Roll your own VU meter for pennies!

Stompboxes circuits published in magazines, books or on DIY electronics websites.
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merlinb
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Post by 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.
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Get a blade between the seams and pry the back off to reveal the treasure:
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The meter is pretty tiny. Perfect for a pedal!
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To build the driver circuit you need to know how much current it takes to make the needle go to full scale, 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. Asjust 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).
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You can also make your own scale for the meter. The old scale was stuck in 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!
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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.
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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!
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culturejam
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Post by culturejam »

Very awesome project, as usual, Merlin! :applause:
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RnFR
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Post by RnFR »

very cool project! it actually put a smile on my face reading it. :thumbsup
one question- I've always wondered the proper way to scale the driver circuit to the meter. any tips on that one? is it as simple as adding resistance or adjusting the gain of the driver?
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danielzink
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Post by danielzink »

What I've been looking for (for quite a while) is something like this circuit - that I can hook up to the speaker leads (or just one lead - the other to ground ?) of a cobbled together amp I have. So the meter would bounce up and down based on speaker output.

Think this project would work ?


Dan
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merlinb
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Post by merlinb »

The circuit I posted was just for *testing* the meter, it's not an actual driver. Sorry if I caused confusion.

A textbook driver (for a 9V circuit) would be:

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R3 is purely a protection resistor; it ensures that when the opamp's output goes all the way to the rail (actually about 7V) the current is limited to the FSD value.

The scaling is adjusted with R2. The smaller R2 is, the smaller the input signal needed to drive to full scale (R3 would also need to be adjusted along with R2). The values shown here give FSD when the input signal is 200mV peak, because 0.2V/750uA = 267 ohms, which is the value of R2 (near enough).
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juanro
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Post by juanro »

Just ocurred to me that if you move R3 to the other side of the VU meter you can just use a trimmer to adjust it.
See modified example:
VUcircuit.jpg
VUcircuit.jpg (15.5 KiB) Viewed 2233 times
Now R3+R2 could be the trimmer.

Regards.
Juanro
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RnFR
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Post by RnFR »

exactly what I was asking for. thanks guys! and honestly, for something like this cheapie, personally I wouldn't even bother with actual numbers. green/yellow/orange/red would probably do me fine! of course scaling the meter would still be necessary, even for something as ambiguous as colors.
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Post by garcho »

You're the best Merlin, thanks!
...and weird on top!
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Post by atreidesheir »

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


Could we all just abide by the pirate code or a gentlemen's agreement that such items will from this day forward and forevermore be referred to as "booty".
"reveal the booty"
COME ON

come on
come on

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juanro
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Post by juanro »

A quick update, I've found these in DX, very cheap, with a cool "retro" rounded look:

http://dx.com/p/icon-current-flow-vu-meter-500ua-6901

I've ordered some, lets see.

Juanro
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