How it Works The Final Circuit Front Page
Making the sensor
The current sensor used in this circuit is made from a 'Type A' reed switch and a length of enameled copper wire. There are numerous different sizes of reed switch so it is impractical to give exact details. This is how I did it.
Cut a piece of paper a little longer than the chosen reed switch, cover it in a suitable glue and roll it around a former that is slightly thicker than the reed switch. Allow this to dry so that it forms a solid tube.
Cut two two circles of substantial card and pierce each with a central hole large enough to push over the tube. Glue these in place to make a bobbin.
When the bobbin is thoroughly dry, take a length of enamel insulated wire. This can be bought from an electronics hobby shop but a cheaper source is from the secondary winding of a discarded battery charger transformer.
Cut a length of wire several feet long, bare both ends and start to wind it onto the bobbin. Neatly wind about 20 turns onto the bobbin. Now connect a charged battery and a bulb of the same wattage in series with the coil and insert the reed switch into the tube of the bobbin and use a meter or continuity tester to see if the switch is closed. Keep adding a few turns at a time until the switch closes reliably each time the lamp is lit. Then add a few more turns 'just for luck'.
Once you are satisfied with the coil, cut the wire to length and secure the winding to the bobbin using PVC tape. You should now have a sensor that looks a bit like this:-