# Special DC Voltmeter application

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#### eodell

##### New Member
Good morning gents,

I apologize, but it's been many years since I have been out of school designing circuits, but I could use some help for a design solution.

Basically what I want to do is use a modified analog DC voltmeter to indicate the position of a mechanical control. I was able to find an outfit that specializes in modifying the gauges so that department is all set, except for the fact that unfortunately the only gauges they could supply that would match the form factor I needed read 8 to 16 volts DC rather than something more useful like 0 to 12.

My original plan was to use a 1k linear potentiometer and add a trim pot at either end to "calibrate" so that the range of motion of the control would (roughly) be represented by the range of the gauge. Accuracy is less important than the appearance of function. By the way, my input voltage is 14.7 volts DC.

What I am wondering is, will the trim pot solution actually work, or might it be better to set up a set of resistors switched in parallel to give me say, 5 points on the dial?

Your time and attention are appreciated.

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#### crutschow

##### Well-Known Member
You could use an op amp differential type circuit such as this to provide the offset to give you a 0V to 10-12V output for an 8 to 16V input. The offset cancellation voltage (plus voltage) is applied to the minus input and the signal goes to the plus input.

If you have only one power supply then you should use a single-supply type op amp such as the LM358.

##### Well-Known Member
My take is you have 8 to 16 volts that represent something. You want to convert this 8 to 16 volts to some engineering units? Should that be the case and if this is a commercial application I would buy a meter along the lines of for example the Omega DP41E (or Newport equivalent) and scale it to read whatever you want. They are also available to operate on the DC voltage you mention of 14.7 VDC. Additionally with an option meters like this offer a linear scalable DC output of 0 to 10 volts or 4 to 20 mA if needed. Good meters like this aren't quite cheap.

Another suggestion would be as Carl suggest using an op amp circuit.

Ron

#### colin55

##### Well-Known Member
You simply put a zener in line with the voltage and maybe trim it with diodes and a small-value resistor.

#### JimB

##### Super Moderator
I am sorry but I am a bit confused by some of your statements.
A bit of clarification would go a long way.

Basically what I want to do is use a modified analog DC voltmeter to indicate the position of a mechanical control.
OK, I think that I understand that bit.

I was able to find an outfit that specializes in modifying the gauges
The "gauges" are the analogue voltmeters?

the only gauges they could supply that would match the form factor I needed read 8 to 16 volts DC rather than something more useful like 0 to 12.
The analogue voltmeters need 8 volts to show the "zero scale" reading and 16 volts to show the "full scale" reading?

My original plan was to use a 1k linear potentiometer and add a trim pot at either end to "calibrate" so that the range of motion of the control would (roughly) be represented by the range of the gauge.
So, the 1k pot is coupled to the mechanical mechanism whos position is to be measure?

Accuracy is less important than the appearance of function.
What does this mean?

By the way, my input voltage is 14.7 volts DC.
Input from where?
Do you mean the available power supply?

JimB

#### eodell

##### New Member
Good morning again everyone and thanks so much for these responses!

To give further detail and answer some questions:

The mechanical control is something we call an expression pedal, not unlike the accelerator in a car. What I'm looking to achieve is for the needle of this gauge (its a modified analog VDO automotive 8-16 DC voltmeter) to roughly emulate the movement of the pedal. What this pedal actually does is actuate a mechanical multiple-contact roller assembly.

The reason the VDO gauge was chosen was because the form factor was able to be modified (quite heavily, in fact) to match the original mechanical instrumentation. What we're looking to do here is simulate the original function. I can either pickup up 14.7 DC input voltage from the roller assembly at certain points in the linear travel of the pedal (less desirable) or mechanically connect the linear 1k slide pot assembly to the control and us that as input for a circuit. The DC supply in the existing system puts out 14.7 volts. THe VDO gauge reads a range of (again, roughly) 8 to 16 VDC. The gauge does have an internal trim pot for calibration, but the range is limited.

It seems to me the op amp solution seems the most likely to produce the desired result (thanks for that crutschow) but what I am fuzzy on is working out the R values relative to using the 1k linear pot.

Thanks again, gentlemen for your attention and I look forward to your responses.

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#### JimB

##### Super Moderator
OK, that make sense.

As the meter needs 16volts for FSD, you will need more volts than the 14.7 which the existing equipment supplies.

As a very quick and dirty calculation, if you use the 1k pot and also use its full travel, with a 20v supply, connect a 1k resistor at each end of the 1k pot you will get 8 to 16 volts from the slider of the pot. See the attachment.
This calculation ignores the current taken by the meter.
If we know the resistance of the meter it should be possible to implement an accurate solution just using resistors and a stable supply greater than 16 volts.

JimB

#### eodell

##### New Member
Hi JimB and thanks very much for this. I'm out of the shop until the weekend so I can't get a reading on the meter to calculate consumption.

Your idea looks like a much simpler solution closer to what I had in mind originally and has in turn given me some ideas and I would welcome your feedback. Certainly I can easily procure s small 24VDC supply to use for this application. Further, it might be possible to increase to local power supply to 16VDC or higher without affecting anything else.

Assuming a 24VDC supply voltage, wouldn't it be possible to make either (or both) of those 1k resistors trim pots to give me further latitude in dialing in the range?

#### JimB

##### Super Moderator
Hi JimB and thanks very much for this. I'm out of the shop until the weekend so I can't get a reading on the meter to calculate consumption.
OK

Your idea looks like a much simpler solution closer to what I had in mind originally and has in turn given me some ideas and I would welcome your feedback. Certainly I can easily procure s small 24VDC supply to use for this application. Further, it might be possible to increase to local power supply to 16VDC or higher without affecting anything else.
Whatever supply you use it needs to be stable to stop the meter calibration drifting due to changes in supply volts. Use a simple three-terminal voltage regulator.

Assuming a 24VDC supply voltage, wouldn't it be possible to make either (or both) of those 1k resistors trim pots to give me further latitude in dialing in the range?
Yes trim pots are a possibility, just keeping it simple for now.

JimB

#### colin55

##### Well-Known Member
You simply put a zener in line with the voltage and maybe trim it with diodes and a small-value resistor.

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