This circuit  is used as  Burglar alarm. LDR is kept at such a place that when thief enters our house then a shadow will fall on the LDR. A small beam of light source is also needed to supply continuous signal to LDR. This circuit uses a popular timer IC which is 555.To detect the presence of robber we have used LDR and a source of light.

LDR is a special type of resistance whose value depends on the brightness of the light, which is falling on it.The source of light and LDR is so adjusted with a reflector that light will directly fall on the LDR, but when robber enters inside then it will block the beam of light and LDR will be under darkness. Sensitiveness can be adjusted by variable resistance. Divided voltage is given to pin 2nd of 555.

Pin 4,6& 8 is connected to the positive supply and pin 1 is grounded. When ligth falls on LDR it will ON Buzzer and LED when no light will fall on LDR it will turn OFF the Output Devices. LED1 is used for proper power supply indication. LED2 is used to represent Output.It is power by 9V battery for portable use. The circuit is economical in power consumption.

Circuit Diagram:

Component Values:

  • Potentiometer :
  • R1= 100 Kilo Ohm
  • Resistor:
  • R2,R3 = 470 Ohm
  • Color Code for 470 ohm: Yellow-Violet-Brown-Golden
  • Light Emitting Diode: LED1,LED2
  • Light Dependent Resistor: LDR
  • IC1= IC 555
  • Buzzer
  • Battery: 9 Volts


To make a  complete working circuit foloow the given steps.

 Step 1:

Mount IC 555 on Bread Board as shown in image.

Step 2:

Connect pin 1 of IC 555 to Ground slot with wire  as shown in image.

Step 3:

Connect pin 4, pin 6, pin 8 to VCC through Red wire as shown in image.

Step 4:

Connect Light Dependent resistor (LDR) to pin 2 of IC 555.

Step 5:

Connect second terminal of LDR to Ground.

Step 6:

Take potentiometer R1 of 100kilo Ohm and connect its Red wire to pin 2 of IC where LDR is already Connected. connect Brown and Black wire of Potentiometer to VCC.

Step 7:

Connect Resistance R2 of 470 Ohm on Bread Board as shown in image. One terminal of R2 is given to Vcc.

Step 8:

Mount Led2 on bread Board near Resistor R2.

Step 9:

Connect Anode of Led is given  to Resistor R2 and Cathode is given to pin 3 of IC.

Step 10:

Mount Buzzer on Bread Board. Connect Positive terminal of Buzzer shown by red wire to pin 3 of IC. Connect negative terminal of Buzzer to Ground side.

 Step 11:

Connect Batter of 9v to the Bread board as shown in image. Observe the output.

At intial stage both Buzzer on LED will be ON. When we put some shadow on LDR the sound of Buzzer and Brightness of LED decreases. When there will be no light falls on LDR both Buzzer and LED will be in OFF condition.

Working Video