Simple oscillator with one relay.

There is a small circuit for to generate a frecuency of 5khz. All that you need is a relay and a voltage source, the values are not critical and adding in paralell a capacitor you can modify the frecuency. The applications of this circuits are several, in oscillator, timers, rings, etc.

Astable multivibrator using relays.

Here is a unusual astable circuit using just relays without transistors or valves. It's very rude and funny but it works. You can modify the frecuency with the variable resistor P1 between certain limits and alterate the wide of the pulse with P2. But be careful because if you get at the limits of the resistors the circuit don't works and you should to reset the circuit by turn the resistors and put off the source and before to start again!. As in another circuits you can modify the value of the capacitor and to experiment with several relays. But the best work is with the 3300uF capacitor, with relays of 50-300 ohms and sources of 5 volts to 12 volts depending of the types of relays that you use, This circuit can be used like warning-light, encoder, alarms, clock, etc.