My chair's been stolen one too many times at work, making working at my desk rather difficult. My first plan involved a microwave transformer, nails and a current limiting device, but an absence of available microwaves made this impractical. Then I looked at my
Thunderbirds in your Pocket noisy thing and made a plan.
After discussion with "The Proffitt", and an explanation that finally taught me the difference between PNP and NPN transistors, I had a circuit that would simulate a button press when a wire became disconnected.
The switch is made from a wire loop with crimps that disconnect when the chair is pulled away.. This switch is normally closed, pulling the base of the NPN low, preventing current from flowing. When the switch goes open, the pullup resistor (N.B. 12k, not 10k as shown, as I pulled that one out first) pulls the transistors base high, letting current flow from COM (the common 4.5V of the voice box's key matrix) to S2 (the switch's input), thus simulating a keypress. The transistors stays closed until the switch is reconnected, making the alarm continuous until the chair is returned to me.
With my four components, and a few bits of wire, I started work on the voice box.
Two bits of wire were soldered to convenient points on the key matrix, ensuring they'd route between the keys, so the buttons could still work.
I then free-formed the rest of the circuit and glued it into a hold I cut in the unit. The header has the switch contacts on two pins (shorting the pins keeps the sound off) and a ground jumper on the other two - by rerouting the ground the box stays off until the ground line is connected to the battery, letting me pull the power if our accounts department get too annoyed.
I then subtly hid the box, and put the wire loop round my chair, completing the circuit, waiting for a burglar to strike:
