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.
Circuit diagram:
Mains Slave Switcher II Circuit Diagram
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.
Warning:
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.
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