Doorbell

William Murdoch, a Scottish inventor, installed a number of his own innovations in his house, built in Birmingham in 1817; one of these was a loud doorbell, that worked using a piped system of compressed air.

In most wired systems, a button on the outside next to the door, located around the height of the doorknob, activates a signaling device (usually a chime, bell, or buzzer) inside the building.

A common signaling device is a chime unit consisting of two flat metal bar resonators, which are struck by a plunger operated by a solenoid.

These do not consume standby power, but require the user to change the batteries, which are usually large primary cells located in the bell box.

In recent decades, wireless doorbells have become popular, to avoid the expense of running wires through the building walls.

When the button is pushed, the transmitter sends a radio signal to the receiver unit, which is plugged into a wall outlet inside the building.

In larger metropolitan cities, a trend has developed over the past decade that uses telephone technology to wirelessly signal doorbells, as well as to answer the doors and remotely release electric strikes.

For simple single-pole, single-throw doorbell buttons, the chime device employs a rectifier diode and ballast capacitor at the voltage input stage of the circuit.

With the rise of the Internet of Things in the 2010s, a number of internet-connected bell systems, known as smart doorbells have appeared on the market.

These consist of a single unit which is located in place of the traditional push-button, and in addition to a physical button, contains a high-definition camera, passive infrared sensor and Wi-Fi capability.

When a notification is received, the user will typically see a live video stream from the smart doorbell, showing who is at the door and potentially allowing a 2-way audio conversation.