Audio Optocoupler?

Discussion in 'General Electronics Chat' started by yonubear, Oct 14, 2014.

  1. yonubear

    Thread Starter Member

    Jan 10, 2013
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    ok all I hate to ask this but I need to findout what everyone thinks of Audio Optocouplers I am building a optoisolated interface for my ham radio and was hopig to isolate the audio. I have found some on ebay but I have never used optocouplers for audio so I don't even know if they are what I need or will even work. this is the only info they give Made by Silonex, Part # NLS-7053

    Thanks and Help Please,
    Yonu
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    There is nothing wrong with the concept but until we see the full details of what you are trying to achieve we cannot tell you if it will work or not.
     
  3. yonubear

    Thread Starter Member

    Jan 10, 2013
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    basically due to the size of the enclosure I am placing the circuit in I don;t have room for isolation transformers so I figures optoisolators. one line ill be the adio from the radio the other will be audio to the radio on the pc side I plan to have to pots to adjust the audio.

    Hope that helps,
    yonu
     
  4. Sensacell

    Well-Known Member

    Jun 19, 2012
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    I have seen some cool circuits that utilize dual opto couplers, the second one is included in the feedback loop to linearize the the transfer function of the whole setup.
    Basically the circuits have both LED's in series, one coupler is included in the local feedback loop and the other has identical circuitry across the isolation barrier.
    It all depends on having well matched couplers. With this scheme you should be able to use small, cheap garden variety couplers.
     
    Last edited: Oct 15, 2014
  5. crutschow

    Expert

    Mar 14, 2008
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    Below is a circuit using a single, standard opto coupler to generate a linear transfer function. The trick is that it uses a current, not a voltage drive to the opto, since the current transfer ratio of an opto coupler is fairly constant over a practical current range.

    As the simulation shows the output voltage follows the input voltage quite closely, with the distortion products being more than 30dB below the fundamental, likely more than adequate for ham audio. To remove the output DC bias if desired, just AC couple it through a capacitor.

    Linear Opto Coupler.gif
     
    Last edited: Oct 15, 2014
    ericgibbs and atferrari like this.
  6. JDT

    Well-Known Member

    Feb 12, 2009
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    If the audio quality is not that important then the transformer can be quite small.

    Find an old PC modem card (remember those?). Rob the little line transformer you will find on it.
     
  7. yonubear

    Thread Starter Member

    Jan 10, 2013
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    crutschow thanks for the diagram and the info I have somewhat of an idea of what you are saying but not totally sure.

    JDT I actually considered that as modem transformers are what I used in my current interface. However they are to large.

    To give an idea the case I am using is roughly half the length of an Altoids tin so once I squeeze in the usb ttl board, the usb audio interface, the extenal connectors, and the rx/tx pots I barly have room for the optocouplers for the ttl and ptt controls.

    Thanks,
    Yonu
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Listen to Crutschow.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    An opto coupler is fairly linear with a current drive so I used transistor Q1 with emitter resistor R1 to act as a linear voltage-to-current converter which converts the input voltage to a collector current (since the collector looks like a high impedance current source). The simulation shows this, with the collector current following the input triangle-wave voltage and the opto output voltage being a close replica of input voltage. Any significant distortion would show up as a noticeable curvature or other deviation in the output as compared to the input.

    Note that emitter resistor R1 can be adjusted in value to allow for variations in the current transfer value of the opto coupler which affects the input-output voltage gain of the circuit.
     
    Last edited: Oct 16, 2014
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    Optocouplers can be a bit slow - maybe even for audio frequencies, so worth checking that detail on the data sheet before going ahead.

    Photo diode couplers are faster than photo transistor ones.

    If all you need is speech bandwidth, you shouldn't have much trouble.
     
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Crutschow's circuit is fairly linear because it uses a standard optocoupler with an LED and a photo transistor, and both halves share the same device physics. The Silonex part has a photo resistor, not a phototransistor, and will not have nearly as good linearity. Also, photoresistors are WAY slow, probably too slow for this application. The part is intended to be used as a variable resistor like a volume control, with value changes that are much slower than the bandwidth of the signal it is controlling.

    ak
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    As you mention linearity, I've seen an appnote describing a setup using 2 optocouplers - one is the isolated transmission to the next stage, the other is included in a nfb loop with the transmitting stage. Can't remember offhand which company published it.

    You also mention photo resistors - they're the slowest of the lot, and could quite likely cramp the treble response.
     
  13. yonubear

    Thread Starter Member

    Jan 10, 2013
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    ok now I am totally lost guys

    Thanks,
    Yonu
     
  14. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Simple translation...
    Find an optocoupler based on a photodiode instead of the one you have that is based on a photoresistor. If you are in a pinch, you can use one based on phototransistor but that will only be good enough for voice audio - not violins or piccolos.
     
  15. crutschow

    Expert

    Mar 14, 2008
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    The OP stated it was for a Ham radio so it likely doesn't need to go over 5kHz for voice. But an opto with a phototransistor should be fine even for music audio if it is configured in a follower configuration, since the Miller capacitance that reduces the frequency response in a common-emitter configuration doesn't come into play. The configuration I posted should be good to above 10kHz.
     
    Last edited: Oct 17, 2014
  16. ericgibbs

    Senior Member

    Jan 29, 2010
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    hi yonubear,
    There is an linear analog opto coupler, HCNR200, see pdf.

    I do like Carl's circuit solution, using a regular opto-coupler.
     
  17. ian field

    Distinguished Member

    Oct 27, 2012
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    There have been designs published for light-beam phones that rely on modulating the current through a flashlight bulb - if that's fast enough for limited BW audio, you might even get away with an LDR.

    But there's a *LOT* of types with various response times, so always bear that in mind.

    My google searches on the RoHS compliance of cadmium-sulphide LDRs has only produced ambiguous results, I've heard of (but not seen personally) a component supplier listing LDRs as "not RoHS compliant", some suppliers have dropped them from stock altogether - I managed to score a small stash on Ebay before they start changing hands at collectibles prices.

    RoHS issues could be a problem if the project is to be offered for publication in a magazine, or as a product for sale anywhere in Europe.
     
  18. bertus

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  19. yonubear

    Thread Starter Member

    Jan 10, 2013
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    ok I am on hold while I order parts I do have a somewhat related question I am using pc817 optos to trigger the ptt and isolate the serial lines if I pull the +v from the 3.3v rail on the usb-ttl adapter to I need resistors before it enters opto?

    sorry I know that is a odd/dumb question,
    yonu
     
  20. ericgibbs

    Senior Member

    Jan 29, 2010
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    Yes, always add a series resistor with an opto [LED], check the d/s for required forward opto emitter current and its forward voltage drop.
    Subtract the Vfwd from the 3.3V and the then Rser= (3.3v- Vfwd)/ mA
    E
     
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