INTERRUPTION ALARM

This circuit is useful to protect the door from theft. The LED and the LDR are placed on the opposite sides of the door. When the light falling of the LDR breaks alarm sounds. The circuit is most effective during day time. If the intensity of yellow LED is low use a white LED or a lamp.

Light Dependent Resistor (LDR) is used in the circuit to detect the shadow of the moving Person. The resistance of LDR is as high as 2 Meg ohms in dark while it decreases to 100 ohms or less in light. When the day light or light from the LED illuminates the surface of LDR, it conducts, so that the pin 2 of IC1 becomes high and output pin 6 becomes low. LED D2 and Buzzer will be off. IC1 is used as a voltage comparator so that its output becomes high only when its input pin 3 gets more voltage than pin2. Preset VR1 adjusts the voltage level at pin 3 at a particular level.

When the face of the LDR gets low light in dark or due to the shadow of a person, voltage at pin 2 becomes lower than pin3. As a result, IC1 gives a high output activating LED 2 and buzzer.

Note: After assembling the circuit, expose the LDR to light. If buzzer sounds, adjust the wiper of VR1 till the buzzer just stops sounding. The sensitivity of LDR is now adjusted to that particular light intensity. Move the hand over the LDR. Buzzer should sound. It is necessary to adjust VR1 for each light level. If the distance between LED1 and LDR is too large, light from LED fails to make LDR conducting.