Basically just a tripwire based switch made from stuff lying around the house. Can also be used as a remote switch for things you don't want to be next to when you turn them on.

Step 1: Tools and stuff, you will need...

Clothes Peg
Thin copper plate
Thin card/ plastic
String/ invisible thread
Crocodile clips
Device you want power
Pen knife or some other kind of cutting implement.
This is wonderfully simple, not to mention high current too! I built a similar gizmo out of a few parts and an N-channel MOSFET, its solid state but given the size only allows 300ma
We used to make these very simply. Instead of the plates and such (which are more durable I suppose) we just wrapped each wire around the jaws of the clothespin. Then when you pulled the insulator (we used half of another clothespin or a wooden peg) the two wires touched directly. Another way was to use small wood screws as the contacts. I used them in a class project in college where we had to make a rube goldberg machine. As the grand finale this is what we did with two tripwires. A falling ball bearing (about 1/2" in diameter) fell into a cup, which tripped one of these tripwire pegs, which turned on an air type popcorn popper. A bowl set on a "teeter totter" piece of wood underneath. When the bowl below the popcorn popper got full, the weight of the corn tipped the "teeter totter" which turned it off. The teacher was amazed. You can use these to turn something off very simply. Just use the insulator piece as part of the circuit. In other words, the electricity is flowing between the piece hooked to the trip wire and one side of the clothespin.. The opposite side of the clothespin has nothing. You pull on the string and the circuit is broken. One last thing, these work real well to set off those really loud "personal alarm" key chains (the ones the sell by pepper spray and such). Works well in many circumstances, such as paintball games.
ok. so instead of the copper plates for the ports i just drove a screw into each side of the peg. the pointy parts were facing out so that the flat parts would touch each other when the card was yanked out. you would attach the clips to the pointy parts of the screws. i hooked it up to the flash component on a isposable camera and just about gave my mom a heart attack.
what could i use to make it beep when someone trips it?
hey, could i use this to turn on my bedroom light just by walking by?
I will design something to do it My idea is a pulley with a string taped to the swith
sorta lol but you'd have to reset it every time, and you'd need to tamper with the mains voltage in the lightswitch. (something I don't suggest doing)
thx i'll try not to shock myself too much
could pennys work?
what else will work
could aluminum foil from say, a soda can work as a replacement for the copper slate?
Yes. You might have to sand the sides down first though.
i dont have copper.... :-( can i use other metal, like alumiunum foil?
you may like aluminum tape.it works well AND is versatile.
Any conducting plate would do I guess. Reinforcing the foil with tape on one side would be a good idea or it could tear (obviously not the sides that need to touch to make the connection).
i love this instructable because i hooked mine up to a buzzer and instead of yarn i used fishing line so its invisible so ive caught my brother so many times ~Nsalt94
you could also drill holes through the clothespin and screw in some conductive bolts for the electrodes, rather than copper plates
what happens when it trips?
Depends on what you have it wired to. If it was wired to the squib it'd go bang. To an alarm, it'd make some noise.
aww. i see! cool

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Bio: Making things to entertain myself comes second to entertaining my offspring, but my mind is always working something out.
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