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Thread: CB Radio Effects Unit. Can anyone help with a wiring puzzle?

  1. #1

    CB Radio Effects Unit. Can anyone help with a wiring puzzle?

    Hi Teensyers,

    Iím working on a maker project for a friend, but am having a hard time figuring out some technical details. If anyone has any ideas on how to solve this problem Iíd be super grateful.

    The concept:
    A CB radio effects box which uses a teensy and audio shield to add delay or reverb to the userís voice. My goal is to make a standalone box which you could plug your CB mic into and then plug the output of the box into the CB radioóbasically like a guitar effects pedal. Microphone->Effects Unit->CB Radio. (See the concept diagram below.)
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    The problem:
    While I thought this would be a really simple project, Iím stuck on how to wire the CB mic so that the transmit/receive button on the mic still works as normal. Iíve tried a couple ideas, but Iím just not sure how to wire this project so that

    1. There are no loud pops when you push in the mic button
    2. The delay trail or effects from the Teensy stop transmitting after the mic button is disengaged

    My current wiring (see the Wiring Diagram below) isnít working very well. First off, there is a loud pop every time I push in the mic. (Iím assuming thatís because the ground of the mic input is connect directly to the ground of the line output?). Also, once I let the button back out, the signal from the Teensy still transmits. So for example if I have delay on my voice, it continues to transmit the delay echos.

    Click image for larger version. 

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    Also, here's a diagram of how a CB transmit/receive button works and how itís wired.

    Click image for larger version. 

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    Any ideas on how I might be able to wire this differently?

    Thanks so much for any ideas!

  2. #2
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    Man, did we have fun with a tape loop and a CB radio - 40 something years ago.

    A few questions:-

    What kind of CB, AM, SSB, FM whatever.

    Do you have it's schematic to explore whats on the inside of the Mic socket?

    What kind of Mic is it? If Dynamic, no need for Mic bias.

    From memory, some CB mic PTT switches will short Mic in receive mode so might be where the plop comes from. Suggested remedy would be to remove the 2.2K mic bias resistor from the audio board.

  3. #3
    Hey, thanks for the response!

    Yeah, apparently they used to put analog delay in old CBs back in the day as a kind of identifier for your voice.

    The CB is a Cobra 29, with a 4 pin aviation socket. Here's a pic and a schematic for that mic, which is dynamic.

    Click image for larger version. 

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    Click image for larger version. 

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    Could you say a little more about mic bias? I'm not sure I totally understand.

    Thanks again!

  4. #4
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    Yeah, CB was a busy marketplace for all kinds of gimmicks although some delay or reverb can greatly assist digging signals out of the noise floor at the receive end especially with SSB.
    Still have two of those radios, one was heavily hacked, both still work.

    On with the show:-
    Looking at the Electret mic and the SGTL5000.
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    The Electret has an internal FET buffer and needs some V+, provided by the SGTL via the 2.2K acting as a pullup. The Oscilloscribble depicts what you might see on point X. The audio floats on the DC potential of VMICBIAS, provided by the SGTL5000.

    Strippping it back to basics, here we have a dynamic mic and the relevant part of a PTT switch.
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    Point X is at +v MICBIAS so when the switch is closed, X gets pulled to ground by the low resistance of the mic, and back to +v MICBIAS when released.

    A 'scope, upper trace on point X and lower trace on pin 15 SGTL night look something like this:-
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    The negative and positive- going spikes asociated with press and relase arise from discharging and charging the .1 uF cap and are likely to be of much higher amplitude than the desired audio signal so I would expect them to sound pretty big.

    Let's think of audio levels. A 600 Ohm dynamic mic won't give you much more than a few tens of millivolts unlike an electret and we need to present a reasonable signal to the SGTL which can be pretty darn noisy if you have any ground loop isues. In the next schematic we have added a 600 ohm - 10K mic transformer which by it's nature will give us some more level to play with as well as avoiding one possible ground loop between the Teensy and the radio.The 2.2K needs to be removed so as to not load the output of the transformer, we want every darn millivolt...
    Click image for larger version. 

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    Now, while we're thinking of audio levels and ground loops, another transformer between the SGTL's line out and the CB's Mic in is worthy of mention, the radio expects to see a 600 ohm mic at dynamic mic levels so the transformer does the impedance match and will drop the level somewhat but you may need to decrease it further with a voltage divider betwen the SGTL's line Out and the output transformer. We also eliminate another possible ground loop here. Hope all this makes some sense.

    1970's planet CB *social media* was hugely entertaining. Hooked up guitar multi-effects, yeah , waa-waa, fuzz, flanger whatever so all "on - air" setting up/testing was dumped into a 50 ohm dummy load, monitoring with another receiver.
    Last edited by MatrixRat; 01-29-2021 at 02:56 AM. Reason: typo

  5. #5
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    Another thought, on the road and staring me in the face is the Mic for a Uniden UH-012 so I took it apart. Has an electret mic and DPDT PTT switch.

    Might be a simpler way of providing a mic that the Teensy Audio board is designed for.

    Another thought emerged from the distant past. Some CB radios need to have the mic disconnected in receive mode (as the Cobra mic does) otherwise wierd stuff happens, so this disconnection likely would need to happen between the Audio Board output and the Cobra.

    Vaguely recall adding a miniature DPDT relay to the internals of the Cobra to sort that out.
    The actual radio we did it to is long gone but I have it's schematic minus the additions however it won't take much to figure out.

  6. #6
    Thanks so much for all the info! I think you're right, that maybe the way to go is building the circuit into the actual mic instead of a standalone unit. I'm having a hard time getting my head around all the implications of putting the teensy in a different box.

  7. #7
    "1970's planet CB *social media* was hugely entertaining. Hooked up guitar multi-effects, yeah , waa-waa, fuzz, flanger whatever so all "on - air" setting up/testing was dumped into a 50 ohm dummy load, monitoring with another receiver."

    That's hilarious. Maybe I can find some new version of this and see if folks are doing anything like it.

  8. #8
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    Another thought on the way back to civilization.

    How about an electret handheld mic and read the PTT switch state with a digitalInput pin and use two NPN low-side switches driven by the Teensy to do the PTT stuff for the Cobra and mute the audio board's output in code?

  9. #9
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    Just been exploring the Cobra schematic and the NPN low-side switch will not work for Pin 4 as the PTT switch more or less pulls the bottom end of the radio's speaker to ground for receive so is switching an AC signal. A relay driven by the teensy may be a better solution.

  10. #10
    Sorry for the delayed response.

    I think that a relay (or pair of relays) is the only way I can conceptualize getting the PTT to work. Thanks for that idea. I'm not really well-versed in analog electronics, so I'll have to do some experimenting, but conceptually it seems doable.

    Thanks again for brainstorming with me! It's been really helpful.

  11. #11
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    No problem. Not mentioned yet the possibility of ground-loop induced noise considering the low level audio being sent to the radio and how the Teensy is powered. An isolated 12-5v reg may be in order and as for the relay, I'd think seriously of using a 12v item with optocoupler isolation from Teensy digital out pin.

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