Timer.
You will need:
• an old alarm clock
• bulb
• two pieces of insulated electrical wire with bare copper ends
• adhesive tape
• a battery--C or D size
These are very simple to make--and deeply satisfying. For the timer, any windup alarm clock will do--preferably one with with plastic hands. The idea is to use the clock to complete a circuit and turn on a light. You want the bulb to turn on in twenty minutes--to win a bet perhaps, or to frighten your little sister with the thought that a mad axe murderer is upstairs.
First, remove the plastic front of the clock. Tape wire to each hand, so that when one passes under the other, the bare ends will touch. It should be clear that a circuit can now be made with a time delay of however long it takes the minute hand to travel around and touch the hour hand.
Attach one of the wire to a positive battery terminal. Tape a flashlight bulb to the negative terminal and the end of the other wire to the end of the bulb. Test it a few times by touching the hands of the clock together. The bulb should light as the wires on the hands touch and complete the circuit.
Bear in mind that the hour hand will have moved by the time that the minute hand comes around, so it's worth timing how long it takes for the minute hand to, say, fifteen minutes before the hour. You can then terrify your young sister with the take of the man with the hook for a hand.
Tripwire.
This is almost the same thing, in that it uses a battery circuit with a bulb linked to a switch--in this case a tripwire. With a long enough wire, the bulb can be lit some way from the actual tripwire for longer warning times.
You will need:
• clothes pin
• wine cork
• tin foil
• fishing line or string
• batter, bulb and insulated wire, as with the timer setup.
• adhesive tape
Wrap foil around the ends of the wooden or plastic clothes pin. Attach your wires with tape to those foil ends, running both to exactly the same battery-and-bulb setup as the alarm-clock switch above.
The important thing is to have a nonconducting item between the jaws of the clothes pin. We found a wine cork worked quite well. The wire itself must also be strong enough to pull the cork out--if it snaps, the bulb won't light. Fishing wire is perfect for this, as it's strong and not that easy to see. It is also important to secure the clothes pin under a weight of some kind. Only the cork should move when the wire is pulled.
When the cork is pulled out, the jaws of the pin snap shut, touching the foil ends together, completing the circuit and lighting the bulb to alert you.
This works especially well in long grass, but its main disadvantage is that whoever trips the switch tends to know it has happened. Enemy soldiers would be on the alert, knowing they were in trouble. Of course, in a real conflict, the wire would have pulled out the pin to a grenade.
Pressure Plate.
One way of setting up a trip warning without the person realizing is with a pressure plate. Again, this relies on a simple bulb circuit, but this time the wires go to two pieces of cardboard held apart by a piece of squashable form such as you might find in children's blocks. A bit of sponge would also be perfect.
This time, tape foil squares over the bare ends of the wires on the inner surfaces of the cardboard and set up a simple bulb and battery circuit as before. With only light pressure from above, the two bits of cardboard come together, bringing the foil squares into contact. The circuit is made and the warning bulb comes on. Enjoy.
