Switching Audio with 5.2 volt PTT line and ADG436BNZ-ND

Discussion in 'The Projects Forum' started by 314159265, Jul 23, 2009.

  1. 314159265

    Thread Starter Member

    May 29, 2009
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    I had a previous post on this project (#156850), but the switch has changed, so I'm hoping the new title might attract someone who is familiar with the analog switch that I ended up using.

    I've tested everything I can think of - there is continuity between the connections, and nothing is shorted out. However, the only output is static. I'm not even really sure where to begin troubleshooting this one.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    There is no schematic there.
    Just a bunch of text.

    Why not keep the web-mail box free??
     
  3. 314159265

    Thread Starter Member

    May 29, 2009
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    I've attached a hand-drawn schematic. I didn't know how to make an electronic copy, so hopefully this will be good enough. I've also got the datasheet in a PDF file that I can attach if anyone can't access it online.

    Here are the connections as I envisioned them working.

    If this switch doesn't work, do they make a DPDT relay that can be tripped by the 5 volt PTT line? I've found a few, but most of those have high resistance (60+ Ω) which would probably drop the volume to a level where it would be useless.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    Your schematic makes no sense.
    It does not show the part number for the IC.
    It does not say what kind of microphone.
    Resistor values are not shown.
     
  5. 314159265

    Thread Starter Member

    May 29, 2009
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    The part number for the IC is ADG436BNZ-ND. I assumed incorrectly that it was enough to put it in the title of the post.

    The microphone is a Radioshack #270-101 Microphone element. It operates on 4-10 volts DC.

    The resistor value for the important resistor is shown (2.2 kΩ). The resistor coming from the speaker has an as-of-now undetermined value. It will be whatever value needed to roughly equalize the output from the speaker with the output from the microphone.

    I'm not an electrical engineer. I'm a liberal arts student working on an electronics project. I've seen some schematics, but I don't know how to draw up a proper one. I came to this forum hoping to find some experts that would be willing to help me out with this.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    The analog switch IC is not made for switching a speaker. The high speaker current will destroy it.

    A resistor in series with a speaker does not equalize it, it just makes it sound faint.

    The mic is not powered. The IC is also not powered.

    A mic preamp circuit and a power amp circuit are needed.
     
  7. flat5

    Active Member

    Nov 13, 2008
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    First off, the mic is not powered correctly.
    No DC is getting to the mic because the 4.7uf is in the wrong place.
    Also, change the 2.2k to 10k.
    Here is an idea of how to power the mic.
    http://www.epanorama.net/circuits/microphone_powering.html#basics
    I just don't like the 2.2k resistor. I'd use at least 6.8k

    (note: a PM was sent to me asking for my help. so I'm trying)
     
  8. 314159265

    Thread Starter Member

    May 29, 2009
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    Fortunately, I haven't connected the speaker yet, because I was planning on using some trial and error to match the volume of the mic and the speaker. If the resistor does not reduce the current, what do I need to do to equalize the microphone and the speaker?

    By not powered, do you mean that the IC should not be powered, or is not powered correctly? Since I drew and scanned the schematic I posted, I did add +5 volts to the VDD input, and connected VSS and GND between each other and the negative for the 5 volt input.

    The output from this project is going into a computer. I've used the microphone in a standalone setup, and found the volume to be sufficient. Is the preamp needed because of the switch? Could the switch be reducing the volume of the audio to the point where the computer can't hear it?

    I think I understand this now. The capacitor is currently between the microphone input and the power input/audio output. However, it should be the last stage the output goes through before reaching the switch. Is that correct?

    That would explain why the microphone was working in a standalone setup, but didn't work when I added in the switch.

    General Thoughts
    It's looking like I'm going to have to resolder a lot of the project, so it might be a little while before I can get this back together to test.

    Do you think I would be better off using a DPDT relay that operates on the 5 volt PTT input? I understand how those work better than the switch. Would it have a negative effect on the audio? Would it handle the speaker better? The reason I didn't use on initially was because I couldn't find one that operated n 5 volts. Now, I've found several that might work, but none are available locally.

    I would prefer to use a solid state component (and it's always fun learning about new things) but realistically an electromechanical relay would last for at least a few years before it wears out. It would also be powered directly off the PTT line, negating the need for me to add additional power beyond that for the microphone.

    Thanks for the replies!
     
  9. flat5

    Active Member

    Nov 13, 2008
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    When I looked at your schematic the first thing I thought was to just use a relay. Try to draw what you want to do using a relay.
    It can be a very low power type.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    Your sketch showed an unpowered microphone connected to an unpowered analog switch IC which was connected to a speaker.
    No computer's sound card with its amplifiers was shown.

    If you power the mic correctly then it will work.
    If you power the switching IC correctly then it will also work.
    If you feed the mic into a preamp (maybe on the sound card) then feed the preamp into a power amp (I don't know if your sound card has a power amplifier) then a speaker will work.
     
  11. 314159265

    Thread Starter Member

    May 29, 2009
    13
    0
    One of the advantages of a relay would be very low resistance in the microphone lines, and might not even need a preamp. It could also handle the more powerful speaker output, allowing the equalization to be done in the software, rather than with the hardware. I'm going to try fixing the capacitor, since it would take a week or so to get a 5 volt relay without paying a big penalty in shipping fees.

    I've attached a diagram of how the relay would be used. I didn't show power to the microphone on that one - just assume that the mic would be powered. I was just sketching it out fast so I could get it posted. The goal is to convey the general concept.

    The speakers are actually the speaker output from the radio. That is what is getting switched with the microphone. During normal operation, the speaker output is routed to the interface output, which goes to the sound card. However, when the 5 volt PTT line drops to 0 volts, and the radio starts transmitting, the audio from the microphone is connected to the computer sound card. It's not actually going to any speakers; it's going into a recording system.

    Here's an updated sketch with everything except the preamp. How should I go about putting a preamp in? I found this option online. Does this one look like it will work? I'm not going to be able to fit some of the larger preamps on the breadboard, so I'd need to resolder everything. The sound card can amplify the mic enough when it is connected directly; however, I have not tested it through the 12 Ω resistance the switch provides.

    I read somewhere that the microphone has a built in preamp. Will there be any issues adding a second one to the circuit, or will it just counteract the switch?
     
  12. flat5

    Active Member

    Nov 13, 2008
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    On the relay pdf I think you have the ptt backwards. The nc & no.
     
  13. 314159265

    Thread Starter Member

    May 29, 2009
    13
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    That's what I thought initially, but if you look at the voltages, I think I've got the connections right. Normally, the PTT voltage is 5.2 volts when receiving. When transmitting, the voltage drops to 0 volts, since PTT is shorted to ground. If the relay is normally closed at 0 volts, the normally open side would spend more time closed, since most of the time the radio would be receiving and there would be 5 volts on the PTT line. Does that make any sense?
     
  14. flat5

    Active Member

    Nov 13, 2008
    403
    17
    No.

    You don't show a ptt switch. I see "5volts" connected across the relay coil.
    Then you say the ptt is shorted to ground to open the relay.
    You mean the 5v is shorted to ground? That would be the worst possible design :)

    Edit: If you can provide a more complete schematic we can get a better picture of the project.
     
    Last edited: Jul 31, 2009
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