I made this electric mousetrap to help Silverfish and others who are trying to build an electronic parachute release timer. The salient points about the picture are:

• The coil
  I used a fat self-tapping screw under the trigger, with a winding of varnished wire from an old TV yoke (actually the wire is triple-strand, but I think that single-strand would be better). There's a plastic washer under the head of the screw which helps retain the winding.

• The retaining screw
  I placed another wide-head screw with the edge of the head positioned to stop the trigger settling too far away from the magnet.

• The pivots
  I also had to adjust the pivots of both the trigger and the retaining wire so they had less slack in them.

• The extra nut
  I attached a small nut on the trigger to see whether additional iron would help make it more sensitive, and it seemed to help.

Advice about using a transistor to trigger the trap

A circuit, and a few words to the wise:

• Since the battery can deliver 300ma for short periods and this won't trigger the trap, I expect that a peak current closer to 1.5 amp is needed - you'll need to choose a transistor with at least this rating.

• You'll need a flywheel diode across the coil in the reverse direction of current flow. This provides a path for the inductive kick from the coil that might otherwise pop the transistor when it turns off.

• Depending on the polarity of the output of the 555 circuit, you might like to change the polarity of the transistor/diode. This circuit shows a negative earth with an NPN transistor.