LDR-based automatic lights  flicker due to the change in light  intensity at dawn and dusk.  So compact fluorescent lamps (CFLs)  are unsuitable in such circuits as flickering may damage the electronic  circuits within these lamps. The circuit  described here can solve the problem  and switch on the lamp instantly when  the light intensity decreases below a  preset level. The circuit uses popular timer IC  NE555 (IC1) as a Schmitt trigger to give  the bistable action.

The set and reset  functions of the comparators within  the NE555 are used to give the instantaneous action. The upper threshold  comparator of IC1 trips at 2/3Vcc,  while the lower trigger comparator  trips at 1/3Vcc. The inputs of both the threshold comparator and the trigger  comparator of NE555 (pins 6 and 2) are tied together and connected to the  voltage divider formed by LDR1 and  VR1. The voltage across LDR1 depends  on the light intensity.

 In daylight, LDR1 has low resistance and the input voltage to the  threshold comparator goes above  2/3Vcc and its output becomes zero,  which resets the internal flip-flop of  IC1. But the input to the trigger comparator is still more than 1/3Vcc, which  keeps output pin 3 of IC1 low.

Triac  BT136 connected to output pin 3 of IC1  remains quiescent due to insufficient  value of current for firing it. Thus lamp  L1 remains ‘off’ during daytime.  At sunset, the resistance of LDR1 Increases, and the voltage at the input  of the threshold comparator decreases  below 2/3Vcc and that of the trigger  comparator goes below 1/3Vcc. As a  result, the outputs of threshold and  trigger comparators go high, which sets the flip-flop. This changes output  pin 3 of IC1 from low to high. Triac1  gets the necessary gate current  through resistor R2 and fires.

Thus it completes the power  supply to the lamp through  Triac1. LED1 glows to indicate  the high output state of IC1. Power supply to the circuit  is directly derived from the  mains through capacitor C4.  This capacitor delivers current in the circuit. Diodes D1  and D2 rectify the AC from  capacitor C4 and capacitor  C3 provides the necessary  smoothing. Zener diode ZD1  provides rectified 15V DC for  the circuit. Bleeder resistor R4  removes the stored voltage of  the capacitor when the circuit  is unplugged. Assemble the circuit on any general-purpose PCB and enclose in a  plug-in type adaptor box. Connect  the live and neutral points to the pins  of the adaptor box.

Provide in the box  5mm holes for LDR1 and LED1. Plug  the unit at a place where daylight is  sufficient to inhibit the circuit operation during daytime. Light from The lamp should not fall on LDR1 at  night. Caution. The circuit carries 230V  AC and most of its points are at mains  lethal potential. So do not touch any  point in the circuit when it is powered And adjust the preset only with a plastic or insulated screwdriver.

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