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).
This means that the Live or Neutral feed must be broken to allow the
resistor to be inserted. This circuit, which is intended for switching
power to a work bench when the bench light is switched on, avoids
resistors or any modifications to the lamp or slave appliances by
sensing the electric field around the lamp cable when this is switched
on. The lamp then also functions as a ‘power on’ indicator (albeit a
very large one that cannot be ignored) that shows when all of the
equipment on the bench is switched on.
The field, which appears around the lamp cable when the mains is
connected, can be sensed by a short piece of insulated wire simply
wrapped around it and this is amplified by the three stage amplifier
which can be regarded as a single super-transistor with a very high
gain. The extremely small a.c. base current results in an appreciable
collector current which after smoothing (by C3) is used to switch on a
relay to power the other sockets. Power for the relay is obtained from a
capacitor ‘mains dropper’ that generates no heat and provides a d.c.
supply of around 15 volts when the relay is off.
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 circuit itself is not isolated from the mains supply so that great
care should be taken in its construction and testing. The sensor wire
must also be adequately insulated and the circuit enclosed in a box to
make it inaccessible to fingers etc. when it is in use.