This circuit is of a highly sensitive photoelectric switch, using a 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.
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 potentiometer is adjusted such that the relay remains de-activated. In this condition, the LDR1 and the potentiometer network provides 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 thus conduct to active the relay. Diode D1 has been used to damp out the high reverse voltage surge generated by the relay coil during interruption of the relay. The resistor R1across point ‘P’ and pin 7 of the IC1 prevents chattering of the relay RL1.
PARTS LIST OF PHOTOELECTRIC SWITCH USING NE555
|Resistor (all ¼-watt, ± 5% Carbon)|
|R1 = 3.3 KΩR2 = 470Ω|
R3 = 22Ω/1W
VR1 = 100 KΩ LIN. Pot.
|C1 = 0.01 µFC2 = 1000 µF, 12V|
|IC1 = NE555T1 = SK100|
D1 = BY125
|LDR1RL1 = 6VDC, 100Ω Relay|
SW1 = ON/OFF Switch
B1 = 9V battery