How to turn on and off with certain frequency (for example 20 Hz) a simple circuit?
To switch the supply at 0.2 Hz you could use a relay driven by a 555 timer.
Alternatively you could try a simple relay oscillator.



simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. A very crude relay oscillator.
How it works:

On power-up RLY1 coil is energised causing the relay to start to pick up.
When the relay armature is partially closed contact 1a will open and cut the feed to the coil. The relay armature will start to release.
When the armature has released far enough contact 1a will close again and the cycle starts again.
You would use contact 1b to switch your circuit.
D1 protects the contact 1a from arcing and also slows down the release of the relay.

Frequency of oscillation will depend on the relay chosen and may not be very repeatable from cycle to cycle.