Epic ulti-multi

While working towards enhancing the core functionality on my synth I’ve now had several moments when I thought “I wish I could split this signal”. Time to build a multi!

Now me being me - I cant just make something simple right off the bat!

Hopefully I can cram everything I want into a 10x20 panel.

Building on the concept of the Bat Meter I picked up this from ebay several months ago and figured it would be a neat little addition to a simple multi:

Since then I have come to the conclusion that I want to have more blinkie lights. Sadly the meter itself is not backlit and if anyone has any idea on how to pull that off, I would be super interesting in hearing about how to do it.

One of the downsides of the Bat Meter was that it struggles to keep up with the rapidly fluctuating voltages on a synth. My idea was to incorporate a pair of 10 bar LED displays. I am already stretching my knowledge technical knowledge here so I decided to look for any help I could get and came across these. These simple PCB’s seem to have everything I could need and more - most likely for less than I could source the parts individually. And they even come in blue and red. I imagine they could just be run off the output of a TL074 - but I’m not sure how to handle a negative value.

I’m not sure how useful it would be, but a over voltage LED of some sort so if I hit or pass +/- 12 volts might be interesting(?).

I also have a 4 position rotary switch…

…but I have the feeling it wont work for what I need. I would like to use it to select which of the four banks the LED bars, overcharge LED’s, and the voltmeter would monitor but I think I need a four pole.

Red/Blue bipolar LED’s seem to be a little more difficult to come by - the first vendor I found no longer delivers to the USA, but I did find this on ebay. Pretty much a lifetime supply if I decide to go for it as I only need four for my design.

I think in the end I’ll need 6 TL074’s (if I understand how these things work).

The last feature I would like is for my inputs to cascade. I know there is a eurorack multi that does this - if you plug something into the first input ALL of the outputs down the line receive the signal until you plug something into the second input, which feeds down to the other two and so on. I would LOVE to have this functionality but have no clue where to even start with it.

I’m still in the spitballing stage here - bring your comments and feedback please!


Just invert it… one more opamp.
And add a diode to each bargraph to get only the positive part to the LED drivers, they probably won’t appreciate negative voltages.


Is it just 4 position or a full rotary locked at 4?
Like the design.

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You can pop open the front of the meter, and then add some leds under the black part. Drill a hole on the back to pass the wire.

I agree with eric, just invert the signal and put a diode.
Note that you have different versions of the LM3914 (LM3915 and LM3916), that output in dB or linear, depending on what you want.

Maybe not 12V, as you would never hit it, but 10V for sure!

In the end, it’s only one signal, time four. So even 1 Pole/4 Pos would work.

Use the switch included in the jack socket! (I can’t draw anything right now, but I will tomorrow if no one already did so)


I did some checking - pole A goes to poles 1-4, B -> 5-8, C -> 9-12. (Triple pole quadruple throw?)

So it is actually overkill for what I need, I suppose!


Yeah, the one you linked is adjusted for battery state indication which is probably not what you want for audio levels, maybe search for a VU meter.
The LM3915 datasheet says it can withstand up to ±35V without damage so you don’t even have to worry about negative voltages.

Stuff is often cheaper on AliExpress, if you don’t mind the wait (the bar graph display you linked to comes from Asia anyways). E.g.


Attempting to plan out the circuit a bit and I have some voltmeter questions.

Should the current running into the meter be buffered?

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Pretty much all of the parts are in now and I am having a go at 3d printing the faceplate.

I have yet to assemble the voltmeters -and this leads me to another question: if I wanted them to read only the + or - voltage, would I just use diodes?

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All meters are basically current meters, the magnetic field that current produces in its coil makes the dial move. They can be used as Voltmeters if you add a series resistor of a known value. The meter plate should then be changed from Amperes to Volts where the resistor value R multiplied by the current value gives the Voltage.

The internal resistor of the copper coil/winding in the meter should be as low as possible so that its value relatively speaking does not add a significant value to the circuit you are measuring the current in. Alternatively if you add a series resistor to convert a ampere meter into a voltmeter, the total resistor value of the meter should be as high as possible so that it does not form a load on the circuit you are measuring the voltage across.

So, when you are thinking of buffering that would relate more to the latter than the former use. You could imagine an amplifier with a very high input impedance that would only minimally load a circuit but would pass the measured voltage through to the metering part.

Where would you want to put diodes and for what purpose?


I ended up getting two of the line level indicators from aliexpress (listed above). The idea was to have one follow the positive voltage an second one the negative.

From what @eric said, I think I could I feed them a signal similar to this. I do have opamps available. So possibly a tl072 for the two line levels and a 1 for the voltmeter?


I would sketch this out but I’m on my phone! Just trying to wrap my head around it for now.


I think the switch I have is in fact perfect for what I need, oddly enough. I can switch between the 4 banks and I can use the three poles - one for the volt meter and one for each line level.


