automatic lawn lights circuit schematic

Automatic Lawn Light with LDR

Circuit of a compact and true solid-state automatic lawn light is described here.
The circuit can be used to switch on incandescent garden light bulbs at desk and switch off them at dawn. A 10 mm encapsulated light dependent resistor (LDR) here works as the twilight detector.

The whole circuit can be housed in a very small plastic cabinet.
For powering the circuit AC 230V household supply is needed. With a little skill and patience, you can easily modify this circuit to drive a number of white LED strings, instead of the incandescent bulb load at the output.

When ambient light is normal, transistor T1 is reverse biased by the low resistance of LDR. Multi-turn palstic trimpot P1 sets the detection sensitivity. If ambient light dims, transistor T1 turns on to drive the triac T2. Now the lamp load at the output of T2 energises.
When the ambient light level restores, circuit returns to its idle state and light(s) switched off by the circuit.

Working voltage for the circuit is derived directly from the AC supply input through components D1, R1, D2 and C1. This obviates the requirement of a bulky and noisy step-down transformer.

If you wish to operate the light bulb(s) on a little reduced power,just replace the triac T2 with a suitable silicon controlled rectifier (SCR). This may give a long life to the incandescent load. Finally, the LDR should not be mounted to receive direct sunlight. It may be mounted at the top of the enclosure, pointing to the sky say southwards.

LDR Lawn Lights Circuit Schematic

automatic lawn lights circuit schematic


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  • jasonlewiz

    what is the meaning of an arrow pointed in p1 100k?

  • Mdila

    i prepare this circuit. the bulb get lighted even in day time. what I have to do for this ..


  • freakcircuit

    Sir thank you for such a good circuit. can we add LED as a indicator, what is the voltage and current at collector of sl 100 for adding LED as indicator?
    Thanks in advance..

  • Pawan Yadav

    Sir i make your circuit this is working well but there is a problem 10 k resistance is hot and output voltage not as per as input i m using BT136 pls tell me what i do pls tell me

  • Hakin ropson

    Pls where can i get bt44

  • duncan

    Hi i wanted to simulate the same circuit using multsim simulator i cant find the ldr pliz advice.

  • Tarig

    Hi Sir I need understand how to working this Circuit
    Plz Explaining me.
    Thanks lot

  • Kapil

    Sir i am not basically making exact same circuit. I am using MSP430 and instead of switching light at twilight i m looking forward to vary the intensity during day time as well!! since im using it fr application of room,hotels etc and outside lawn as well!!
    But MSP430 is not able to drive TRIAC BTA-16…
    so where should i give my PWM controlled MSP output?
    Istriac sufficient or do i need any other cintrolling device along with it like some driver,opto-coupler etc
    Sir im controlling fan as well but same problem DC motor is not rotating even at 2V output of MSP im using ULN2803 as driver but still…
    What should i do? replace ULN2803 with any other driver or what

    • xdpereira

      Why do you want to use a complex microprocessor for a simple task? The output of the triac is already PWM (at 50 Hz), and pulse width depends on the phase of the trigger input at its gate. So you don’t need to give it a PWM input.
      As for your motor problem, as far as I know, the ULN ic’s need a 5v input.You could use an opto to interface a 5v supply.
      Or you could use a cheaper PIC series ic, which has a 5v output.

  • AKRaja

    Hi Sir,
    Can you please post the link if you completed the Lawn Light Circuit, which can handle electric lamp loads (incandescent and/or other types)upto 1000W.

  • Nonsco1023

    Sir you are God sent for this forum keep up sir