As a guide, a one-inch reed switch with 40
turns reliably switched on with the current flowing through a 150-watt
lamp (approx. 625 mA) but larger reeds may require more turns. If the
master appliance draws less current (which is unlikely with power tools)
more turns will be required. The reed switch is used to switch on
transistor T1 which in turn switches the relay RE1 and powers the slave
appliance. Since reed switches have a low mechanical inertia, they have
little difficulty in following the fluctuations of the magnetic field
due to the alternating current in the coil and this means that they will
switch on and off at 100 Hz.
C3 is therefore fitted to slow down the transistor response and keep
the relay energised during the mains zero crossings when the current
drawn by the appliance falls to zero and the reed switch opens. C1 drops
the mains voltage to about 15 V (determined by zener diode D1) and this
is rectified and smoothed by D2 and C2 to provide a d.c. supply for the
circuit. The relay contacts should be rated to switch the intended
appliance (vacuum cleaner) and the coil should have a minimum coil
resistance of 400 R as the simple d.c. supply can only provide a limited
current. C1 drops virtually the full mains voltage and should therefore
be a n X2-class component with a voltage rating of at least 250V a.c.
The circuit is by its nature connected directly to the mains supply.
Great care should therefore be taken in its construction and the circuit
should be enclosed in a plastic or earthed metal box with mains sockets
fitted for the master and slave appliances.
Author: Elektor – Copyright: Elektor Electronics Magazine