but i am not getting the result.
    as soon as i start the supply piezo starts beeping without any delay or any consideration if LDR is exposed to light and it also beeping in dark.
    please reply as early as possible

  • sushant

    what are the values of the resistors for making the alarm ring within 10 seconds when the light falls on the LDR

    • Ilko

      I put two trim pots into the circuit instead of the resistors in question and get alarm working as desired. Then, just measured resistance of each pot. Easy…
      If you don’t want to go that way, you may use the equation

      Time (sec) = 1.1 x Resistance R3 (Ohm) x Capacitance C1 (Farads).
      The current values of R3 and C1 give

      Time = 1.1 x 2200000 x 0.00001 = 24 sec

    • Ilko

      The best way is to try by yourself. I purchased a solder-less breadboard on eBay and enjoyed my lab test.

  • Ilko

    Sometimes the fridge with a “French door” won’t completely close unless I push the door. The tiny opening (crack) that can exist between the door and the fridge frame won’t trigger the fridge light though. But it is enough to make the fridge running non-stop.
    If I would like to use in your schematics instead of photo resistor a mechanical or magnetic switch, what changes should I make to the circuit? Is something like my drawing OK? If yes, could you give me a “ballpark” resistor’s nominations, please?

    • sarath

      is R2-1 and R2-2 are photo resistors / R 1 is photo resistor.? I want complete circuit values clearly

    • Ilko

      I’ve got answer to my question (got it by trying).

      The upper resistor – R2-1 (the one that is being short-circuit by switch) should be 470K Ohm and the lower – R2-2 – 560 Ohm. Works for me as designed.

  • kanchan

    why dont u use the IC in monostatble mode?

  • nikhil

    why pin no 5 of 555 timer ic is not used?

    • Jim Keith

      The use of pin 5 is optional. If high accuracy timing is required, the addition of a capacitor from pin 5 to circuit common is recommended.

  • Rodney Jackson

    Read the value of the LDR. Is it the correct value? or is it perhaps open circuit? Have you checked the wiring closely? maybe you have skipped a pin somewhere.

  • mamush

    it maze technology I learns so good ides thanks you. can other main project set this email sent.

  • Hasi

    i constructed this circuit on breadboard but i am not getting the result.
    as soon as i start the supply piezo starts beeping without any delay or any consideration if LDR is exposed to light.
    please reply as early as possible

  • Sayali

    i constructed this circuit on breadboard but i am not getting the result.
    3V is not available so i connected regulator to 9V battery so it is supplying 5v supply to the circuit.
    as soon as i start the supply piezo starts beeping without any delay or any consideration if LDR is exposed to light.
    please reply asap

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