When I was about eight or nine, I built a very crude doorbell for my bedroom. It was a simple electric circuit containing a battery, a switch, and an electric motor standing on a large cardboard box. When a caller pressed the switch, the battery fed power to the motor and made it spin around with a buzzing noise (a bit like the vibrating alert on a cellphone or pager). Standing on the box, the motor made a reasonably audible but rather dull humming noise.