LED VU Meters

Electronics enthousiasts, like you and I, are crazy about meters, right? That’s why I made these LED VU meters Of course, they are all LEDs, but I wanted to give them the look of a real old-fashined VU meter. I used a famous IC, the LM3915 Specially designed to control 10 LEDs, and with this input here, the signal source… The higher the signal source, the more LEDs turn on. This is the LM3915, that’s the logarithmic variant. There’s also the LM3914, that is linear… that one is used, for instance, to measure the voltage of the battery but the 3915, the logarithmic variant, is very suitable for audio applications, for instance: VU meters Here’s an example of the 3914, the linear variant. I attached this potentiometer to the input, with the red wire to the plus, and the black wire to the minus And the white wire, the whiper, is connected to the input of the IC. So, when I rotate the potmeter, you see that when I increase the voltage, more and more LEDs light up. This is called ‘bar-mode’ …and this is called ‘dot-mode’ Of course, I have used this dot mode The internet has lots of schematics for VU meters, but most of them are garbage. Look at this one, you’ll find this a lot, or this one, or this one, or this one, straight on the speakers. I made a similar circuit so I can show you what the problem is. It looks like it works ok, but maybe you notice that the LEDs aren’t convincingly on or off. In the higher regions, they seem to flicker a little And if you switch to dot-mode, it doesn’t work. Because this should be dot mode, but it really is bar-mode. So that’s not right. All those faulty circuits connect the audio signal, which is an AC signal, directly to the IC. And that’s not right, because the IC expects a DC signal. So what you see here is that all those LEDs, even the first one, is constantly flickering between on and off Sure, the louder the signal, the more LEDs are on, that’s correct, but they all are flickering on and off really fast. What you should do is add a peak-detector before all this. A peak detector is a kind of rectifier that follows the peaks of the signal. The circuit around this transistor here, and this diode, that’s my peak detector. Now you see that the lights are burning a little better. And when I switch to dot-mode, you see that it also works. I’ll try to show you what the peak detector does. Ok the blue signal is the music signal. And the yellow signal is from the peak detector. Let me adjust the blue a little. The blue is an AC signal, and the yellow is a DC signal Of course, it still fluctuates. It follows the peaks of the music You can see it also dampens the signal a little bit. For instance, this first blue peak is very sharp, but the yellow peak is less steep. So the impact of the drum tapers off a little bit. That’s what you want, otherwise the meter will be much too jittery. So, this is the effect of the peak detector, and if you offer this signal to the IC, you offer a DC signal, and that’s how it should be. Let’s take a look again at how the IC works and how you can adjust it. This is the block diagram, and what do we see? 10 comparators in a row. Every LED has its own comparator. And what do they do? They compare the input signal, here’s where the input signal comes in; it gets buffered and then offered to all comparators The comparators compare the input signal to a certain reference voltage Here’s where we connect the reference voltage, we offer it ourselves. For instance 10 Volts. Those 10 volts get divided by this large voltage divider of 10 1k resistors. That’s how it gets divided among all 10 comparators So the bottom one here gets a reference voltage of 1 volt. This one gets a reference voltage of 2 volts. Here, 3 volts, 4 volts, and so on So, if we offer 1.1 volts as input, this bottom comparator will go off, and this LED will turn on Because it has 1 V here and 1.1 V here, so it’s higher, so the LED will turn on. At, for instance, 4.5 Volts, this comparator will go off. These ones too, of course, because 4.5 is also higher than 3 Volts, 2 Volts and 1 Volts. So, in bar-mode all these LEDs will turn on. And in dot-mode, an extra piece of electronics ensures that only this LED turns on On the logarithmic variant, model 15, the resistors aren’t divided as neatly into 1k steps. On that model, the resistors are divided differently so that you get a logarithmic scale. So, it’s very important that the voltage we input here, the reference voltage, is as high as the maximum input signal we are expecting. Because, at the maximum input here, we want a nice, full output signal with all LEDs burning. That’s why you need to choose the reference voltage very carefully. I made it adjustable with a potentiometer. I put a potmeter here, that lets me tweak the reference voltage and how sensitive the meter will be. Then there’s this: you can see that the LEDs are connected straight to the power. There’s no resistor that limits the current That’s not needed, because in the IC there’s a very nice power source, that precisely controls how much current flows through the LEDs. And you adjust it with this resistor here It says here: “This load determines the LED brightness” For me, this resistance is 470 Ohm. Which comes down to about 20 mA per LED They drew one LED per channel, but of course you can also put 2 LEDs there … because the power source ensures that 20 mA gets pulled through both LEDs. You do have to make sure that the voltage is high enough to feed all LEDs. For me, the power voltage is 12V, which is enough to power about 6 LEDs per channel. So I have 6 LEDs in a row, and the power source pulls 20 mA through each of them, no problem. If you want to connect even more LEDs, your power voltage will need to go up, and the IC can’t handle that. It can handle a max of 15V I think. So 12V with 6 LEDs is a nice value. This is my final diagram. Here’s the rectifier, the peak detector, with a transistor that amplifies it somewhat. With the diode to rectify the current. And these two components here, this resistor and this capacitor. They govern the amount of dampening, or how quick the meter responds. If you make this capacitor larger, you get a very slow meter. I think 470n is a nice value, because you get a meter that’s still responsive, but not too jittery. Here’s the potmeter that’s used to adjust the reference voltage. So that’s where you adjust that the meter will only go red at the loudest sounds. And here at pin 1, there’s a 10k resistor It’s a bit of a strange story. These chips are known for sometimes leaking current from channel 1. So in some cases the LEDs on channel 1 will constantly glow dimly. That’s annoying. But, on my breadboard I didn’t see it. When I soldered the same chips on the PCB, they did have the problem. So I added this resistor to solve the problem. So, it wasn’t in my first design, but I added it later. Here’s two connectors, one is input and one is output. That makes it possible to connect two prints to each other, so that you get a left and a right. This is the print that goes with it. This is the front. There you can see where the LEDs are soldered, and the IC slot. All other components are at the back. Now we’re looking at the back side of the print. There’s the connector, the transistor, all components… the diode is here, under the IC slot. … and some resistors. So you see I kept most of the components at the back side Because it looks nicer So here are the connectors, here’s one on the edge, there’s the other one. So that we can connect two of them to make a left and a right channel. So these are my prints. I connected two together. These are the ones that I’ll end up using. I made some changes in the design. This was the first design. What I showed you before was the adjusted design. You can see a couple of last-minute changes. This is the resistor for the current leaking. This wire here connects the two reference voltages together. Because the potmeter here adjusts the reference voltage for this channel … but on this side you want to use the exact same reference voltage, so that you don’t have to adjust twice. In my newest design, the reference voltage is shared through its own connector here, so you only need to use a potmeter on one of them. By the way, I’ll remove this potmeter from the board because I want it on the enclosure. Here’s the connector that ties everything together. And this cross connection is not necessary in my new design. So everything can connect one-on-one. This has to do with the following: this is where everything goes in, left and right. The left channel is used here And everything gets transfered to this side. And this side should use the right channel. But if you connect it one-on-one, it uses the left channel. So that’s what the cross connection was for. But in the new design I swapped the connectors so that you can connect everything one-to-one and this one will use the other channel So this one left, this one right. I also made an enclosure for the whole thing This is the back of the box. I also put a power unit in there. With a simple 9V transformer, 100 mA, rectifier and damping e-cap. So it’s not stabilised, but that’s ok, it’s good enough. Here’s the potmeter, for adjusting So now I press the power in its place. Because the back side fits very nicely. So the prints are glued in place, and also the screen with a few drops of epoxy glue. So now it’s completely done. For now, thanks for watching, and see you next time! Thanks for watching

