I have 36 Ah battery. I will charge it through solar change controller or through inverter.

I want to connect this battery to two 9 watt DC LED garden lights which will turn on and off automatically from dawn to dusk.

Plenty of circuit diagram is on the net but they are little different from one another.

I have BC 547 and LDR. What resistor I should use? I have little knowledge about electronics. Please share the circuit diagram.


2 Answers 2


I think a good starting point is: https://www.build-electronic-circuits.com/night-light-circuit/

It shows a simple circuit like so: simpe night light

I would use a potentiometer in place of R1 so that you can tune the circuit to the correct light level.

But the problem with this circuit is that, the BC547 cannot drive enough current for 2 9W LED in this configuration. (it can drive ~100mA MAX).

What I would advise to is remove the LED from this diagram, and use the current from the BC547 to turn on a relay. Or, replace the BC547 with a large 10 amp Power Mosfet.

Relay can be driven like so: enter image description here

Alternatively, you could keep the small BC547 to turn on a large Power Mosfet like so: Connect the emitter to the gate of the power moset, and from there connect a resistor to ground: like so: (I didn't have model for MOsfet, but he large mosfet should be a power mosfet, not BJT) enter image description here

  • \$\begingroup\$ If OP's "9 watt DC LED garden lights" actually are complete luminaires, ready to be staked in the ground and hooked up to a 12V supply, then the "current limiting" resistor may be superfluous and wasteful of power. (But, I'm not an EE, and I don't know much about designing with MOSFETs.) \$\endgroup\$ Sep 5, 2023 at 14:21
  • \$\begingroup\$ All LDRs and solar panels in my old solar garden lights failed because they got sunburned and blocked the light. All my new solar garden lights use glass covered solar panels to detect light and dark that do not get sunburned. \$\endgroup\$
    – Audioguru
    Sep 5, 2023 at 15:11

The BC547 is too wimpy to do control 18W worth of lights, at least by itself.

Since you need a relay or other kind of high-current switch to make this work, you may as well source a pre-made CDS-controlled 12V relay to do this. Search for "12v solar activated switch".


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