Arduino-Based Shadow Alarm

Shadow Alarm

Arduino-Based Shadow alarms are usually used for protection against theft. A shadow alarm is a device that sounds an alarm when a shadow falls on it. Described here is a simple circuit of an Arduino based shadow alarm. This compact shadow alarm unit is capable of sensing a moving shadow in a restricted area, and can be easily installed on wall, window or door to protect your valuables from theft. Constant lighting is required in the confined area to detect the moving shadow.

Step 1: Get the parts

T1, T2 – BC547

D1,D2 – 1N4007

LED – 5mm light emitting diode

R1,R4 – 1k resistor

R2,R3 – 470 ohm resistor

Shadow Alarm

VR1 – 4.7k preset

LDR – Light dependent resistor

C1 – 470uf, 25v electrolytic capacitor

C2 – 100uf, 16v electrolytic capacitor

Batt.1 – 9v battery

CON1 – 7 pin male header

CON2 – 1 pin male header

Arduino Uno


Note: I have used Freeduino instead of Arduino Uno but I have also tested with Arduino Uno .

Step 2: Make the alarm system

1) Take a dotted PCB having 16 x 11 holes

2) Write numbers and alphabets on the PCB as shown in the above figure

3) Place the components on the PCB as shown in the above figure

4) Solder the components

5) Solder the wires

Step 3: Prepare the shadow sensor

1) Cut a female to female header pin wire into two

2) Take a LDR and solder the wires as shown in the above figure

Shadow Alarm Schematic

Step 4: Programming

1) Download shadow_alarm.ino from here

2) Connect Arduino to your PC

3) Click on upload

Step 5: Connections

1) Connect the 1st pin of the shadow alarm circuit to Vin of the Arduino.

2) Connect the 2nd pin of the shadow alarm circuit to pin 10 of the Arduino.

3) Connect the 3rd pin of the shadow alarm circuit to the LDR and other pin of LDR to Gnd.

4) Connect the 4th pin of the shadow alarm circuit to 5v of the Arduino.


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