Photoelectric switch using ne555

This circuit is of a highly sensitive photoelectric switch using NE555 timer IC. When the LDR₁ is kept in light, the switch remains off, but when the light source is removed, the controller circuit gets switched on.

This switch has vast application. With little modification, it can be used as a burglar alarm, a corridor visitors monitor, an automatic street lighting system, and as an automatic water tap system.

Circuit Description of | Photoelectric switch using NE555

The NE555 timer functions here as a Schmitt trigger. A NE555 IC comprises two comparators-a threshold comparator and a trigger comparator. These two comparators control the flip-flop.

When light falls on the LDR₁, the 100 KΩ potentiometers are adjusted such that the relay remains de-activated. In this condition, the LDR₁ and the potentiometer network provide about 1/3Vcc at point ‘P’, and the trigger comparator is referenced to make the output level high. With high-level output, the operation of T₁ as well as of the relay ceases.

When the light beam is interrupted, the voltage at point “P” rises to about 2/3Vcc, and the threshold comparator resets the flip-flop. The output now goes to the low state to provide a proper forward base bias for transistor T₁. T₁ is thus conducted to activate the relay. Diode D₁ has been used to damp out the high reverse voltage surge generated by the relay coil during the interruption of the relay. The resistor R₁across point ‘P’ and pin 7 of the IC₁ prevent chattering of the relay RL₁.

Check out other interesting projects using LDR posted in bestengineeringprojects.com

1. Self Resetting LDR Alarm using Timer IC 555
2. Light Sensor Switch Circuit using LDR and 741 IC
3. Automatic Light Operated Switch using LDR and IC741
4. Automatic Fence Lighting with Alarm
5. IC 555 Based Automatic evening lamp

PARTS LIST OF PHOTOELECTRIC SWITCH USING NE555