Refrigerator Alarm Circuit

Posted on Feb 5, 2014

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

 Refrigerator Alarm Circuit
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to 220K ©. 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 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.

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