Adding a recorder output to an analog panel meter

[ChristofferB]

Hi all!

I guess this is a general issue on many instruments, but I'd like to add a voltage output to my RM5/1 ratemeter, to log the count rate as a dc voltage either with an arduino or a chart recorder (just to keep it 1970's).

-I can get to the meter terminals
-The meter is driven by a current (I assume) so I guess whatever I add should be very high impedance as to not alter the meter reading
-Current needs to be converted to a convenient voltage (say 0-100 mV or 0-1V)

-No schematic on the ratemeter, it's all transistors. I'd rather not mess too much with the base board.
-Instrument runs on 3V.

[update] Meter is a Crompton instruments 200uA F.S.D. one


Is there a neat, simple way of doing this? This MUST have been a common mod to do to instruments in the 1960's and 1970's right?

Thanks in advance!

[sandalcandal]

Might not work depending on the accessibility of the gauge during use but have you considered a software based solution? https://medium.com/@nayak.abhijeet1/analogue-gauge-reader-using-computer-vision-62fbd6ec84cc

[sandalcandal]

Otherwise would run some tests to the the input voltage range of the gauge. Get the instrument to sweep over its full range while u measure input to the gauge or disconnect the gauge and carefully feed voltage to the gauge to get a mapping of voltage to meter reading. Using an Arduino as an ADC with serial output is probably the easiest solution but depending on your accuracy needs you might have to get a dedicated ADC (or difference MCU with better ADC). If impedance requirements and/or the voltage range for the gauge doesn't match well with the ADCs range you'll need to make some basic amps using op-amps or instrument amplifiers to match the ranges.

[ChristofferB]

I know the meter range, it's 200 uA full scale. The main issue is piggybacking off the meter without loading it down.

[ChristofferB]

I guess the main question REALLY is: I thought this was a smart way of making a count rate to voltage converter. Is this really more work than just building a rate to voltage circuit?

[sandalcandal]

I know the meter range, it's 200 uA full scale. The main issue is piggybacking off the meter without loading it down.

Would also be good to know the impedance. As along as the input impedance of the ADC or amplifier stage before it is high compared to the gauge there shouldn't be a problem. This off the shelf ADC breakout from Adafruit has 100Meg input impedance on its highest gain setting. A quick look for galvanometers in this range gives impedances in the 100s of ohms or less.

Edit: No experience with making Geiger rate counters from me 

[KK6IL]

The solution may be simple. Almost all analog meters are fed from a voltage which varies proportional to the meter deflection applied through a fixed resistor to convert the voltage to current.  Trace the meter leads back to the circuit, locate this resistor, and with the equipment in operation, verify the voltage does indeed track the meter reading. Take this voltage through a resistor, or better yet, a buffer amplifier to the recorder output connector. If memory serves, I think we used a 1K resistor back in the Wavetek voltmeters back about 1970.

Warning, do not apply an ohmmeter to the meter movement - the ohmmeter current is often high enough to destroy the meter. The easy way to find the magic resistor would be to disconnect the meter leads from the meter, and measure resistance from the meter leads to each resistor. One meter lead is usually connected to ground. Little trickier to identify the circuit if the current limiting resistor is between the meter and common.

[vk6zgo]

Hi all!

I guess this is a general issue on many instruments, but I'd like to add a voltage output to my RM5/1 ratemeter, to log the count rate as a dc voltage either with an arduino or a chart recorder (just to keep it 1970's).

-I can get to the meter terminals
-The meter is driven by a current (I assume) so I guess whatever I add should be very high impedance as to not alter the meter reading
-Current needs to be converted to a convenient voltage (say 0-100 mV or 0-1V)

-No schematic on the ratemeter, it's all transistors. I'd rather not mess too much with the base board.
-Instrument runs on 3V.

[update] Meter is a Crompton instruments 200uA F.S.D. one


Is there a neat, simple way of doing this? This MUST have been a common mod to do to instruments in the 1960's and 1970's right?

Thanks in advance!

If it is a plain old moving coil meter (PMMC), it will be quite low in impedance.
You can see if it is affected by a fairly high parallel resistance/impedance as follows:

(1) Set the instrument up to give a convenient meter reading.

(2)Connect a DMM across the PMMC terminals, on DC volts(the former may well autoscale to mv while you are doing this test) manually record the both the DMM reading & that of the PMMC.
 .
(3)Whilst still maintaining the same input to the instrument, disconnect the DMM & see if the PMMC reading increases.
If there is no discernible change, you know that 10-11 Mohms are not a problem, & can design your connected circuit with such an input impedance.

If you think this is a excessively high value of Z, try hooking progressively lower values of resistor across the PMMC, until there is a discernible drop in its reading.
You then know your lower limit for Z.

[RoGeorge]

- if it's stationary, a Raspberry Pi Zero + Webcam + Timelapse snapshots or video to work on any instrument and avoid opening the original.

- if it must be a voltage output, then locate the series resistor (the uA is usually driven by a voltage source series with the resistor and the indicator's coil), and that's the voltage to log.

[shooter]

The output voltage to the gauge itself is low, however with a opamp, or a ADC you can make it good to be able to put it in an arduino (NANO) and work with that. As it is pretty small (compared with the old days maybe possible to built it in the old meter.

[ChristofferB]

Ok i might have made this harder than it needed to be. The meter was connected in series with a 1.5k resistor to gnd, so it is a voltage it measures. and being a 200uA fsd meter we can calculate the FSD voltage. About 300 mA.

I just connected the meter + and and to my plotter and it works perfectly. No change in meter reading with the additional load. Works a charm! Thanks for your assistance!