There are two sets, and they work the same way. The bellpush/button has a long-life battery inside and sends a signal to the bell(s) which ring. The bells plug into a 3-pin socket so don't need a battery. They all work, but the two sets are not interchangeable.
The Lindam ones are quite old, one of the ringers is a bit yellow from the sun, but they all work well and are reasonably loud. There are three ringers which all work off the same bellpush at the door so you can have the ringers in different parts of your house to make sure you can hear them wherever you are when the bell goes. There are 4 chime choices for each ringer.
The grey and white one is a lot newer and a bit quieter, but loud enough. It has a choice of 36 chimes.
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Wireless door bells/chimes (Ravelston EH4)
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