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I have a Yaesu FT-857 Amateur Radio transceiver which has an output for an Analogue Moving Coil meter.

The radio will output a current varying between zero and 1 milliamp (0–1mA), depending on the signal strength received by the set. This is intended to drive the coil of an analogue meter. 1mA indicates a very strong incoming signal, values below are weaker stations.

The spec doesn't give a voltage, but I've measured that this varies between zero and five volts (0–5V). While it does appear that the voltage also varies with signal strength, it's not mentioned at all by the spec, and I'd like to stick to measuring the current, for the definitive value.

What's the best way to measure/sample this signal, so that I can use the Current value in an Arduino, to create a digital coil meter with a 7-segment display?

Note: This transceiver does provide the value digitally using CAT, a serial data link. For reasons I won't go into, I only have the analogue meter line available.

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  • \$\begingroup\$ 1mA through a 5kohm resistor gives a voltage drop of 5V. \$\endgroup\$ – Ignacio Vazquez-Abrams Feb 3 '15 at 1:13
  • \$\begingroup\$ It's a good bet that the radio uses a voltage source internally, connected to the external interface through a 5K resistor, so there's really nothing wrong with reading the voltage directly. \$\endgroup\$ – Dave Tweed Feb 3 '15 at 1:31
  • \$\begingroup\$ Fair enough... What's the best way to measure this voltage? Ideally with isolation from the radio... \$\endgroup\$ – Nick Feb 12 '15 at 13:00
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Ideally with isolation from the radio.

It is easier to measure the meter voltage.

The following circuit doesn't give true isolation between the radio and your Arduino circuit. However if the radio electronics is isolated from the mains the circuit could work well. You will have 100K ohms separating the radio from the Arduino. Also, the op-amp circuit won't load down the meter (well, 200K isn't much load across meter, but will have an effect).

The amplifier shown has gain of -1. Connect V4 to the positive of your meter and V3 to the minus of your meter. Diode D1 is to prevent negative voltage from reaching your Arduino. Check the Vout for proper operation before connecting to your Arduino.

You can also put a capacitor across D1 to smooth out the signal, or use averaging in the Arduino program.

The +Vcc (V1) to be 5 volts so the Arduino won't ever experience an over voltage from V1. The Op-amp is close to rail to rail, but output voltage can only go to 4.9 volts (and dependent upon what op-amp you choose).

enter image description here

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  • \$\begingroup\$ If the 'meter output' turns out to be a true current source (like a transistor collector), the input impedance of this circuit might be too high (will saturate the current source). A 100 ohm load across the input, and 10k values for R1 and R2, would be safe. Since it's a transmitter, some RF filtering at the input is also recommendable. \$\endgroup\$ – Whit3rd Feb 14 '16 at 7:41

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