Electronics Puzzler Solution: Wireless ChimeBy Forrest M. Mims III
The Mystery Power Supply is a Small Solar PanelSince no wires can be connected between the foyer and interior of the bank, the circuit needs to be powered by a supply that will last at least 15 years. A small 6-volt solar panel will provide plenty of power if mounted on the foyer's window so that it faces outside. Since the circuit need only work during banking hours, darkness is not a concern.
How It Works
S1 is a floor switch in the lobby of Big Bucks Bank. The circuit below drives a near-infrared LED with a stream of pulses when a visitor steps on and closes S1. The infrared pulses are directed toward a receiver that activates a chime when they are received. The frequency of the pulses is controlled by the 47K resistor and the capacitor. The 100 ohm resistor limits current through the LED.
Solar powered infrared transmitter designed to send a signal to a receiver when S1 is briefly closed.