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Study the finished switch. As you can see, the switch is normally closed since the contacts normally touch each other. This is the switch’s “triggered” condition — in other words, this is what the switch looks like after it has “sensed” an “intruder.”

To set the switch — and make it ready to respond to an “intruder” — slip a small square of thin cardboard between the two contacts. The cardboard will hold the contacts apart and break the circuit. The second diagram shows you how to place the cardboard square. Now, punch a small hole in one corner of the square and tie a length of light string through the hole. This completes the switch.