w2aew @ youtube.com writes:
This video describes and demonstrates a fun little circuit that is designed to create a automatically switching, dual-range analog voltmeter which is intended to be built into a variable power supply. By using two ranges, it permits accurately setting a low voltage such as 3.3 or 5V, as well as accurately setting a higher voltage like 24V. Setting a low voltage using a high voltage meter is not very precise, hence the reason I put this together. The circuit is demonstrated, and the schematic is reviewed to describe the operation.
Of course, there are many ways this can be done – this is just one example. It uses one of my favorite little analog ICs, the LM10 op amp and reference. The LM10 (designed by the legendary Bob Widlar) is used as a voltage reference and comparator with hysteresis. A zener diode is used as a shunt regulator. There’s an indicator LED to show when the meter is in the high range, and a 2N7000 enhancement mode n-channel MOSFET is used to change the resistors associated with the analog meter.
Auto-ranging Analog Voltmeter for a variable power supply – [Link]