This is a very simple touch sensor switch that is build with the 555 timer IC.
You just need to touch the metal plate and the relay gets energised and is kept in this state for about 100 seconds, then is released.

This kind of sensor switch is suitable for making touch operated bells, buzzers of small toys which operate for a small time and then switch off automatically.

The input impedance of the trigger is very high and the touch sensor switch can be triggered by the voltage induced in the human body. The relay is a 12 V DC relay operating at currents less than 200 mA.