Modifying analog voltmeter

Is it practical to modify an analog panel mounted voltmeter such as this one?

What I'm looking for it to get the full deflection (or close to it) at 5V DC instead of at 300V AC. I like the design (and price) of this one and I'm unable to find something similar looking for the comparable cost with 0-5V range.

I'm not particularly bothered by the loss of precision my modification would cause, since the intended application is an analog gauge for a simulator cockpit in a game. I'd be controlling the voltmeter with a PWM through a low pass filter and can tweak the output and the background graphics as needed. I have never taken apart anything similar, so I'm not sure what the insides look like. My local electronics store stocks some ugly ones, but the price is a kidney and a firstborn and I'm not really that curious.

Are my hopes that the range of the meter is determined by some resistor and that the needle coil is the same for all ranges grounded? Is it feasible to make a modification as described above?

That one doesn't have a linear scale - typically such a device just has a rectifier and series resistor to turn AC peak voltage to small current.

Find a DC meter with a linear scale as a foundation, its a better starting point.

Practical or not, I think it will be fun :P

They are all the same inside. Take out the electronic components, and add a single resistor to match it for 5V. Search Ebay for : vintage panel meter The bare meter (without electronic components) is linear. You only need to add a resistor and print a new background.

You can use them to weigh an eyelash : https://www.youtube.com/watch?v=n90whRO-ypE

Are my hopes that the range of the meter is determined by some resistor and that the needle coil is the same for all ranges grounded?

Probably... But, not all coils will be "the same". Since it's an AC meter, there is at least one diode inside too.

Mechanical meter movements are usually "current meters", but with constant coil resistance, current is of course proportional to current voltage.

The two issues are -

  1. Can you get the thing open to get direct access to the coil?

  2. What is the current & voltage sensitivity of the movement? i.e. Can you drive the meter to full scale with 5V, and are the current requirements low enough that you can make it work with a resistor network, or will you need an amplifier (or transistor/MOSFET)?

At that price, it's probably worth buying one and doing some experiments. If you have a multimeter, you can measure the coil's resistance and you can connect a 1.5V battery to see what happens. If you want to be super-careful, start with a resistor in series with the battery and meter coil.

That one doesn't have a linear scale - typically such a device just has a rectifier and series resistor to turn AC peak

It's a 300V scale with 150 in the center... That looks linear to me!!!!

Thanks guys. I will just order a few and see if I can hack them open without too much damage. The one I linked has some screws on the sides, so that part is looking good.

Peter, that video is so cool!

Do you know the size ? It could be 7.2cm, but I don't know. These are all the SO45 panel meters : http://kdindustrial.ecol.xyz/ I assume they all use the same bare meter inside.

Here are the best photos, you can zoom in and see the wires of the coil. http://www.aliexpress.com/item-img/2PCS-SO-45-DC200MA-Panel-Meter-Moving-Coil-Type-For-Vintage-300B-2A3-KT88-KT66-211/1466618201.html

No, I don't know the size, but it's only important if it's freakishly big, which I doubt.

Here is the size : 47mm http://www.amazon.ca/Plastique-0-300V-voltm%C3%A8tre-tension-analogique/dp/B00RKCMPS6

That is very small.

Well, I don't have too much room anyway. Seriously, it's for a game, and entire project is in it's infancy. If I find that it works well and I like the entire approach I might redesign entire interface. For now I'm just looking for a few dials and switched for basic instruments and data.

Hi Shpaget, probably the ac voltmeter you’re using don’t have diodes inside, this is easy to be done using a moving and a standing still coils, anyway, you need to know what you have, a coil or a magnet, if you have a magnet, as in the other answers you could read, you only need the full scale current and place a calculated resistor to get this current having 5 volts, then you’ll have a deflection to the end of the scale, this current probably is in the order of microamps.

If at opposite you have no magnet, just make it, look for the terminals of the standing still coil, (or the other), and apply to it a dc current, then you’ll have a magnet and everything will work, if in doubt give more info and I will try to help you more.

Regards:

JME

Thanks. I ordered a few, so when they get here (should be in couple of weeks) I'll give a status update.

DVDdoug: It's a 300V scale with 150 in the center... That looks linear to me!!!!

Its not even slightly linear, the scale from 0..100V is half the width of 100..200V

Hi, Do I have this situation correnct.

You need an analog meter, similar to the picture posted, that has a 0 to 300 scale. (forget the AC bit that is only for show, simulator) But you want to control it with arduino 5V, PWM.

By a 0 to 5V meter, use a LP filter. If necessary rescale the internal series resistor to get 5V. You will have to open the meter to re draw the 0 to 5 to 0 to 300 scale anyway.

Tom.... :)

Thanks for the idea Shpaget.
I knew I had one, and it needs some cleaning. It’s a great idea to use it with an Arduino project. Although to show the number of emails in my Inbox would be too difficult…

Vintage panel meters are no more than 5 dollars on a flea market.

meter.jpg

Tom, the idea is to use a voltmeter to get something like a vertical speed indicator. For the sake of circuit simplicity and to avoid using high voltages I figured using a 5V voltmeter would do the job. The only reason I'm not using meters like these is because these are ugly, and the ones I like are 0-300V. I could concievably use the casing of the 300V meter, and slap the 5V mechanism inside, but that would require more modification than just replacing a resistor.

The alternative approach would be to use a stepper or a servo motor, but then I have to make the casing, and I don't have a workshop with the tools I would need.

Would an ammeter work for this purpose as well? They all look like they have the same needle coils.

You're welcome Peter, thank you.

Inspection of the scale and what is visible of the movement of that meter indicates that it is not a D'Arsonval movement. The symbols under the AC Voltage mark specify it as "moving iron" which means it does not contain diodes, and to be used in vertical mounting only.

So the question is - does it contain a range-setting resistor, if so is the actual movement sensitive to 5V or a significantly higher voltage and does it respond to DC anyway? Since it has no frequency designator, it probably does (respond to DC).

Oooo, sorry Shpaget, I think Paul__B is right. And that's why it measures AC without diodes : http://en.wikipedia.org/wiki/Ammeter#Moving-iron A search for moving iron came up with the same panel meter : http://www.alibaba.com/product-detail/SO-45-Moving-Iron-Instruments-AC_438825505.html

No need to apologize. I ordered a few different ones anyway, so I can experiment. Even if they turn out to be unsuitable, it doesn't matter. Either way I learn something new, and that's basically the reason I do all this stuff.

In the meantime, I remembered I had a crappy old multimeter I bubblewraped months ago and prepared to send to Dave Jones from EEVBlog, but never got around to actually ship it. So I hooked it up to an Arduino and got instant results. The meter is so crappy that on 10V range I get (pretty much) full deflection on 4,5V I get from my USB port. It's perfectly broken! No need to change resistors.

Now I have an analog fuel gauge! Logarithmic scaling for vertical speed indicator was too much trouble to implement, and to be honest it needs more than 45° needle deflection in each direction. Real ones have almost 180°.

Aside, but I attempted to purchase some interesting things from Alibaba early this morning, managed to set up an account, only to be told after selecting items which quoted free shipping to Australia, that on "checkout", they could not be shipped to my region.

Perhaps this only applies to Australia, but they clearly won't be getting much business from us (of course, it does not help that the bottom has dropped out of our dollar as China is not so interested in buying our minerals even at rock-bottom prices). :astonished: