Ein-Transistor-Mischstufe (BC547B)

Immersion heaters, which are used to heat water during the summer months when the central heating system is shut down, consume considerable amounts of power if left on continuously, even when the hot water tank is well-lagged. It is quite easy to switch on the immersion heater and forget about it, particularly if the switch is in some out-of-the-way place such as the airing cupboard.