Wired Doorbells can be powered by batteries or transformers connected to a mains supply. The transformer drops the voltage to between 8 - 12v a/c - a safe voltage for external bell pushes. Transformers are normally external to the doorbell and are often positioned in the consumer unit, but can be positioned anywhere (they are ugly boxes are not normally visible). Some units have internal transformers which only require a mains connection. Battery wired doorbells are perfectly acceptable and for typical use batteries should last about 3 years - but they have the benefit of a simple DIY installation. Some units can either be powered by batteries or external transformers and the sound is produced by 2 different methods - mechanical or electronic. Mechanical action doorbells have a hammer hitting metal to produce a ding-dong, ringing bell or buzzer sound, whilst electronic chimes have pre-recorded melodies ( up to a choice of 17 for some models ). Unless they are sold as kits, a doorbell will need a bell push button and bell wire as well - the bell wire must be good quality to prevent "voltage drop" at these low voltages (a symptom is that the doorbell will sound feeble).