Radio 1968, N5
A simple relaxation oscillator can be built using only three components: a potentiometer, a photoresistor, and a neon lamp. The optical feedback in the oscillator is between the photoresistor and the neon lamp. Ignition of the neon lamp leads to decreasing the resistance of the photoresistor. The voltage drop across it reduces to a value lower than the ignition voltage of the neon lamp and the lamp goes out. The subsequent increase in resistance of the photoresistor will increase the voltage drop across it to a value higher than the ignition voltage of the neon lamp - the neon lamp lights up, etc.
The frequency of the oscillator changes by changing the supply voltage and the value of the resistors R1 and R2. To maximize the feedback the neon lamp and the photoresistor encapsulated in an opaque or dark box.
Popular Electronics, 1965, volume 23, N2.