Picture of Musicator Jr - Mk 2
A simplified Sound-to-Light display using a few inexpensive parts.
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Step 1: The Circuit - Overview

Picture of The Circuit - Overview
If you're familiar with the original Musicator-Jr, you'll know it uses a dual op-amp to convert sound from an electret microphone to display on LEDs. This new design uses a much-simplified circuit and is able to support up to 40 LEDs.

Like the USB-Musicator design, this circuit makes use of the LM431 "Programmable Zener" circuit. Although packaged like a transistor in a familiar TO92 form, there is a half-dozen transistors inside which forms a powerful op-amp and driver, which we will make use of here.

We have added a 2N4401 transistor as an output stage, which will allow us to drive LEDs up to 600mA. So it is not advisible to substitute any other device for it.

The basic design is for a 9-volt battery, but 12-volt operation is also possible with a few value changes.

A point-to-point diagram is supplied below, a larger / clearer version is also avilable here.

qs (author) 4 years ago
A slightly different display, based on AkoSiGloc9's suggestion - check it out below:

violentorchid9 months ago
They took down your video for copyright reasons, that sucks. You can probably argue it since you aren't making money.
qs (author)  violentorchid9 months ago

It can also be argued that only Beethoven can do that, since he wrote the original!

EET19822 years ago
Hi how are you? I built your circuit on my breadboard. It works great with one exception. I can get the middle blue and white lights to light up beautifully. But the red ones hardly visable. I have each row in parallel like you have it. I have everything identical to your circuit. Hmm...
2012-08-11 22.59.08.jpg2012-08-11 22.59.05.jpg
qs (author)  EET19822 years ago
Unfortunately I cannot see enough details in the photos, but since all the LEDs are connected in series, they should be lighting up evenly - it may be that you have some blue/white LEDs going directly to ground. That is incorrect: make sure that EACH white or blue LED is connected to TWO red LEDs before going to Ground.
EET1982 qs2 years ago
Thanks for the reply. I'll just try different Red LED's. Thanks again for the great instructable!!! I will be making your "Using AC with LEDs (Part 3) - The BIG light" once I find the BR. Take it easy!
EET19822 years ago
Very nice. I'll put this one on my to do list lol. Thanks again :).
qs (author)  EET19822 years ago
You are so very welcome. Happy building!
Polter2 years ago
Hello! Great instructable! Can I use a 500 trimmer instead of the 470k (based on the pint-to-point diagram)? Or I must use it in parallel with a 47k resistor?
qs (author)  Polter2 years ago
Hi, thanks for the comment.

If, by 500 you meant a 500K-ohm trimmer then that is fine, nothing else is needed, since you can always adjust the pot to a lower resistance value.

btw, if you had tried to parallel a 47K resistor to it, you would end up with a value around 43K. NOT what you'd have wanted.
phatpaul2 years ago
Interesting circuit. I'm trying to figure out how it works.
It seems the IC is operating as a comparator. It's comparing the IC's internal 2.5V reference against the audio signal's AC component + a DC bias generated from the forward voltage on the 2 lower LEDs.
The voltage of the LEDs should add up to be around 2.5V then. So the performance of this circuit depends on the Vf of the LEDs you use?

Also there's negative feedback that keeps the DC bias at 2.5V on average. So an LED with lower Vf will be driven at higher current even when there is no audio- to maintain 2.5V at the feedback pin. That means the LEDs will be on even when there is no audio...

I also noticed your R-feedback resistor value is specified as a range. Does the value of R-feedback control the frequency response or the amplitude or both?

I've built several sound to light circuits. I typically use an audio amplifier IC to drive a opto-isolator. But I'm interested in getting different responses. I'll have to play around some with this circuit.
qs (author)  phatpaul2 years ago
For this circuit, you have to think in linear terms.

The overall circuit IS an Op-amp and, with no input, it will try and hold the voltage at the top of the 2 LEDs at 2.5v. This gives each LED about 1.25v. With less than 1mA through them, they're barely lit, if at all. Remember that they are connected in series, so both LED must share the same current, regardless of any differences in Vf.

When a signal arrives from the mic, it changes the equilibrium and the output changes such that Vout = 2.5 - (Vin* ( 1 + (Rfeedback/Rin) )) where Rin is the impedence of the Electret and associative components, usually about 2k-ohm. The range of Rfeedback tailors the amplification to bring the 0.5uV signal of the mic to something visible on the LEDs.
fheisler3 years ago
Seems like a wonderfully simple circuit, but... I tried building it with 9v, R-feedback=165K and two blue LEDs rated at 3.5V each. The top LED stays fully lit, and the bottom LED (replacing the two lower lights in your diagram) responds to the mic as expected. Extra two-LED chains are also always fully lit, but it seems like the voltage drop should be close enough... Any clues?
qs (author)  fheisler3 years ago
The fact that the LEDs do not light up equally means there is a problem with the connections. The top LED is probably shorting somewhere.
fheisler qs3 years ago
Thanks for the quick reply! It ended up being a faulty LED that lights up fine but I guess has a bad lead or draws way too much voltage...
AndyGadget4 years ago
Clever use of the programmable zener. I see your other version of the musicator gives more detail on the operation. A possible enhancement which springs to my mind would be a three channel version with RC / RL filers on the input to give bass, mid and treble displays in different colours. I love the simplicity of this though.
qs (author)  AndyGadget4 years ago
Thanks for your support!

After the original Arduino-driven Musicator, I've been interested in building a frequency sensitive version based on the LM324, but I was discouraged by the parts count.

I'm working on some ideas but time is in real short supply here...
jptrsn qs3 years ago
I have been thinking about this idea as well. I wonder if it would be possible to grab a pre-made crossover from a speaker box to take care of the filtering for you? Seems like it might make the project less DIY, but more straightforward.

Looking forward to seeing more projects. Keep it up!
qs (author)  jptrsn3 years ago

Just posted a 3-channel version (using direct audio output) here:

Turned out the filter was not as demanding as I had been afraid of. My first iteration used op-amp based active filters but the part count was way up there... This one is fairly simple but IMO very effective.
AkoSiGloc94 years ago
whats this symbol

+ 1u or v?
qs (author)  AkoSiGloc94 years ago
It's a 1 uF (1MFD) capacitor at any voltage. If it is polarized (Electrolytic or Tantalum), then the + should be towards the Electret mic side.
AkoSiGloc9 qs4 years ago
Is there a way and I hope this is not asking too much. Can you do a wiring diagram of which one connect to which I know you got the schematics but its hard for me to follow. Thanks!
qs (author)  AkoSiGloc94 years ago
Here is a point-to-point diagram (also added in Step 1) - hope it helps. A larger copy is downloadable here.
AkoSiGloc9 qs4 years ago
thank you i appreciate this so much i will let you know how my end product will end ill post up and instrucable as well thanks!
Blofish4 years ago
You are my Idol ! LOL
I love your work
I agree.

Damn, this looks so easy I'm almost ready to start building one!
qs (author)  Blofish4 years ago
Aw shucks! Thank you.
Dipankar4 years ago
Nice work my friend.
Keep it up.
qs (author)  Dipankar4 years ago
Thanks buddy! Glad to see your work featured so prominently!
sw4 years ago
Very innovative!

Nice work.
qs (author)  sw4 years ago
Thank you!