

Shadow Alarm using LDR


Himanshu Choudhary
Shadow alarms are widely used for security purposes. A shadow alarm is a device that generates an alarm whenever a shadow falls on it. Project can be easily installed on the windows and doors. This circuit demonstrates the principle and operation of a simple shadow alarm using LDR.  Following is the block diagram of the same.
Circuit Diagram





This circuit in this project is based on LDR which is used as detector to detect the shadow. When the shadow falls on the LDR, its resistance goes high so voltage across it increases. This voltage reaches at input negative pin of comparator LM324, where it is compared to the reference voltage which is at positive pin. So when negative pin (input) voltage is higher than positive pin (reference) voltage comparator gives high output and buzzer sounds.

Here, reference voltage is set by using 10K preset, we set this voltage around 2.5-3.5 volts. 


Circuit Diagram


Resistor Image


LDR | Light Dependent Resistor


Presets | Variable Resistors
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Piezo Buzzer
Piezo Buzzer
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IC LM324

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