What's The Power Supply?by Forrest M. Mims III
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).
Click here for the answer.