Can high-brightness white LEDs and light-dependent resistors (LDRs) be used to make an oscillator? I decided to have a go. For the experiment, I chose type PGM5516 LDRs which have a relatively low resistance (5 kΩ to 10 kΩ at 10 lux). A ring oscillator configuration was used, with each LED placed directly next to its neighboring LDR (see circuit).
The first test, without the capacitors, was unsuccessful. I had hoped that the natural slow response of the LDRs would be enough to start oscillation, but the oscilloscope proved me wrong. Only by adding the 220 µF electrolytic capacitors could I get the oscillation to start, using a power supply of between 3 V and 9 V. The oscillation frequency rises with supply voltage. At 3 V the circuit must be kept in darkness (and the result is a very low-energy running light!). At 2.7 V the circuit draws 0.9 mA and is disturbed by even the smallest amount of ambient light.