Use this refrigerator alarm circuit to prevent leaving the fridge door open by sending out an alarm signal every time the door is left open for some time.
The sensor of the alarm is an LDR. After a short delay, it triggers an alarm signal and keep the signal on until the door is shut close.
When the light falls on the LDR, it becomes low ohmic. The voltage at the junction of LDR/R2 charges the C2 via the R1 slowly. After about 10 seconds, the capacitor voltage reaches the input threshold of U1. Then U1 generates a square wave signal with a frequency of several Hertz. The time delay can be shortened by decreasing the value of R1 down to 220KΩ. U2 inverts the signal from U1. This inverted signal switches the U3 tone oscillator. When there is no light hitting the LDR, U3 is turned off. Finally, U4 amplifies the tone signal generated by U2 and drives the ceramic transducer. The frequency is doubled at the transducer.
The light sensitivity of the LDR can be adjusted with P1. The alarm tone freq. can be adjusted to the resonant freq. of the ceramic transducer through P2.
The maximum volume level can be reached when the tone freq. is equal to the ceramic transducer’s resonant freq..
The refrigerator alarm circuit can be powered with a small 9 volts battery. The current consumption is around 0.6 mA at standby and 5 mA when the alarm is activated.
Sent by Luca Marcel, MD.
R1 = R3 = 1MΩ
R2 = 1KΩ
R4 = 10KΩ
C1 = C2 = 10µF
C3 = 0.47µF
C4 = 0.01µF
P1 = 10KΩ
P2 = 25KΩ
D1 = 1N4148
U1…U4 = 4093