Want to hook up a perm analog meter/gauge to my amplifier, for volts, amps, etc. - AVS Forum
Receivers, Amps, and Processors > Want to hook up a perm analog meter/gauge to my amplifier, for volts, amps, etc.
jjminch's Avatar jjminch 01:09 AM 05-31-2013
Yea I realize this is a bit odd. I'm building my son a robot shaped ipod dock. Just using a little 20w, 2ch amp for it.

So to make it more robot like, I want an old school analog gauge hooked up to the amplifier. I took a multimeter and got 1.5 volts AC to one speaker under small load.

That small of a measurement won't even show on most panel gauges. So what gauge do I need to find (volts, amps, ohms, and at what range) that will show the most "movement" and can easily just be placed in line with one channel of the amp?

For reference sake I want something that looks like this. http://www.amazon.com/0-250V-Rectangle-Analog-Panel-Meter/dp/B005FJ7V6C/ref=sr_1_1?s=electronics&ie=UTF8&qid=1369983615&sr=1-1&keywords=Ac+volt+meter

And here is the sketch of what I'm building.

Ross Ridge's Avatar Ross Ridge 12:40 PM 05-31-2013
Sounds like what you want is a VU Meter. Where you'd get one and how you'd integrate itwith your project I have no idea. That's something that you should probably discuss on a more appropriate forum.
jjminch's Avatar jjminch 04:35 PM 05-31-2013
Thanks for the info. Any idea on a better forum?

I found a VU meter but unsure of where this meter gets its measurements from. I'm not technically adapt enough to modify a circuit board or something.

Kelvin1965S's Avatar Kelvin1965S 05:04 PM 05-31-2013
You need to wire the meter in parrallel with the speaker, but you will also need to put a resisitor in line with the meter to protect it from too much current.

If the meter you linked to is 500uA then this (usually) means that when there is 500uA passing through the meter it will read full scale (ie maximum). As it's late in the UK at the moment (and I've enjoyed a few glasses of wine biggrin.gif) I'll need to come back to this in the morning to work it out. I will need to know the speaker impedance and how many watts the amplifier is putting out. I could then work out what resistor you should use (and figure out how to put a simple circuit diagram up for you to follow).

I think it's great that you're making this for your son, so I hope I can help you sort it out. smile.gif
jjminch's Avatar jjminch 10:27 PM 05-31-2013

Thanks in advance for the assistance. I'm looking forward to the project and will try and help the best I can.

Using this design with the same filter and drivers, but with different dimensions (same cabinet volume) and slightly different amp.
Drivers http://www.madisoundspeakerstore.com/approx-3-fullrange/aurasound-ns3-193-8a1-3-black-cone-wide-range/?mobile=0

And amplifier http://www.amazon.com/Lepai-TRIPATH-TA2020-Stereo-Amplifier/dp/B003P534SW/ref=cm_cr_pr_product_top
I think actual tests showed 12w a channel with 8ohm load. Sounds pretty good for 25 bucks I think.

So based on your post I should be able to "tap" into a speaker output line before the filter (coil and resister) and run that along with 12v power to the meter?
Kelvin1965S's Avatar Kelvin1965S 04:05 AM 06-01-2013
OK, so the speaker is an 8ohm load and the amp putting out up to 12 watts. We don't know if this is peak watts or RMS, for now I'll work it out based on it being RMS (the meter may under read if the 12 watts is measured as peak, which is safer for the meter). Ideally you want to see the meter jumping about without having the amp up full, so some experimentation of values might be needed (or add an extra variable resistor perhaps).

First to workout the current flowing through the speaker at 12 watts/8 ohms which is 1.22 amps (Watts = current squared x resistance). This also means that there will be 9.8 volts across the speaker (Watts = voltage x current).

(Use this link if you want to confirm the calculations: http://www.csgnetwork.com/ohmslaw2.html)

The specs show that the meter needs 500 uamps for full scale deflection which is 0.0005 amps. So with 9.8 volts across the speaker we will need a resistor of 19,600 ohms to give 500 uamps (0.0005 amps). We will also need to add a diode since the meter measures DC and we're monitoring an AC signal.

You need to connect the meter in parrallel with the speaker, but with the 19,600 ohm resistor (or nearest available value since this isn't a standard resistance) and diode in series with the meter to protect it from excessive current (the diode protects it from reverse current).

I need to draw something out to show how you connect it all up and add it to this thread, which I will do later on (I have some jobs I should be getting on with smile.gif ).
Kelvin1965S's Avatar Kelvin1965S 02:48 PM 06-01-2013
OK here is the circuit diagram based on the figures I worked out above. I've added an extra variable resistor to allow adjustment and links to Radio Shack for the parts (guessing this is easier for you since I'm in the UK and don't know any other USA parts places).

The 15K ohm resistor

IN4003 diode

Variable resistor 10K multi turn (adjust with a small screwdriver and just connect one end and the middle connector)

This is the picture of the circuit, hope it makes sense, but I'll be back on line tomorrow to explain if not.

Ross Ridge's Avatar Ross Ridge 03:02 PM 06-01-2013
I don't know about this model of VU meter specifically, but my understanding is the they're supposed have their own integrated rectifiers, ones that have much a lower activation voltage than common diodes.
jjminch's Avatar jjminch 05:59 PM 06-01-2013
Hey Ross when I searched I couldn't find any that had the diodes and rectifiers built in. There were quite a few DIY boards for them, so I don't think they come with them.

The circuit makes sense, seems pretty simple actually. My only question is if it should tap into the speaker wires before or after the filter (not a crossover since its a single driver design). My guess is before.

Lastly you stated 9.8v is going to the speaker above. Do you think that is enough to power the 12v gauge bulb requirement, or do I need to find another source?

Thanks again for the tech support, this is going to work out great.
Kelvin1965S's Avatar Kelvin1965S 06:31 AM 06-02-2013
It needs to be connected across the speaker (ie after the filter) because you need the meter to measure the voltage across the speaker not any voltage drop (if any) across the filter.

You'll only get 9.8 volts across the speaker at maximum output, most of the time it will be much less. I imagine that the 12 volt supply is to light up the lamp in the meter, so you would need to arrange a separate supply for that (not from the speaker terminals).

If you search for a 'VU' meter it may already have the rectification built in, so you would just require the resistor (and optional variable resistor). However, it might need different values for the resistor, so if you change the meter, let me know and I'll work out the new resistor values.
jjminch's Avatar jjminch 09:57 PM 06-02-2013
That makes sense. I found the same model vu meter with no light. I might get that one and just put a little LED in there. My micro amp has only 5v going to it, so that would probably be easier to sort out. I might get a little USB one since ill have a hub in there.

Will order the parts and update when I can. Going to lay everything out and test it before I do any cabinet building.
Kelvin1965S's Avatar Kelvin1965S 09:20 AM 06-03-2013
Sounds like a plan, good luck with it all and let us know how it turns out (or if you need any further help). If I don't respond on the thread it's probably my short memory redface.gif so feel free to PM me.
jjminch's Avatar jjminch 07:32 PM 09-21-2013
I got everything hooked up and the VU meter worked! It looks really cool, and ill post some better pics up when done.

One thing I didn't figure is this is a nursery radio. So the VU meter barely moves since it's mostly listened at lower volumes. The lower quality amp starts to distort when it's at 60% so I need to add a resistor. PM out to Kelvin on what suggested value for an additional variable resistor.
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