There are many situations where two or more pieces of equipment are used
together and to avoid having to switch each item on separately or risk
the possibility of leaving one of them on when switching the rest off, a
slave switch is often used. Applications which spring to mind are a
computer/printer/scanner etc or audio amplifier/record deck/tuner
combinations or perhaps closest to every electronics enthusiast’s heart,
the work bench where a bench power supply/oscilloscope/soldering iron
etc are often required simultaneously.
The last is perhaps a particularly good example as the soldering iron, often having no power indicator, is invariably left on after all the other items have been switched off. Obviously the simplest solution is to plug all of the items into one extension socket and switch this on and off at the mains socket but this is not always very convenient as the switch may be difficult to reach often being behind or under the work bench. Slave switches normally sense the current drawn from the mains supply when the master unit is switched on by detecting the resulting voltage across a series resistor and switching on a relay to power the slave unit(s).
The last is perhaps a particularly good example as the soldering iron, often having no power indicator, is invariably left on after all the other items have been switched off. Obviously the simplest solution is to plug all of the items into one extension socket and switch this on and off at the mains socket but this is not always very convenient as the switch may be difficult to reach often being behind or under the work bench. Slave switches normally sense the current drawn from the mains supply when the master unit is switched on by detecting the resulting voltage across a series resistor and switching on a relay to power the slave unit(s).
The output current of this supply is limited so that the voltage drops
substantially when the relay pulls in but since relays require more
current to operate them than they do to remain energized, this is not a
problem. Since the transistor emitter is referenced to mains Neutral,
it is the field around the mains Live which will be detected.
Consequently, for correct operation the Live wire to the lamp must be
switched and this will no doubt be the case in all lamps where the
switch is factory fitted. In case of uncertainty, a double-pole switch
to interrupt both the Live and Neutral should be used.
The sensitivity of the circuit can be increased or decreased as required by altering the value of the T2 emitter resistor. The sensing wire must of course be wrapped around a section of the lamp lead after the switch otherwise the relay will remain energized even when the lamp has been switched off. The drawing shows the general idea with the circuit built into the extension socket although, depending on the space available an auxiliary plastic box may need to be used.
The sensitivity of the circuit can be increased or decreased as required by altering the value of the T2 emitter resistor. The sensing wire must of course be wrapped around a section of the lamp lead after the switch otherwise the relay will remain energized even when the lamp has been switched off. The drawing shows the general idea with the circuit built into the extension socket although, depending on the space available an auxiliary plastic box may need to be used.
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