50 LED VU meter - Help

[GamerAndds]

Im just wondering if anyone can look through it and "in theroy" will it work? Im not worried about the little 555 timer circuit but basically its for when my spk relay comes on. This is a small part of a huge project, and will need to be acurate for power (watts) monertering.

Thanks all Andre

[kripton2035]

I just think it would be easier and cheaper to use one of these display :
http://stores.ebay.fr/ledbargraph?_trksid=p2047675.l2563

[dom0]

You honestly think that you'll get 150 dB SNR?

[GamerAndds]

You honestly think that you'll get 150 dB SNR?

The size is what im going for here, putting two 3 inch display on something thas 21 inches looks verry silly, now putting two 5 inch meters on ssomething thats 21 inches looks better- and honestly its more evenly spaced with 5 inches between the ends and the other meter! 5 inches is roughly 50 leds... so it works out. Lighting up two LED's per output is not an option, each must be controlled by itself. I was thinking about using LM3914 instead...

kripton2035- I will have a look at it, thanks!

[dom0]

You honestly think that you'll get 150 dB SNR?

The size is what im going for here, putting two 3 inch display on something thas 21 inches looks verry silly, now putting two 5 inch meters on ssomething thats 21 inches looks better- and honestly its more evenly spaced with 5 inches between the ends and the other meter! 5 inches is roughly 50 leds... so it works out. Lighting up two LED's per output is not an option, each must be controlled by itself. I was thinking about using LM3914 instead...

kripton2035- I will have a look at it, thanks!

1.) In that case, just spare the lower ~60 dB and wire them up to always light, they'd do it anyways.
2.) Note the difference between 3914 and 3915.

[DrGeoff]

You'd be better off with 1dB increments, 3dB is too coarse for that many steps.
You could try using quad comparators and a resistor network to set the steps to whatever you want, they don't have to be equal.

[GamerAndds]

You honestly think that you'll get 150 dB SNR?

The size is what im going for here, putting two 3 inch display on something thas 21 inches looks verry silly, now putting two 5 inch meters on ssomething thats 21 inches looks better- and honestly its more evenly spaced with 5 inches between the ends and the other meter! 5 inches is roughly 50 leds... so it works out. Lighting up two LED's per output is not an option, each must be controlled by itself. I was thinking about using LM3914 instead...

kripton2035- I will have a look at it, thanks!

1.) In that case, just spare the lower ~60 dB and wire them up to always light, they'd do it anyways.
2.) Note the difference between 3914 and 3915.

Yes the LM3914 is linear where the LM3915 is log based- as I stated above though I really dont want to run any more that one segment at a time. Another thing is that this will be bar/dot (dot mostly) type of meter...

[GamerAndds]

You'd be better off with 1dB increments, 3dB is too coarse for that many steps.
You could try using quad comparators and a resistor network to set the steps to whatever you want, they don't have to be equal.

Would the LM3914 work better in this case as its values between comparators the same (I think its 1K if my memory is right), DrGeoff?

[DrGeoff]

The LM3914 does not contain a log amp, so it shows a linear display (ie not dB). This is probably of no use to you.
The LM3915 has 3dB steps (from memory) which is not of any use with 50 steps, as this represents 150dB of dynamic range. Even if your highest measurement value was +24dB, your lowest value would still be -123dB. Since it is unlikely that your noise floor will be below this (you might be getting -70dB, for example) then your lower 10-20 leds will be illuminated all the time.

[GamerAndds]

I thought I posted the circuit I made here-

[GamerAndds]

The LM3914 does not contain a log amp, so it shows a linear display (ie not dB). This is probably of no use to you.
The LM3915 has 3dB steps (from memory) which is not of any use with 50 steps, as this represents 150dB of dynamic range. Even if your highest measurement value was +24dB, your lowest value would still be -123dB. Since it is unlikely that your noise floor will be below this (you might be getting -70dB, for example) then your lower 10-20 leds will be illuminated all the time.

You mean to do something like what this guy did?

Video at

My problem with this is that it does not show dot mode? How could I acheive this effect?

[DrGeoff]

Yes, that's one way to do it, although I'd use LM339 or LN2901 quad comparators instead of the LM324 op amps.
A voltage divider from a reference setup for 1dB steps into the comparator will give you 40dB of range in this instance with 40 LEDs.

An alternative way to do it, used in many consoles and mic preamps, is to drive the LED string with a constant current source and use the comparator output to short each LED node of the string to ground. It means no resistors needed in series with each LED, as the LEDs are in a series string.

As for dot mode, can't say I've ever had to use it. I'm not sure how useful a display that is.

[Richard Crowley]

The LM3914 does not contain a log amp,...

The LM3915 (3dB log) and LM3916 ("VU" meter) chips don't have log amps, either.

In fact none of the LM391x chips have amps.

The difference in behavior is entirely a function of the resistor ladder values..

The most fascinating digital VU circuit I ever saw used a sawtooth generator, a SINGLE comparator, and a long SHIFT register.

[DrGeoff]

The most fascinating digital VU circuit I ever saw used a sawtooth generator, a SINGLE comparator, and a long SHIFT register.

As a sort of oversampling SAR with visual output?

[ocw]

"The LM3915 has 3dB steps (from memory) which is not of any use with 50 steps, as this represents 150dB of dynamic range."

Just feed three different chains of LM3915's.  Feed one chain of two directly, add a one dB pad before a second chain of two in series and a two dB pad before a single LM3915.  If those IC's are accurate you end up with 50 LED's with one dB per step after you mount the LED's appropriately.

