A thermostat can be thought of as a relay whose contacts are switched automatically in
response to air temperature. A mechanical timer is a relay which is switched according
to a pre-set timing sequence. The term ‘electronic controller’ can be used to describe
anything from a self-contained solid-state thermostat which contains no moving parts
and a remote temperature sensor to an intelligent computer-based controller which
controls and monitors many parameters with control and monitoring strategies which can
be changed at will be the operator.