A very simple thyristor signal generator circuit which uses a single thyristor, two resistors, a capacitor and a mini-loudspeaker. Once the circuit is switched ti a current source, current flows through R1 and switches on the thyristor. The thyristor conducts current and chargers capacitor C.
Since the charging current is inversely proportional to the charging of the capacitor, it slowly decreases until such a level that the thyristor switches off. Once the thyristor is off, capacitor C discharges through R2 and the whole process is repeated. The periodice switching on and off of the thyristor generates the oscillating signal. The frequency of the oscillation is determined by the C/R2 combination.