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.