Ultra simple and economical circuit of an automatic garden/lawn light is presented here.This compact circuit is built around common,inexpenxive and easy to use electronic components.This circuit is basically a 230VAC mains operated white LED lighting unit with built in twilight sensor mechanism.
Working of the circuit is straight forward.AC 230V supply available across the input of the circuit is down-convereted to a safe and steady value of near 7.5 VDC by the series drop capacitor - behaves as a voltage dropping ‘resistor’ - (C1),half-wave rectifier (D1),zener diode (ZD1) and the buffer capacitor (C2).This method is cheaper and smaller compared to using a step-down transformer.This DC supply is fed to the gate terminal of the lamp control triac (T2) through a medium-power driver transistor (T1).In normal condition,ie if the ambient light level is more than the threshold value set by the sensitivity control trimpot (VR1),T1 is reverse biased by the light detector (LDR) and the triac remains in off state.
In case of darkness, state of T1 is reversed to fire the triac and as a result input ac supply is routed to the bridge rectifier assembly (D2-D5) through the triac.This brige rectifier unit converts the high voltage ac supply to a low value dc supply with the help of the second series drop capacitor (C3).Here 15 standard white LEDS (LED 1 - LED 15) - wired in series - are connected across a buffer capacitor (C4).Buffer capcitor (C4) ensures that there is sufficient voltage to light the LEDs when the available mains voltage is less than the forward voltage of the LEDs and it takes care of the surge that occurs when the mains is switched on.This current pulse could otherwise damage the LED string.Resistor R3 limits the operating current of the LED string.
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