Dennis L. Eggleston: Basic Electronics for Scientists and Engineers
Page 95, Figure 3.41 Zener DC voltage
indicator.
"The circuit makes use of the fact that zener diodes can be obtained with a variety of breakdown voltages, and the diodes in the circuit are labeled with their respective breakdown voltages. As the input voltage increases, we eventually reach the point where the leftmost zener can break down, and the resulting current flow turns on the first light. At a higher voltage, the next zener also breaks down and its light also turns on, and so on. Thus for higher input voltage, more of the indicator lights are on. This type of display is often seen on stereo systems and cell phones to give, for example, an indication of the received signal strength."
