Simple Refrigerator Door Open Alarm Circuit with IC 555

Sometimes people forget to close the fridge door.

This causes loss of cooling and waste of electricity.

To solve this problem we can use a Simple Refrigerator Door Open Alarm Circuit with IC 555

When the door is left open a buzzer sound comes to remind the user to close the door.

This project uses two 555 timer ICs, LDR and a few passive components.

It is small, cheap and can run from 3V battery.

Circuit Working:

Parts List:

Refrigerator door alarm circuit is explained here with simple working.

Main aim of this circuit is to give buzzer sound when fridge door stays open for more than a set time.

Circuit uses two 555 timer ICs and both 555 work as oscillators.

LDR is used as sensor and it is a light sensitive device.

In dark it has high resistance so when light fall on it the resistance goes low.

This change is used to trigger alarm and LDR is connected with timing capacitor C1 of IC1.

When fridge door is closed inside light gets OFF.

LDR has high resistance so capacitor charge normally.

When door is kept open then lamp glow and LDR resistance drop and capacitor stop charging and start discharging.

This action makes IC1 start the timing and after around 25 sec IC1 output goes high.

This output enable IC2 and then IC2 oscillates and buzzer makes beeping sound.

Cycle repeat until door is closed.

Make circuit on good PCB.

Time delay can be changed by changing R and C values for that we need to see 555 datasheet.

Place LDR close to fridge lamp for best result.

Formulas with Calculations:

Formulas for IC1 Astable mode delay generator.

Astable frequency formula for 555:

T_high = 0.693 × (R_A + R_B) × C

T_low = 0.693 × R_B × C

T_total = T_high + T_low

f = 1 / T_total

where,

• R_A is R3 = 2.2MΩ
• R_B is LDR + R1 combination works like R_B with diode path
• C is C1 = 10µF

Because of large R3 and C1 the IC1 produces a very slow oscillation.

This slow output is used as timing delay before alarm activates.

Roughly the delay is about 20 to 25 sec.

Formulas for IC2 Astable mode buzzer oscillator:

IC2 also works in astable mode.

Its output frequency sets the buzzer tone.

T_high = 0.693 × (R4 + R4) × C2 = 0.693 × (2 × R4) × C2

T_low = 0.693 × R4 × C2

T_total = T_high + T_low = 0.693 × 3R4 × C2

f = 1.44 / (3 × R4 × C2)

Now add the values:

• R4 is 1MΩ
• C2 is 100nF (0.1 µF)

f = 1.44 / (3 × 1M × 0.1µF)

= 1.44 / (0.3)

= 4.8 Hz

This low frequency makes buzzer sound in pulses like beep-beep instead of continuous tone.

Note:

IC1 astable is very slow oscillator which works like delay timer for 20 to 25 seconds.

IC2 astable is fast oscillator which drives buzzer at 5 Hz beep rate.

How to Build:

To build a Simple Refrigerator Door Open Alarm Circuit with IC 555 following steps are needed to follow for connections:

• Gather all the components as shown in circuit diagram
• Connect pin 1 of 1C1 555 to GND of the circuit
• Connect pin 2 of IC1 to LDR and diode
• Connect pin 3 of IC1 to pin 4 of IC2
• Connect pin 4 and pin 8 of IC1 to positive supply of 3V
• Connect pin 6 of IC1 to pin 2 of IC1
• Connect diode D1 between the junctions of resistor R1 and LDR R2
• Connect pin 1 of 1C2 555 to GND of the circuit
• Connect pin 2 and pin 6 of IC2
• Connect pin 3 of IC2 to output of the buzzer one end and other end of buzzer goes to GND
• Connect pin 4 and pin 8 of IC2 to positive supply of 3V
• Connect resistor R3 between the junction of pin 3 and pin 2 of IC1
• Connect resistor R4 between the junction of pin 3 and pin 2 of IC2
• Connect capacitor C1 one end to resistor R3 and pin 2 of IC1 and other end to GND
• Connect capacitor C2 one end to resistor R4 and pin 2 of IC2 and other end to GND

Conclusion:

This Simple Refrigerator Door Open Alarm Circuit with IC 555 is very easy to make.

It prevents electricity loss and keeps food fresh by reminding the user to close fridge door if they have kept it open.

It uses low cost components and easy to build on small PCB.

Delay of around 20 to 25 seconds avoids false alarm when door is opened for short time.

References:

Simple Alarm For Fridge Door