Here's a terrific electronic buzzer you can use for a doorbell or signaling device.
When you complete the wiring connections, press on the Key. You'll hear a loud "buzzing" sound from the Speaker. Now touch the Relay case while you hold the Key down. What do you feel? (Don't worry - it won't shock you or hurt.)
You felt the Relay case vibrating. This is because the Relay is rapidly opening and closing its internal switch.
Look at the schematic, can you explain why this is happening?
When you press the Key, electricity can flow. When current reaches the Relay, the internal switch activates and opens the circuit. This cuts off current to the Relay, meaning the circuit once again is closed. But as soon as current again reaches the Relay - you guessed it the Relay's switch opens the circuit, and repeats the entire cycle.
The capacitor also plays an important part by changing and discharging each time the circuit opens and closes. So in this circuit the Relay is a bit like a dog chasing its own tail — it keeps going around and around but never manages to catch it.