Using an LDR, this Philips circuit directly activates a relay when light strikes the sensor. The relay is 45 V with 20k coil resistance. The 220 nF capacitor must be polyester for 400 V.
The frequency of this type is oscillator is given by the RC network. Calculations can be found in...
Found in old documentation about neon lamps, this circuit produces very low frequency signals that...
This circuit was obtained from an American publication of 1982, but can be assembled with ease...