[DrGeoff]

Just feed three different chains of LM3915's.  Feed one chain of two directly, add a one dB pad before a second chain of two in series and a two dB pad before a single LM3915.  If those IC's are accurate you end up with 50 LED's with one dB per step after you mount the LED's appropriately.

Great idea. I hadn't thought of that one.

[matseng]

The most fascinating digital VU circuit I ever saw used a sawtooth generator, a SINGLE comparator, and a long SHIFT register.

That is a really cool design. Just shift in ones at a steady rate in relation to the sawtooth frequency and hit 'load' when the comparator says so. In can't really be simpler than that. 

[Richard Crowley]

I have lost the reference where I saw this.  It was an online version of an electronics magazine I can't remember. Indeed the design was brilliant.

I found a similar (but different) circut in an AES (Audio Engineering Society) paper preprint.  I paid the $20 to see what it was, and it is an interesting design, but considerably more complex.  http://www.aes.org/e-lib/browse.cfm?elib=10257

[bktemp]

I have lost the reference where I saw this.  It was an online version of an electronics magazine I can't remember. Indeed the design was brilliant.

Maybe this?
http://www.eetimes.com/document.asp?doc_id=1225971

[Richard Crowley]

That's it!  Thanks for finding it.       

My Google-fu isn't what it used to be.   

[GamerAndds]

"The LM3915 has 3dB steps (from memory) which is not of any use with 50 steps, as this represents 150dB of dynamic range."

Just feed three different chains of LM3915's.  Feed one chain of two directly, add a one dB pad before a second chain of two in series and a two dB pad before a single LM3915.  If those IC's are accurate you end up with 50 LED's with one dB per step after you mount the LED's appropriately.

Can you draw up a quick schematic, Im pretty sure I know what your saying but just want to make sure...

[JuKu]

Just feed three different chains of LM3915's.  Feed one chain of two directly, add a one dB pad before a second chain of two in series and a two dB pad before a single LM3915.  If those IC's are accurate you end up with 50 LED's with one dB per step after you mount the LED's appropriately.

Interesting idea, enough so that I read the datasheet. :-) The LM3915 is not accurate enough for 1dB step for the whole range, but it is for the top 10dB. By my experience, unless you are building a measurement tool, a VU meter needs to:
- show the difference between a really faint signal and no signal: Only a few steps under -48dB, and none under -60 or -70dB.
- 3dB/step to -20dB
- 1dB/step over -20dB. Half-dB steps are nice, but luxury.

So, the above idea is sound, with some limits.
--
[ Some random thoughts about VU meters:
Nowadays with digital equipment, a peak hold function (that LM3915 does not have) is useful on inputs: You want to know if you overshoot a recording and by how much. At the very least, a holding "clip" indicator I a must, if recording. Even in live sound, if you heard the clipping you'd like to know what channel caused it.
The more channels you have, the more individual channel cost matters. Turned out that for me, the most economical solution was is a distribution mixer, a multichannel soundcard, a PC, an big monitor and http://www.darkwooddesigns.co.uk/pc2/Multi.html software.
Numerical values (i.e. a digital meter) are not needed, unless you want to do measurements.]

[DanielS]

Just feed three different chains of LM3915's.  Feed one chain of two directly, add a one dB pad before a second chain of two in series and a two dB pad before a single LM3915.  If those IC's are accurate you end up with 50 LED's with one dB per step after you mount the LED's appropriately.

Can you draw up a quick schematic, Im pretty sure I know what your saying but just want to make sure...

Since each LM3915 does 3dB steps, what he's suggesting is running three pairs in parallel, each offset by 1dB at the input and interleave the LEDs... A01 B01 C01 A02 B02 C02 .. C16 A17 B17.

While this might work in theory, it could be troublesome in practice since LM3915 are not precision parts.

If I had to do it, I think I would go with the microcontroller and shift register option: you can have an almost arbitrary number of LEDs, whatever scale you want and whatever bar graph behavior you want.

[ocw]

The mid-range LED accuracy of the LM3915 is just -0.5/+1.0 dB, so my suggestion was for a novelty project that might work.  And, unless a single impedance is used for measurement, the required pads' accuracy would have to not depend on the circuit impedance.  For something with greater accuracy, use one of the other ideas.

[GamerAndds]

Well Im using this meter on my amplifier, and if I could I would use a 5"x4" analouge vu meter  - problem solved. However due to space constraints sadly I cant do this... also I dont want to cut my solid 1/4 inch front aluminum plate  It looks beautiful!
This is what I want:
A 5" LED display with 50 segments controlled induvidally (The two meters will then be pefectly spaced on the front).
Dot or Bar mode- however the amp will mostly be kept in Dot mode.
Relativly accuracy for wattage markings- 1W 5W 10W 50W 75W 100W 250W 500W 750W 1000W(0db) and "overdrive" marking (which are where the leds will start to get brighter).

There are some other things like the last couple of leds will be brighter in concecutive order but I can take care of that myself using the LM39xx series by changing the drive current of the ic and using independant resistors on each of the last segments.
The purpose of this is to show wattage really, and to show a "0 db mark @ 1000W" for more of a refrence to what I designed the amp to drive if that makes any sense...

Thanks all for your input!

Yes its 1000 watts per channel (2 channels driven) on 110Vac, its a breaker tripper if your not running it on a 20 amp service