Here's how I combined voltage and amperage readings of a DC system into one meter to switch between. It's really simple but not entirely accurate across tens of volts (as you can compare between my pictures). If you can suggest how to improve that please do. Parts used are:
- DC meter and shunt (20A/75mV in my case, you can see the shunt I got behind the analogue volt meter)
- One 100K resistor and a 500Ω potentiometer/trimpot
- LM358 operational amplifier
- I just used standard components, tighter resistor tolerances may give better accuracy on the meter
- The op-amp is being used as a buffer, as an analogue volt meter will draw too much current from the voltage divider pulling the output voltage too low to read with any useful accuracy. Note the op-amp will "pass on" the voltage dividers output voltage and also supply the meter the currrent it needs to do a proper reading.
- Use a double pole double throw switch to switch the meter between parallel with the shunt and the op-amp output with GND for V and A