Make your own VU meter for almost nothing!

[merlinb]

VU meters are cool, but moving-coil meters tend to be quite expensive, and often too large to fit in our favourite pedal enclosures. But, since we don't need a precision instrument, we can use the cheapest crappiest meter around, and one place you will find one is in one of those cheap battery testers. I bought this one on ebay from a Chinese seller for £1.50!


Get a blade between the seams and pry the back off to reveal the treasure:


The meter is pretty tiny. Perfect for a pedal!


To build the driver circuit you need to know how much current it takes to make the needle go to maximum, which is called the full-scale-deflection (FSD) current. Hook it up to a battery (with a protection resistor so you don't accidentally burn it out), a pot and your DMM arranged as a current meter. Adjust the pot until you get full scale deflection. With mine it turned out to be 750 micro amps (0.75mA). (Now write it on the back of the meter so you don't forget). I also measured the resistance at 750 ohms (coincidence).


You can also make your own scale for the meter; the clear front panel should pop right off. The old scale was stuck on with double-sided tape, so I used a craft knife to slice it off. Then I put some blank paper behind the needle and, knowing the FSD current, incremented the current in 10% steps (e.g., 75uA) and marked off the position of the needle. As you can see, this meter follows an approximately logarithmic law (except for some weirdness between 0-10% and 70-80%), which is perfect for audio monitoring!


Then I scanned in my hand-drawn scale and used it to draw a proper scale. I ignored the nonlinearities at the ends- shh...no one will know.


Mount the new scale in the meter and you're ready to build it into your next pedal. HINT: A blue LED behind the meter adds mojo!

[merlinb]

I'm not sure what the cheapest driver circuit would be for a 9V pedal, but this should work:

[Perrow]

On a related note, you can use any VU meters you find as voltage and current meters in your current limiting regulated power supply

[Earthscum]

Way cool, Merlin! I've been wanting to do something like this for awhile now, and have passed up a couple cheap meters... don't know why I thought they'd work drastically different than what I wanted them for. I wanted to do a pair on my tube pre (meter in and meter out). Now I can keep my eyes peeled for a matching pair again.

Thanks much!

[runmikeyrun]

this is way cool!  What a great idea.

When I built a couple circuits into old Civil Defense geiger counters I tapped off the output to send signal (pre-volume pot) to the radiation meter, which showed output level.  I made a simple bridge rectifier out of 4 diodes to give the meter DC vs the AC voltage coming off the pedal.  No need for smoothing caps, we're not talking a precision instrument!  I'm not sure if these meters will work on AC or not, something to keep in mind.  Also, I soldered a trimpot in line (post-rectifier pre-meter) as a variable resistor to adjust the current to the meter.  That way if I modded something later I didn't have to replace a resistor, just fine tune the pot.

[davent]

Very cool, thanks for sharing! How 'bout a meterscale drawing program (freeware?). Haven't tried it but looks handy.

http://www.tonnesoftware.com/

Edit; Very, very basic free version but the option filled version is not free...

[PRR]

> moving-coil meters ... cheap battery testers. ... £1.50!

Thanks. Wonderful. Great tip.

eBay "battery testers", Buy It Now, Price + Shipping Lowest
Many look just like Merlin's, some are a fancy case with maybe the same meter, and then there's other stuff (buzzer, flashlight).
Prices are indeed around £1.50 even in weak US Dollars:
Universal Battery Tester Checker AA AAA C D 9V Button $1.99 Free Shipping
. . .
Universal 9V AA AAA C D Button Battery Checker Tester $2.39 Free Shipping  

Now I can re-create the power room from "Forbidden Planet" without a Hollywood budget.

> this meter follows an approximately logarithmic law

Strange. In battery terms, it expands the "bad" zone and compresses the "good" zone. Me, I'd want 7V-10V spread-out over half the scale. This seems to cram 7V-10V into ~~15% of the scale.

As you say, this IS good for audio. Gets the -10dB mark well off the -20dB mark.

You can see how they do it. The poles are skewed and notched. As it comes up-scale the field strength gets weaker. (At first I thought it was electrodynamic, Square-Law.)

> not sure what the cheapest driver circuit would be

That sure does work (half-wave peak-catching) and seems to be "cheapest".

Thinking inside the box: you could even use the batt-check meter to check your battery! Use a momentary SPST to connect meter + through 5K to V+. (Opamp slams to ground and D1 will back-bias.) Make marks with fresh and flat batts.

Going downstream of the pedals: resistor, FWB, cap and meter across the loudspeaker output will dance, passively. Not good below 1W but works fine for typical guitar speaker level.

[amptramp]

I picked up a battery meter at a garage sale a couple of years ago with exactly this kind of thing in mind.  The meter face is small but the meter chassis is larger, somewhat the opposite of what was shown in the original post.  I am still trying to determine whether I want to use it for an effect or in the car for monitoring intake air temperatures or whatever.

[Morocotopo]

Wouldn´t it be cool to have a meter in a tube amp to bias the power tubes?

[mark2]

I'm not sure what the cheapest driver circuit would be for a 9V pedal, but this should work:

Does anyone have this schematic, or an alternate suitable driver for a VU meter?