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A simplified Sound-to-Light display using a few inexpensive parts.
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Signing UpStep 1The 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.
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.
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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.
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.
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.
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...
Looking forward to seeing more projects. Keep it up!
Just posted a 3-channel version (using direct audio output) here: http://www.instructables.com/id/A-Triple-Channel-Musicator-the-TriM/
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.
+ 1u or v?
----|(----
I love your work
Damn, this looks so easy I'm almost ready to start building one!
Keep it up.
Nice work.