This is the circuit diagram of the voltmeter I just built.
circuit in pdf format
I built it on a little round pcb & bolted it directly to the back of a MU45 1mA meter. I photo-shopped the meter face to read 10 to 15 volts.
Most of the circuits I have played with tend to become non-linear at the lower end of the scale. This circuit is absolutely linear. Using an LM358, the current drain is approx 1.2 - 2.2 mA. The ICL8062 gives 1.2 volts. The dual op-amp could be replaced with a TSM103, which has the reference voltage built in. You could then do away with U2, R6, C3. U2 could be any reference source; zener diodes, even an LED. But a precision reference is more stable.
The box in the bottom left explains the basic math. For my implementation, the voltage at point B is 1.2 volts. So the voltage at point A needs to be 1.2 volts when the battery is 10 volts. There will then be zero volts across R1 & R2, so no current will flow in the meter. When the battery voltage is 15 volts, the voltage at point A will now be 1.8 volts, so there will be 0.6 volts across R1 & R2. If R2 is adjusted so R1 + R2 = 600 ohms, then the current through R1 & R2 (and hence the meter) will be 1mA.
As built photos tomorrow. Questions welcome.
The LM358 & ICL8062 were on hand, hence their use.
Amanda