24 thoughts on “LED VU Meters

  1. those schematics , you didnt even boder looking, you shoulda pick one much better. check out my channel to see how a real vu meter should dance 😉

  2. Wauw.. dit wil ik ook proberen te maken. Ik heb je schema stiekem gekopieerd maar moet nog even goed kijken hoe je dat met de piek detector hebt gedaan. Ik heb de LM3914 (2x) en daarmee wil ik dit gaan maken en later met plexiglas een mooie balk maken of een toren. denk je dat het met de LM3914 kan werken? we hebben hier in de buurt geen zaakje om de 3915 ff snel te halen ;-(

  3. Hey, Ik ben je kanaal vandaag toevallig tegengekomen. Ik mag mijzelf ook hobbyist elektronica noemen. Meestal kijk ik veel filmpjes in het engels omdat er van deze veel meer te vinden zijn op youtube, maar ik vind het erg leuk om een nederlands kanaal toe te kunnen voegen aan mijn abonnementen. Leuke video's ga vooral zo door.

  4. Fantastisch leuk om eens in het Nederlands te zien (iemand schreef het al) en uiteraard erg mooi ontworpen. Neem aan dat je de ontwerpen laat maken via een service (dus niet zelf maakt)? Ik heb zelf ook een boordje met een LM1915 maar bedacht om deze dan in DOT-mode te zetten maar toch met een enkele LED (ik gebruik op dit bordje felle witte LED's) en dan met strookje spiegelend plexiglas per LED een naald na te bootsen. Dat maakt het ontwerp wel een stuk eenvoudiger lijkt mij en dan krijg je ook een solide naald te zien. Misschien vind je dit echt een China oplossing 😉 maar volgens mij kan dat best mooi zijn. Leuk idee voor een nieuw project?

  5. Ha Bart, allereerst mijn complimenten voor de goede uitleg in de video. ik wil graag de piek detector inbouwen maar hoe en op welke plekken van de ic moet deze gesoldeerd worden en wat zijn precies de specificaties van de componenten voor de detector?
    Dan weet ik wat ik moet vragen in de winkel.

    Ik ben maar een amateur 🙂

  6. Hi. It is beautifull. But one problem language. I did not understood. Anything. But if u could translate in english it will b fine otherwise it all went behind my back. Thank u. And pls pcb diagram

  7. Just got my boards from jlcpcb.com and they look great. Now to built this. Thanks for posting gerber files. You guys dont have to post them. So thanks.

  8. This is my project in 18 years ago and now this in my system audio in my house.thank you.working amazing,what is new?my new ?vu on PIC.go on.

  9. Al fin alguien que explica bien.
    Gracias man.
    How can make a speed sensor with vumeter?
    It is posible?
    Please answer my cuestion!!!🗣

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