Forrest M. Mims Jameco Puzzler 3

What's The Power Supply?

by Forrest M. Mims III

Puzzler 3
Big Bucks Bank needs a totally wireless, stand-alone means to sound a chime when a visitor enters the lobby of its bank during business hours. No wires can be connected between the foyer and the inside of the bank. The system and its 6-volt power supply must be installed in the foyer and should have a working life of at least 15 years. The solution is shown in Fig. 1.

Figure 1. Wireless annunciator transmitter.


The circuit sends a pulsed IR signal from the foyer of the bank, through the glass door and to a receiver inside the bank that activates a chime when a visitor steps on a floor switch (S1) in the foyer. The circuit draws current only when S1 is closed, and its power supply has a life of at least 15 years.

What is the 6-volt power supply concealed behind the puzzle piece? Before revealing the answer, here are some hints:

The circuit draws current only when a customer enters the bank foyer and steps on a switch mat, usually for a fraction of a second.

Lithium batteries might work, but maybe not for a full 15 years.

Power is dissipated in the floor switch when a visitor steps on it, but this power is not used to power the circuit.

The system needs to work only during banking hours (9:00 AM to 3:00 PM).

   Type your answer here: 


Give up?

Click here for the answer.