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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 smo...

The flash module in this circuit comes from a  Fuji disposable camera. The author’s design  is based on various ideas on this subject that  can be found on the Web. The guide number  [1] of the flash is approximately 14 in the air  and close to 6 underwater. The flash intensity  is not adjustable. This flash unit is primarily intended to be used  for underwater photography, which is why  the author fitted it in a  case originally used for  a Nikon Coolpix 7900.  Of course, it can also  be used for dry-land  photography. The flash  module is powered by  a 1.5-V battery, which  must supply approximately 30 to 40 mA  to charge the capacitor. The control portion is built around a  PIC12F675, which is  powered by a 3-V button cell. Its current consumption is practically negligible  just a few  milli amperes while the flash is active and only  600 nA the rest of the time, when the micro-controller is in the standby state. For this rea-son, the unit does not have an on/off button. The flash from ...