I like the design, but there is no way to calibrate the circuit. Depending on the zener voltages and transistor, the voltage could vary in a range that would not be the same from circuit to circuit. I was thinking that a TL-431 in place of one or both of the zeners, with an adjustment trimpot on it would allow calibration. Or vary the 400 ohm resistor. But 400 ohms is an odd value, shouldn’t it be 390 or some other standard value..
This was a first pass at the circuit, but I think you missed the note in the article about not using a 400 Ohm resistor, I used a 200 Ohm resistor in series with a 1K potentiometer, this gives me the ability to adjust the current in the circuit. Since I only made one expanded meter circuit I “calibrated” the meter face, this also allows for me to correct for the first 1.5V where the circuit was not linear.
Sorry. But that’s great. How does it work? Is it accurate enough for the purpose? One thing that scares me is that if you have to pull the meter face off and relabel it, the meter needle might get bent. It’s easy to do, and once it’s bent, forget it. Or the movement might get damaged. I think the way to do it would be to order some meters with custom faces. It might be doable if the meter is already labeled 1 through 10, you could then draw a 2 before all of the digits except for the 10 would have to be 30.
I have now been running it for over a week and checking it against a DMM, the expanded meter has been very close (within a needle width) to where I would expect it to be every time I have checked it. I have only checked operation at 24V and 28.8 so far but I also plan on checking the lower end of the meter for the next week then to 30V.
Taking apart the meter face was not that difficult. Two screws held the cover on and another two screws held the face on. The thing that scared me was dropping a screw into the magnet and movement, which I did not do. I took the meter apart at least 4 times.
I really wanted to add a calibration gain and offset to the circuit but I was so pleased with the initial build of the circuit, simplicity and operation that I decided that I was done with it. I ran through about five different design configuration in simulations, some with gain and offset before testing this circuit out for the first pass.