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What can I put on a cable to just draw power from a headset jack of a mobile device?

  1. Nov 12, 2009 #1
    First of all, I'm not an electrical engineer. I'm just a software developer who has written an application to do push-to-talk over wireless LANs on Windows Mobile devices, mostly for warehouse environments. We've got a prospect right now who wants to run our software on some forklift terminals, but we've run into a problem that I'm hoping someone here can help me answer. And here it is...

    Because the internal speaker of the forklift terminal (which is running Windows Mobile) is not loud enough, we need to connect external speakers to the terminal's headset jack. We have done this in other mobile devices before with no problem. You just plug the speakers into the device's headset jack, and the device detects this and starts automatically routing its audio to the headset jack instead of to the internal speaker.

    But with this particular device, it is apparently looking for more than just the physical presence of a plug being inserted into its headset jack. When we plug passive, non-powered speakers into it, the device recognizes them and works properly, but because the speakers are not powered the audio is still not loud enough. However, when we try powered speakers (currently just plugged into a wall outlet, eventually they'll be powered by tapping into the electrical of the forklift), the device does not notice them and therefore does not switch the audio from the internal speaker to the headset jack where the powered speakers are plugged in.

    So, we tried plugging a little RadioShack splitter into the headset jack, then connected the passive speakers into one of the ports in the splitter, and the powered speakers into the other port, and bingo! The audio routes out the headset jack to both sets of speakers (the powered and the non-powered), and the audio in the powered speakers is nice and loud. So my question is, what can we do in terms of a cable that would simulate these passive speakers? Ultimately we want to have a cable that connects to the externally powered speakers, but also draws power like the passive speakers do (I think). The passive speakers that I'm referring to, in case that matters, are Sony SRS-P11Q's. The powered speakers don't really matter. We have tried many different ones, and they all behave the same way. They work only when plugged into a splitter where the passive speakers are plugged in too.

    My guess is that the passive speakers are actually drawing some power from the headset jack, but the powered speakers, since they are getting power from an external source, are not trying to draw any power, and the mobile device is actually not noticing them unless they try to draw some power. So, assuming that's the case, we want to come up with something we can attach to a cable that would make it draw power, ideally just as much power as these passive speakers are drawing, so we can simulate what they are currently doing. What is the best way to do that?

    Thanks for any help you can provide. And if you have any specific followup questions, ask away!!

    Thanks,

    Brad
     
    Last edited: Nov 12, 2009
  2. jcsd
  3. Nov 12, 2009 #2

    mgb_phys

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    Re: What can I put on a cable to just draw power from a headset jack of a mobile devi

    My guess is that it detects resistance of the unpowered speakers or headphones (typicaly about 32ohms) while the powered ones have an amplifier which has a input impedance of a few mega-ohms.

    A 20-30K resistor on the splitter output should be enough.
     
  4. Nov 12, 2009 #3
    Re: What can I put on a cable to just draw power from a headset jack of a mobile devi

    Wow, thanks for the fast reply sir! I will give that a try. I've also noticed by trying to put a few other things on the "passive speaker" side that I seem to be able to affect the volume going out to the powered speakers. So, what would be the affect of putting a 10K resistor vs. say a 50K resistor or something like that? I would like to see if we can get the powered speakers as loud as possible, but I also don't want to damage anything of course. I'm sorry if that's a naive question, but I really don't have a clue about this stuff. It's a world I've never really dealt with, but I've got to admit I am finding it very interesting!
     
    Last edited: Nov 12, 2009
  5. Nov 12, 2009 #4

    mgb_phys

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    Re: What can I put on a cable to just draw power from a headset jack of a mobile devi

    Putting anything higher impedance than the headphones won't do any harm because that takes less current.
    The input to a powered (amplified) speaker is probably at least a few 100K so there is no difference between a 50k resistor, powered speaker and nothing.

    Note if the socket is stereo you probably want the same resistance between each channel and the common ground - the amp might be sensing on either of them.
     
  6. Nov 12, 2009 #5
    Re: What can I put on a cable to just draw power from a headset jack of a mobile devi

    It sounds then like I don't even need the splitter to do this, correct? Could I just make a cable that has a 20-30K resister in it, where the cable plugs into the powered speakers? Are you saying that might simulate my current setup (as far as what the mobile device is looking for in order to send audio out the headset jack)?
     
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