Question: I've got an old analog (0-5ma) ammeter and want to convert it to measure 0-±5 volts, as an output for my Arduino.

I'm a programmer and my knowledge of electronics is limited. I know what components do but obviously not how to use them properly.

I tried to place some 2k (1 and 2) resistors in series with the ammeter but it doesn't work as expected. This was sortof working but is wasn't a linear movement even with a capacitor attached to + and - (+ and - come from the Arduino PWM/analog output)

![schematic](https://i.stack.imgur.com/8JBQS.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2f8JBQS.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)

I know its a ammeter so it needs current and if theres nothing asking for current there will not be any so i put a very high resistor between + and - but that didn't really change anything.

So how can i change my am meter to measure 0-5v?

[![enter image description here](https://i.stack.imgur.com/kap7W.jpg)](https://i.stack.imgur.com/kap7W.jpg)

This is it opened, the resistance is about 2.9k (if I measured it correctly)

[![enter image description here](https://i.stack.imgur.com/88xE2.jpg)](https://i.stack.imgur.com/88xE2.jpg)

Could it be microAmps?

Hope some one can help, Thanks in advance.

Answer: 
[![enter image description here](https://i.stack.imgur.com/54aFg.png)](https://i.stack.imgur.com/54aFg.png)

*Figure 1. Extract from the FT330 sensor.*

That sensor is capable of sinking 25 mA so there should be no problem with noise.

Your problem is likely with the installation or choice of power supply or connection to the counter. You haven't supplied details of these (despite the request).
The VFD switching frequency specification is a maximum, not necessarily a fixed number. It can be directly proportional to output frequency. It can rise with output frequency over a range, then fall then rise again like a car engine rpm rises and falls whenever the transmission shirts to a higher gear. It can also vary in a more complicated way depending on design. Whatever the case, electrical noise from the VFD is likely the source of the problem.

First look at the routing of all wiring connected to the sensor. If any of the wiring is near any of the VFD wiring, particularly the input and output power wiring, reroute the wiring. If the sensor or it's power supply are in the same enclosure as the VFD or motor, they may need to be enclosed in a separate compartment. You may also need to look at the grounding of the VFD, motor, sensor and whatever receives the signal from the sensor.

It would help to have some kind of diagram showing what all equipment is involved in the system, how it is powered and how it is grounded.