Friday, March 30, 2012

Simple LED VU Meter Circuit

VU meter or a volume unit meter is a device used for indicating the music volume output from an amplifier or a loudspeaker system. It may be also considered as a device for displaying the PMPO of the amplifier at a particular volume setting.
Though the unit looks quite  technical, which is applied as a measuring device of audio power, in real terms these are more like decorative ornaments of an amplifier. Without such devices attached, an amplifier system would look quite dull and without any juice.
The varying response from a VU meter certainly gives a whole new dimension to a sound system making it more dynamic with its features.
Prior to the days when LEDs were not so popular, moving coil meter type of displays were commonly incorporated as VU meters and surely these units with there back lights ON produced a distinctive visual effect as their needles deflected from left to right displaying the varying pitch of the connected audio system.
With the advent of  the LEDs, the moving coil displays slowly got replaced with the ones which incorporated LEDs.
With color effect at its disposal, LEDs became the HOT favorites as far as VU meter were concerned, even today amplifiers employ a LED VU graph for displaying the music power in an amplifier.
For electronic hobbyists who are rather more interested in building a particular required gagdet right at home instead of buying a commercial piece, this cool VU meter circuit will interest them if they are intending to make one for their music system.
The circuit of a simple LED VU meter explained here uses the outstanding chip LM3915 from TEXAS Instruments.
The circuit diagram shows a very simple configuration employing two of the above ICs in the cascaded form for producing a good 20 LED sequencing bar type indication.
The music input is applied across pin #5 and ground of the IC. The music input can be directly derived from the speaker terminals of the music system.
R3 has been stationed for adjusting the typical dB levels between the LEDs for enabling visually more enhanced sequencing pattern in response to the fed music input.
The diagram shows a separate power supply being used for the circuit, however if the amplifier supports a 12 volt stabilized power supply, can be used for powering the circuit as well, this would help to get rid of the extra bulk involving the transformer and the associated rectification circuitry.
The color of the LEDs may be selected as indicated in the diagram or may be altered as desired by the user.
Everything is pretty straight forward and can be simply built over a general purpose board.
Assemble the IC first and then go on fixing the rest of the components and connect then to the relevant pin outs of the IC.
The LEDs should be soldered at the end, such that all of them are arranged in a straight line, preferably at the edge of the PCB.
An external enclosure may be used for housing the assembled circuit or possibly the circuit may be installed in the amplifier dashboard itself, if situation permits the required drilling and fittings.





Please refer to the comments posted by Kieth Russel and me in order to understand the following diagram application:



16 comments:

  1. Swagatam,

    First let me thank you for creating this terrific site, it is an amazing resource for students, hobbyists, and professionals alike. I am a high school electronics teacher (with an ME background so I've been learning on the fly) and often find myself stumped on more advanced circuits.

    I am trying to build a visual sound meter that will show both the "current volume" level (via led bar) and the "max volume" (another led bar w/ pushbutton switch to reset)

    So far (sorry no schematic just on a breadboard) I have an electret mic feeding a 386 audio amp then into a LM3914. The "current volume" display works fine although I would like to broaden the volume range that it reacts to (perhaps even add another mic and average the inputs so it's not so range dependent).

    I have gotten my max volume display to work but in a very sloppy manner and I am hoping you have a better suggestion. Right now I'm using a relay to trigger a transistor/capacitor/resistor combo for each led on the "max volume" display (which does keep them lit but is just a lot of components). I wanted to a trigger those relays with another transistor activated by the 3914 output but couldn't get it to work (may not have been biasing the transistor correctly) and have resorted to using a photoresistor to activate the transistor which then triggers the relay.

    Not a high priority project but just a something cool for the classroom. Hopefully you have time to give it a look.

    Thanks!
    -Keith

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    Replies
    1. Thank you so much Kieth,

      In order to get correct biasing from a 3914 you will need PNP BJTs configured, you may take the help of the design posted in the following link:

      http://homemadecircuitsandschematics.blogspot.in/2013/07/bike-generator-to-220v-converter-circuit.html

      Although the IC has in-built resistors for each of the outputs, attaching external 1K resistors with the bases would be safer and therefore is recommended.

      However I couldn't exactly understand the utility of the max volume" feature, does it refer to the actual volume content from the amp output?

