In just under 7 minutes, the Droitwich Transmitter sends out a special signal every night alongside BBC Radio 4 Long-wave to signal to Economy 7 electric meters to switch over to the off-peak rate. About 1.4 million electric meters in the UK use this technology. 1/...

The off-peak time varies between electricity board region, but in Southern Scotland the off-peak is midnight to 0830hrs 2/...

This Radioteleswitcher from the late 1980s has a nice metallic CTHLUNK sound as the switch is triggered bang on midnight. 3/... There it goes!

The service was due to be switched off in 2017 - the LW Droitwich transmitter relies on glass valves to operate which are no longer in production, but still powering the transmitter. There are allegedly about 10 spares in the world. 4/...

The system is controlled by the Central Teleswitch Control Unit which is housed in an industrial estate in East Kilbride. The CTCU receives codes from each of the regional electricity operators, and forwards commands to the BBC Message Assembler at Crystal Palace, London

For parts of central and northern Scotland, the Longwave teleswitch signal is boosted by the LW transmitters at Westerglen, Falkirk; and Burghead, Moray 6/...

So wherever you are, you can rely on cheaper off-peak electric heating to dry your soggy shoes overnight because of the Radio Teleswitch Service and BBC Radio 4 Longwave delivering its secret midnight signal. (7/7.)

Get those thermionic glass valves warmed up, standby in East Kilbride, Crystal Palace, Droitwich, and Westerglen; it'll be time for the 0830 Radio 4 Longwave signal in an hour to switch us back over to peak rates...

Just sneaked in one last off-peak kettle boil and coffee before the switch over to peak rates. Cheers, Radio Teleswitch Service.