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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

Resistor is a passive component used to control current in a circuit. Its resistance is given by the ratio of voltage applied across its terminals to the current passing through it. Thus a particular value of resistor, for fixed voltage, limits the current through it. They are omnipresent in electronic circuits.

LDR | Light Dependent Resistor

An LDR (Light dependent resistor), as its name suggests, offers resistance in response to the ambient light. The resistance decreases as the intensity of incident light increases, and vice versa. In the absence of light, LDR exhibits a resistance of the order of mega-ohms which decreases to few hundred ohms in the presence of light. It can act as a sensor, since a varying voltage drop can be obtained...

Presets | Variable Resistors
A preset is a three legged electronic component which can be made to offer varying resistance in a circuit. The resistance is varied by adjusting the rotary control over it. The adjustment can be done by using a small screw driver or...
1N4001 Diode Image
Diode 1N4001

1N4001 is a member of 1N400x diodes. Diode is a rectifying device which conducts only from anode to cathode. Diode behaves open circuited for the current flow from cathode to anode. 1N4001 is a 1A diode with low forward voltage drop and high surge current capability. It comprises of diffused PN junction and has low reverse leakage current of 5µA. Its DC blocking voltage is 50V.

Piezo Buzzer
Piezo Buzzer
The piezo buzzer produces sound based on reverse of the piezoelectric effect. The generation of pressure variation or strain by the application of electric potential across a piezoelectric material is the underlying...
LM324 | LM324N Datasheet
IC LM324

LM324 is a 14pin IC consisting of four independent operational amplifiers (op-amps) compensated in a single package. Op-amps are high gain...

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