      And why do we need relays for this? Since LEDs are used, these can be lit using transistors directly....

      Regards.

      Delete
    2. Definitely don't need relays that was just me doing things the hard way. To clarify I want the "max volume" led's to light up along with the first led bar set but STAY ON until a pushbutton switch is pressed.

      I created a partial schematic (single output from the 3914) showing my new setup. I will try to link as instructed below. On a side note do you have suggestions for hobbyist level circuit design software with pcb layout capability? I've tried a few (currently using DesignSpark) but haven't found anything I really like.

      Anyway everything works fine with a single pnp except I can't manually reset my "max volume" led (have to wait until the capacitor drains for it to turn off). I thought I could just short circuit the capacitor leads (where the pushbutton switch would go) but it seems to just recharge the cap through the "current volume" led. I'm pretty sure I could use another transistor to solve the problem but would like to use as few components as possible (will be put on pcb).

      Any advice would be appreciated.

      Thanks again!

      Delete
    3. I am trying to understand it but could do only partially, I think a schematic would help to make the whole picture clear and for making required corrections.

      By the way which capacitor are you referring to and in which diagram?, if you could identify the exact component I would suggest a suitable solution.

      Actually I have never used a schematic or PCB designing software so far, presently I rely on corelDRAW for making these drawings....so not very sure which one could be the best, I'll do some research and let you know for sure, though

      Regards.

      Delete
  2. I shared the schematic with you through google+.

    ReplyDelete
  3. Here is the link just in case:
    https://drive.google.com/file/d/0BwBqyM2fjd9-akJPU1ctd1p2Z3M/edit?usp=sharing

    ReplyDelete
    Replies
    1. Thanks Kieth, that looks great, however I could not relate the positioning and the polarity of C1. Is it for sustaining the illumination on the "max" LED? If it is so then I think C1 must be connected directly in parallel with the "max" LED or simply across the transistor's base/emitter.

      Rest everything seems to be correct with the circuit, the base resistor of the transistor could be lowered a bit for increasing the LED brightness, a 33k would probably work better.

      I am assuming the bat 9V being used for the entire circuit otherwise its negative will need to be made common with the ICs negative supply.

      Delete
    2. It looks like they are using the thin line as positive on their electrolytic capacitor schematic symbol, and yes C1 is to keep the transistor on for illuminating the "max" LED. I tried it directly in parallel (no resistor?) with the "max" LED as well as between the base/emitter but neither kept the "max" LED lit...

      Your question about the polarity of C1 prompted me to try my pushbutton switch to 9V instead of ground which works perfectly (although I don't understand how exactly)

      Only problem now is that with the 100uF cap and 100k res the "max" LED only stays well lit for about a minute as it slowly fades. I have tried various cap/res combos for slightly improved times (max ~5 min) but like you said the high res compromises how bright the led gets. Also I would like to limit the cap to 100uF for space considerations if possible...

      Here's an updated schematic:
      https://drive.google.com/file/d/0B37pDhLa1w2ldXRLcVhDcXlJSlk/edit?usp=sharing

      Delete
    3. The schematic has many faults, let me clarify them point wise:

      I am assuming the 9V supply to be common with the IC also, and this is recommended for simplifying the operations.

      Connecting the cap across the emitter/base of the transistor will definitely keep the max LED illuminated for sometime, however this would also produce a delayed illumination when switched ON, therefore it's not the right place for the cap to be in.

      Are you using simulator for checking the results? I am afraid in that case you may be getting misleading results from it, that's why I never use simulators.

      with a 9V supply the "current LEd" must have its own limiting resistor, a 10K would be OK in series with it, otherwise the IC could dissipate a lot and get warm.

      The present positioning of C1 is correct, except its polarity which needs to reversed.

      The reset button must also accompany a limiting resistor for ensuring better safety to the IC

      I will try to update the corrected diagram soon in the above article.

      Delete
  4. Yes 9V is common for IC and all LED's.

    The schematic I sent does work as desired (albeit needing resistors for "current" led and switch as you suggested) but just doesn't keep the "max" LED lit long enough. Here's a link to my breadboard layout if you are still skeptical: https://drive.google.com/file/d/0B37pDhLa1w2lNDdUeFRIVk9XdEE/edit?usp=sharing

    I modified the circuit as you recommended in your schematic, and it also works as desired, but has the same issue of not keeping the "max" LED lit long enough (~1min with the 100uF cap to ~3min with a 470uF).

