A neighbor moved a year and a half ago and was throwing out some beat up garden lights, so I salvaged them from the dumpster. They were pretty good ones 7 or so years ago when they were new, but the plastic was aging and brittle. I got a few solar cells and circuit boards and some Ni-Cad batteries, which are not supposed to be in the trash. I stuck a good AAA cell in a few and they still worked. So I tried to charge a couple Ni-Cads, and one took a charge after I cleaned the corrosion off its contacts. I took one of the lights apart and cleaned it up and got it to work with the Ni-Cad cell.
These LED lights are made as cheaply as possible, minimal parts, no capacitors so they depend on the low internal resistance of the AAA cell to handle the high current pulses. The LED is turned off during the daytime so it doesn’t discharge the AAA cell. The circuit senses daylight by using the solar cell; if it’s charging then it’s daylight. It’s fast since the circuit is too cheap to have any resistor and capacitor time delay.
I have a two transistor flasher circuit with a 1 Watt green LED on its output, and it flashes brightly. I turned the ceiling light off and brought the lit up garden light close to the flasher. Every time – about once a second – the solar cell ‘sees’ the flash, it turns the lit up LED off briefly. So I’ve turned a simple solar garden light into a detector of light flashes. I wonder if the garden light is fast enough to follow a series of on-off flashes?