by Forrest M. Mims III


The Mystery Power Supply is a Small Solar Panel

Because 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 in Fig. 1 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.



Figure 2. Solar powered infrared transmitter designed to send a signal to a receiver when S1 is briefly closed.

About the Author

Forrest M. Mims III has been an electronics and science writer and photographer since 1969. He received an IR 100 Award from Industrial Research Magazine for developing an eyeglass-mounted infrared travel aid for the blind. He received a Rolex Award in 1993 for developing a handheld instrument that measured the ozone layer accurately enough to find an error in a NASA ozone satellite. He was named by Discover Magazine (December 2008) as one of the "50 Best Brains in Science." See his home page and publications here and follow him at Twitter.