    Any thoughts to improve? Ideally the "max" LED could stay well lit up to an hour.

    ReplyDelete
    Replies
    1. agreed! your circuit may be working but its not technically correct, the one that I have suggested is technically sound,

      anyway a delay of 1 to 3 min looks quite large, and perhaps cannot be achieved using a single transistor, you will probably need another NPN stage with the existing PNP.

      Alternatively you could consider using NOT gates from the IC 4049 (each has 6 nos, so two ICs would be required) for achieving the 1 to 3 minutes or even upto 10 minutes using much smaller capacitors.

      Delete
    2. ....for achieving delays in hours you could probably opt for 4060 ICs on each pinout of the IC 3914, the entire design could look enormous but nevertheless will provide you with all the desired delay effects.

      Delete
    3. Another NPN seems to do the trick quite well (1hr+ as laid out in link below) so I don't think I'll need to add another IC. I'm sure this schematic is far from "technically sound" but it does seem to get the job done with minimal parts:

      https://drive.google.com/file/d/0B37pDhLa1w2laFctUVJXQ0FlLVU/edit?usp=sharing

      I only modeled two of the "max" leds to show how that I am using a common switch to reset all of them at once.

      The only problem I'm having is that the output from my 386 (looks like saturating at +/-1.4V on my oscilloscope) isn't enough to activate the full row of LED's. I grounded pin 8 on the 3914 to set the "Ref Out" to 1.25V. I'm confused because when I use a DC power supply directly into pin 5 of the 3914 and sweep from 0-1.25V the LED's light up perfectly.

      I'm thinking I might need to try the 3915 or 3916 (and/or a better amp) but don't have any on hand so wanted to run this by you before ordering... (i'm not sure why they show a slightly different "typical application" for the 3914, i'm hoping they can be swapped without issue?)

      Also I tried taking out the resistors protecting the switch and each current LED (again trying to limit components) and did not notice any significant increase in power consumption so I left them out... (currently pulling ~70mA with all LEDs on)

      Thanks again,
      Keith

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    4. The design looks fine to me, however without a resistor at the base of the NPN transistor would instantly fry it, I am surprised how it worked for you.

      Also, in order to work with a single reset switch you have shorted all the 10M inputs together, that would mean as soon as one of the inputs of the IC becomes low all the LEDs would light up together....are you sure you have confirmed it practically??? All the 10M points needs to be terminated via individual diodes to the reset switch for implementing this correctly.

      The LM386 stage could be replaced by a single BJT stage as given in the following article (at the bottom) and the REF of the IC could be varied for tweaking the sensitivity.

      http://homemadecircuitsandschematics.blogspot.in/2012/03/how-to-make-vibration-detectormeter.html

      To me all the versions are almost identical with their specs, it won't make much of a difference.

      Delete
    5. I originally had a res protecting the NPN base but noticed that the voltage coming of the PNP collector was so low (~.7V) I that I might get away without it. I believe the 10M is just barely letting the PNP open so in a way protecting the NPN as well...

      Your are correct that I need diodes at each junction leading back to the switch (I only had two "max" leds breadboarded and one was hooked up with a corroded wire which I believe only by chance allowed my most recent schematic to work)

      I will try replacing the 386 with a BJT as referenced but I believe I am already at the lowest possible senstivity (with pin 8 to gnd) for the 3914...

      Now that I think about it the 3915 and 3916 would likely perform even worse since they are effectively "less sensitive" to signal inputs than the 3914.

      Delete
    6. If you measure the voltage at the collector of the PNP after disconnecting it from the base of the NPN you would find the entire 9V present here.

      The base of the NPN is actually sinking the entire 9V to ground and allowing only 0.7V to be measured at its base since its the regular forward drop rating of all BJTs. If you are not connecting a resistor at its base you are forcing it to work under stress, reducing its life span.... the10M may be not allowing too much current , but without a base resistor the NPN is always in an unsafe zone.

      Alternatively to avoid the base resistor you could simply shift the LED/resistor assembly across emitter.ground of the NPN, and connect its collector directly with positive.

      Delete

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