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This is a single-wire earth-return telephone system used in cave rescue communications.

schematic

simulate this circuit – Schematic created using CircuitLab

The ground is a metal casing to make contact with the users hand. Line is clipped to an exposed section of a wire that runs through the cave.

The circuit is shown in receive mode, PTT switches to transmit, the speaker serves as a microphone and speaker. All systems I've seen use a Rocking Armature Transducer for the speaker. There is no base station or similar - you can just connect two or more of these to the same wire and use them.

My electronics knowledge is not good enough to fully understand this. Here's what (I think) I do understand.

  • C3 and C4 are audio coupling (block direct current flow.)
  • R1 and R2 hold + of the op-amp at 4.5v.
  • Earth-return is a misnomer - it actually works off the capacitance of your body.

When receiving

  • R3 is shorted and therefore does absolutely nothing.
  • The op-amp is working as a buffer and does not amplify the input.
  • I'm guessing C1, C2 and R4 are serving to filter the input - perhaps suppress high frequency noise.

When transmitting (this is where I really get lost)

  • C2 is now shorted and does nothing.
  • What's R3 doing - is the op-amp now amplifying?
  • Are C1 and R4 still performing some function?

Am I right so far? Can someone please help me understand the rest?

My goal is to be able to alter the circuit to try out different speaker setups.

Thanks

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  • \$\begingroup\$ Could this get a somewhat more descriptive title? \$\endgroup\$
    – user17592
    Commented May 14, 2013 at 19:29

1 Answer 1

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Maybe look at it like this: - enter image description here

I've removed components that get shorted or open circuited by the PTT switch contacts

In transmit mode, the speaker becomes the mic and the op-amp has a large gain determined by R3 (100k) and the small impedance of the speaker. The Rocking Armature's impedance is 150 ohm so for a rough estimate, the gain is 100,000 ÷ 150 = 666. This means that a 10mVp-p signal on your "mike" gets amplified to 6.67Vp-p.

In transmit mode R4 is superfluous but I left it in because it wasn't exactly open circuited or short-circuited by the PTT switch. C1 in transmit mode is just stabilizing the non-inverting input of the amp.

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  • \$\begingroup\$ Hmm, where's my image/picture gone? There is meant to be a picture in this answer, instead I see a red X and the words "enter image description here". Can anyone else see this picture. Appreciate any feedback \$\endgroup\$
    – Andy aka
    Commented May 14, 2013 at 18:01
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    \$\begingroup\$ I see the picture. \$\endgroup\$
    – DrFriedParts
    Commented May 14, 2013 at 19:26
  • \$\begingroup\$ @DrFried: I don't see the picture, only a placeholder that says "enter image description here". \$\endgroup\$
    – Olin Lathrop
    Commented May 14, 2013 at 19:32
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    \$\begingroup\$ I can see the images just fine. \$\endgroup\$
    – Adam Lawrence
    Commented May 14, 2013 at 19:55
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    \$\begingroup\$ @Jake R4 only has a role in receive mode. It's used to limit energy into the op-amp and provide high frequency attenuation because, with the unit requiring body capacitance to work, there will be a tendency for high frequency audio to be received/transmitted better than low frequency so, it works in conjunction with C2 to attenuate HF stuff and kind-of linearize the audio spectrum. \$\endgroup\$
    – Andy aka
    Commented May 14, 2013 at 21:45

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