I could use a little help figuring out the last bits of this circuit. How do I go about pinning down the specific values of the resistors and the diodes? And am I even on the right track here? I feel like the volt meter is simple enough and I’m not sure I even need a resistor on it. For the two ten segment displays - I really have no idea if it is this simple or if there is some other bit that I am missing. Do I need to invert the op-amp going into the -12v display? Also, I’m not sure if I need resistors on the inputs of the op-amps as well. There are 10m’s on the buffer multi schematic that I was basing some things on.


Any help here is much appreciated.

Got a datasheet for the 10 segment displays? Guessing they’re rather like single LEDs, so I would assume it tells you what maximum current you can use, and what size voltage drop you have across the display.

Then you need to choose R2 and R3 to limit you to that current. Ohm’s law: the voltage drop across the resistor is 12 V (or whatever voltage you’re putting in) minus the voltage drop across the diode and the 10 segment display, and the current going through the resistor (and hence through the display) is the voltage drop divided by the resistance. Or in other words the voltage drop divided by the maximum current gives you the minimum value for the resistance.

Don’t know if you can use the 10 segments with a negative voltage, check the datasheet. My guess is you can, if turned around like D2.

As @Jos said above, the “volt meter” is really an ammeter with a known series resistance, chosen to make the maximum input voltage (12 V or whatever) give you the current that corresponds to full scale on the ammeter. You can use an ammeter, choose R1 accordingly, and put a voltage scale behind the needle, or you can buy a voltmeter that’s got the resistor and the voltage scale in it already in which case you don’t want an additional resistor (R1).

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Pretty simple circuit- no diodes involved. I’m looking at the lm3915 datasheet and there is nothing on there that says I can’t do it - I see a section that says no protection is needed for +/-35v.

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Ah, so the “10 segment display” is actually this discrete board? I’d misunderstood, thought it was a single component. Looks like the voltage is a reference, going into a buffer through a 20k resistor, so you shouldn’t need your series resistors or diodes. And it looks like you would need to put your -12 V through an inverter.

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Not a step I was anticipating! Is there a good way to do that?

Could I just use the -12v as a reference voltage?

Concerning the volt meter:
the amp meter + series resistor will only be capable of showing a positive signal, so you will need to rectify any AC signal you may feed it. I’m assuming a type of meter here where the needle is on the left most side when no voltage is applied. If it is in the center of the meter scale, then it will be able to show negative and positive DC-signals. If you use a rectifier that will cause a voltage drop which you will need to compensate for. It is most practical to use a trim pot in series with the amp meter in stead or R1 (assuming it is a micro amp meter that does not have a shunt resistor) and calibrate the setup using a trusted voltmeter with a high input impedance. Assuming the meter is a 100 micro ampere one, then a series resistor of 100kOhm will allow you to measure up to 10 Volts ( i.e. U = I * R = 100E-6 * 100E3 ).

Concerning the line driver module:
You will have to find out what the max input voltage for the LM3915N (as shown in the images on aliexpress) which seems to be the driving force behind the line driver module. Your input stage should deliver that voltage whenever your signal has its max value. The line driver circuit may not be capable of handling DC values out of the box though as line voltages most often are AC voltages, so you probably need to bridge an input capacitor ( C1 ? ) to get dc coupling back. You may want to check the datasheet of the LM3915 for that.
For monitoring the negative signal voltages you will probably have to invert the negative signal into a positive signal before you feed it to the line driver circuit.
I think R2 and R3 will not be necessary as I expect the line driver to have a high input impedance.

I went back and looked at the post at the start of this topic so now I have a better idea what we’re talking about!

I’d sort of had the idea you were talking about (for the voltmeter and the 10 segs) monitoring rail voltages. Now it’s not obvious to me what your intended use is. Is this something for control voltages or for audio signals?

So you have a Russian voltmeter that appears to be set up for ±15 V. If you’re looking at control voltages then I imagine you can just connect it directly to your signal — though it still may whip the needle around too fast to be very useful. For audio signals I don’t think you can easily use such a meter.

I’m maybe less sure what we’re talking about with regard to the 10 segment displays, since what you linked to there and the board you showed above are quite different though apparently both are based on the LM3914/5 chips. For control voltages I suspect the answer’s the same in either case, connect directly for the +12 V display and through an inverter for the -12 V. For audio signals too, it looks like the LM3915 will work, though it might take more design effort to get it to do whatever it is you want. See the datasheet and scroll down to Application Hints.

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Making some headway!


Sorry I havnt gotten back to this in so long. Been waiting on parts and having a think about the whole thing.

So you have a Russian voltmeter that appears to be set up for ±15 V. If you’re looking at control voltages then I imagine you can just connect it directly to your signal — though it still may whip the needle around too fast to be very useful. For audio signals I don’t think you can easily use such a meter.

So I based the whole idea on this module here:

There is a video of it in use on that page as well - and yes, the meter is pretty cosmetic as rapid fluctuations will not really show up - hence the 10segment bar displays.

As of right now, the most basic functionality is in place - each of the 4 banks outputs from the in to the four outs as expected. Everything is in place for the bipolar LED’s as well.

Hooking up the meter on a whim to my voltmeter, it seems to have a 15k resistance value strangely enough.