Help with... DC amplification?

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joshuabardwell
 
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Help with... DC amplification?

Post by joshuabardwell »

Here is what I'm trying to do. I have a "sound and light" machine. If you're not familiar with it, it plays binaural audio into headphones while flashing LEDs in front of one's eyes in patterns synchronized to the audio. The LEDs are mounted into glasses. There are four LEDs per eye, in two channels (left/right). What I am trying to do is expand the system so that multiple people can use it at the same time.

Splitting out the audio is easy. Headphone amplifiers are common. I'm not sure what the right way to split the glasses out is, though. The "sound and light" machine has a 1/8" stereo jack that the glasses plug into. The glasses themselves have no circuitry at all (other than the LEDs, of course), so I believe that the machine is driving the LEDs directly through the jack.

One approach would be to build, what I guess would amount to a DC amplifier. I was looking at building the 5v regulated power supply from the Boarduino circuit, then using PN2222 transistors to feed the glasses. This would be okay, but I can't help thinking there is a better/simpler/less expensive approach than building from scratch. I got a 20 watt audio amplifier for around $20. I don't know whether it would be appropriate for this situation, though, because it's designed for AC signals. Can you pass DC through an audio amplifier? Is there an inexpensive off-the-shelf DC amplifier or other solution that would work for me?

Thanks.

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adafruit_support_mike
 
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Re: Help with... DC amplification?

Post by adafruit_support_mike »

joshuabardwell wrote:Can you pass DC through an audio amplifier?
Not if it's any good. It would generate heat in speaker coils and burn things out pretty quickly.. and perhaps literally.

Based on your description of the device, it sounds like the glasses have built-in circutry to convert the audio to LED pulses. You'll need to work out what's happening between the input jack and the blinky parts to get a better idea of how (or if) to duplicate the signals.

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Renate
 
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Re: Help with... DC amplification?

Post by Renate »

Oh, this is one of those brain synchronization things, not a music entertainment thing?

So you've got 8 LEDs total on 2 circuits.
The first thing to determine is if it's series or parallel wiring.
You say that there are no additional components.
That would seem to indicate series unless they snuck in some resistors.

You can do 2 different tests.
Measure the voltage peaks coming out of the LED driver unit.
Is it more like 5V or 12V?
Get a 12V supply and a 1K resistor.
Connect the minus to the shell and apply the 12V through the resistor to the tip or ring.
See if the LEDs light up.
Is the voltage when the LEDs glow more like 2V or 8V?

I'd guess that the wiring is series.

If you knew the current driving capacity of the unit it might be possible to just wire a few pairs of glasses in parallel with some build-out resistors.
If you want to do this halfway right, you could build a simple buffer using 2*(N+1) transistor (N=number of glasses).
I'd go with a constant current driver because it's simple enough and only adds two zeners or diode chains.

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joshuabardwell
 
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Re: Help with... DC amplification?

Post by joshuabardwell »

adafruit_support_mike wrote:
joshuabardwell wrote:Can you pass DC through an audio amplifier?
Not if it's any good. It would generate heat in speaker coils and burn things out pretty quickly.. and perhaps literally.
I'm confused. There would be no speakers hooked up to the amp. Just the LED glasses.
Based on your description of the device, it sounds like the glasses have built-in circutry to convert the audio to LED pulses. You'll need to work out what's happening between the input jack and the blinky parts to get a better idea of how (or if) to duplicate the signals.
Such circuitry exists for "audiostrobe" encoded sound files and CDs. These have the optical signal encoded at around 19 kHz, and a circuit extracts it and sends it to the glasses. But in this case, the unit is generating the optical signal directly as part of a program, rather than decoding it from an audio source. I believe that the glasses are entirely passive--containing only LEDs and, probably, resistors, and that the machine drives them directly vs. sending a signal to the glasses that must be decoded. Therefore, I'm hoping that simply amplifying/buffering the signal coming from the machine will be sufficient to drive additional pairs of glasses.

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Re: Help with... DC amplification?

Post by adafruit_support_mike »

joshuabardwell wrote:I'm confused. There would be no speakers hooked up to the amp. Just the LED glasses.
I was talking about audio amps in general.

The usual application of audio amps is to drive speakers, and in that application DC current is bad enough that it can lead to catastrophic failure. A well-designed amp will take reasonable measures to prevent catastrophic failure, so most audio amps are explicitly designed to prevent DC current from reaching the load.

That doesn't change if you connect the amp to something other than a speaker.

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joshuabardwell
 
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Re: Help with... DC amplification?

Post by joshuabardwell »

Thanks, Mike--that's the information I needed. In the mean time, I have been thinking about my situation, and I think the best solution is going to be to use a 5v regulated USB power supply (like this: https://www.adafruit.com/products/501). I have to buy a power supply anyway, and this will save me having to build a voltage regulator circuit to bring 9-12v down to 5v. Once I have a stable 5v supply, I can just build a basic amplifier with a spare power FET that I have laying around.

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Renate
 
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Re: Help with... DC amplification?

Post by Renate »

I'm still in the dark (so to speak) how the LEDs in the glasses are wired up.
If they are all in series, a 5V supply is not going to work.

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joshuabardwell
 
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Re: Help with... DC amplification?

Post by joshuabardwell »

Renate wrote:I'm still in the dark (so to speak) how the LEDs in the glasses are wired up.
If they are all in series, a 5V supply is not going to work.
The glasses that came with the machine seem to run on about 4v, which is output by the machine. To buy them from the manufacturer costs something like $60, which is ridiculous. I plan to build my own, and will wire up the LEDs however works best for the supply voltage I choose. That's the beauty of DIY, no?

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Renate
 
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Re: Help with... DC amplification?

Post by Renate »

I missed the part about you wiring up your own glasses.
Sure, you can wire the LEDs in parallel with resistors.
If you were going to make a bunch of these, it might make sense to knock out a little oval PCB and use surface mount chip LEDs.

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