Fridge Door “Open!” Alarm

This circuit produces an audible alarm when fridge is left open for a preset time.This versatile alarm is based on two 555 timer IC’s.Both ICs are wired as astable Multivibrators.An LDR is connected in parallel to the timing capacitorC1 of IC 1.When door is completely close there will be no light inside the fridge and LDR offers high resistance keeping R1 fully charged.When door is left open ,the lamp inside the fridge will remain glown ,LDR will be illuminated,and its resistance falls.This makesC1 stop charging and starts to discharge ( In simple words ,this is because at low resistance LDR bye passes much of the current in the parrallel path and capacitor gets less current).Now IC 1 starts slightly oscillating and after a preset time of 25 ( time T1) seconds its output goes high.This out put makes the Ic 2 to produce oscillations and results in a beeping sound for next 20(time T2) seconds and cycle is repeated till door is closed.

Assemble the circuit on a good quality PCB .Time T1 & T2 can be adjusted by varying C1, C2, R1, R4 etc (Refer data sheet of 555).Place the LDR close enough to the lamp inside the fridge.

Parts List.

R1_____________10K 1/4W Resistance
R3_____________2.2 M 1/4W Resistance
R4_____________1M 1/4W Resistance
C1_____________10µF 25V Electrolytic Capacitance
C2_____________100nF 63V Polyester Capacitance
D1____________ 1N4001 Diode
IC1,IC2_________NE 555 Timer ICs
BZ1___________ Piezo Buzzer
B1____________ 3V Cell