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 Forum index » DIY Hardware and Software » Les Hall's Projects including eChucK
Microphone Jack as Power Source
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CreatorLes



Joined: Oct 05, 2014
Posts: 84
Location: San Antonio TX USA

PostPosted: Thu May 14, 2015 12:07 am    Post subject: Microphone Jack as Power Source
Subject description: Can it work?
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I would like to power low-power circuits from the microphone circuit. This would avoid all the power dance of batteries, wall sockets, solar, and USB - none of which are truly ideal. If I could just plug in a device and have it "just work" from a single cable, that would be ideal for me personally. I believe is is possible to obtain power from the mic circuit. here's how:

I have a MacBook Pro, which uses a TRRS jack where the connections from the tip to the handle are: Left, Right, Ground, Mic. The Mic connection, the one closest to the handle, takes power input through a resistor of 1k to a few k Ohms of resistance, and receives signal through a capacitor connected to the same terminal.

A circuit to do the job I'm thinking of would look the same on the input side. There would be a resistor to the power input of the chips (possibly with decoupling caps on the local side of that resistor), and a capacitor to send signal out to the microphone input.

As a bonus, the stereo headphone outputs would also be available to the circuit for whatever purposes one might have. Most notably this could be a pass-through to a pair of headphones on a TRRS jack that may also have a microphone input to go to the circuit.

I'll cook up a schematic and post it soon.

Les
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CreatorLes



Joined: Oct 05, 2014
Posts: 84
Location: San Antonio TX USA

PostPosted: Thu May 14, 2015 12:50 am    Post subject: Reply with quote  Mark this post and the followings unread

OK, I drew something up real quick-like. I'm totally new to Gimp so please pardon the crudeness of the attached schematic. Also I didn't feel like drawing this in EAGLE CAD as it's more of an illustration really. OK, to discuss it now...

On the right we have our Mac with it's TRRS jack carrying microphone and power as well as headphones out signals (and of course ground). Note that the output voltage to power the microphone is fed through a low k-Ohms resistor and the mic signal back to the Mac (whack!) is through a capacitor, blocking the DC of the power signal and effectively separating the power from the signal.

On the other side, where our device is, we do a similar thing. We have a resistor going to the terminal labelled "+" to deliver power to our circuit, and I have completed the circuit by adding a capacitor to attenuate signal going into the power of our device. This turns out to be optional in many cases and might well be avoided, I am not sure.

Also on the other side we have our return signal labeled "M" for "Microphone", which is assumed to range in voltage from ground to the "+" terminal DC voltage, whatever that turns out to be. This is way way too large a signal to send into a Mic jack because electret microphones (the types used in earbuds and the like as far as I know) produce very low signals. When I work with an electret mic I have go run it through not one but two opamp stages, each with gain of 100 or more. That's a total gain of 10,000 or more so the signal is about 5V/10,000 or half a millivolt.

So we need to attenuate the signal by 10,000 or more to send it into the microphone input to ensure that it does not swamp the input and clip like a wild beast from Borneo! That's long-winded eggsplionation is why there is a voltage divider on the output.

Then we need to inject the signal into the microphone input of the Mac, so let's use a capacitor as with the Mac side - this will do the job nicely.

Now, to get fancy with this, we want to pass-thru the headphone outputs Left and Right to another jack so we can hear the Mac's output audio and also we want to possibly use those headphone outputs in our circuit as well, wouldn't that be nice? I will cook up a circuit to do that later. For now it is enough to...

POWER OUR PROJECTS FROM THE AUDIO CABLE!!!

Yeah, that was worth shouting out! Until next time, adios!

Les


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LFLab



Joined: Dec 17, 2009
Posts: 491
Location: Rosmalen, Netherlands

PostPosted: Fri May 15, 2015 12:13 am    Post subject: Reply with quote  Mark this post and the followings unread

It's a great idea, phantom powering stuff, and being a bass player I always wonder why it hasn't caught on (eliminating batteries needed for active electronics).
But, isn't that power from the Mac meant to drive the tiny FET preamp in a microphone? Current capability only needs to be like half a milliamp or so (and that's being generous).
It would be great to do it all via USB, plenty of power!
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CreatorLes



Joined: Oct 05, 2014
Posts: 84
Location: San Antonio TX USA

PostPosted: Fri May 15, 2015 3:51 am    Post subject: Reply with quote  Mark this post and the followings unread

LFLab wrote:
It's a great idea, phantom powering stuff, and being a bass player I always wonder why it hasn't caught on (eliminating batteries needed for active electronics).
But, isn't that power from the Mac meant to drive the tiny FET preamp in a microphone? Current capability only needs to be like half a milliamp or so (and that's being generous).
It would be great to do it all via USB, plenty of power!


I totally agree with you LFLab! this is only for very small power consumption devices such as a simple lunetta or maybe one of the processors i use. That's about all!

For USB power, the DC power is like up to half an amp i believe, lower in various implementations. It is available on two of the pins, so no special circuitry is required.

Les
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LFLab



Joined: Dec 17, 2009
Posts: 491
Location: Rosmalen, Netherlands

PostPosted: Fri May 15, 2015 10:38 am    Post subject: Reply with quote  Mark this post and the followings unread

USB-1 = 1amp
USB-2 = 2amp

Very Happy
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CreatorLes



Joined: Oct 05, 2014
Posts: 84
Location: San Antonio TX USA

PostPosted: Fri May 15, 2015 11:37 am    Post subject: Reply with quote  Mark this post and the followings unread

LFLab wrote:
USB-1 = 1amp
USB-2 = 2amp

Very Happy


heh, that's "powerful" information, thanks!

Les
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