It has become necessary to verify the identity of a person or visitor before opening the door as he may be a burglar. This switchless bell circuit will help you to identify who is standing at the door as it that activates through the puff of your breath or with a clap. Only those who know the place of secret bell would puff out before the hole of the switch box and now door can be opened without any fear.
This Transistor based secret bell circuit is based on the property of the transistor to amplify weak signals. In this, the condenser mic fitted inside a hole of a switch box will trigger the bell on detecting the change in air pressure as you clap or puff your breath.