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
Not shown in the picture is the capacitor I used - a 1000uf 16 volt
electrolytic capacitor is paralleled with the 9-volt battery to provide
additional peak current capability - without this the trap won't trigger.
- 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
- 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.
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