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Help with Intercom Circuit

Discussion in 'General Electronics Discussion' started by RobHedley, Nov 9, 2014.

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  1. RobHedley

    RobHedley

    3
    0
    Nov 9, 2014
    Hi,

    Firstly apologies for jumping straight in with my first post with a request for help.

    I am currently building a Race Driving Simulator (Build Thread ). I realised quite quickly that I needed an intercom from the office to the sim as when it is running you can't give any instructions to the driver without screaming your head off!!

    Now, I have to say that my electronic knowledge is limited to a GCSE in Electronics from a long time ago and just learning what I needed to complete task for RC Cars etc With that in mind, I spoke to my good friend google and found a circuit online for a high quality intercom (Here). I then taught myself how to use CadSoft Eagle and designed the circuit schematic.

    The question I have is on the circuit from the link above, the microphone is a triple pin Electret where there is Output, Ground and power. The Microphones I hope to be using (in the headsets and my Race Helmet) only have an Output and Ground. I read a few sites where they are feeding the power into the same pin as the output. I have amended my circuit to reflect this and would like too ask for you guys to tell me if I am being an idiot. The section I am talking about is the far left of the schematic.
    [​IMG]
    Schematic I have created in Eagle
    IntercomCircuit.png (17.7 KiB) Viewed 7 times


    Any advice is greatly appreciated and also if anyone has time, as sanity check on the rest of the circuit (Be Kind). I am more than happy to let anyone that help to come and have a blast on the Sim if they can get down to just south of Guildford (In England).

    Regards

    Rob
     
  2. KJ6EAD

    KJ6EAD

    1,114
    157
    Aug 13, 2011
    Well, you have no power shown. I imagine that could lead to some disappointment were you to build it as shown. :oops:
     
  3. hevans1944

    hevans1944 Hop - AC8NS

    4,534
    2,110
    Jun 21, 2012
    Looks okay to me. I assume DC power comes in through SL1. Not sure what the purpose of R3, 560 Ω resistor, is and why the collector of T1 doesn't have a bypass (filter) capacitor.
     
  4. RobHedley

    RobHedley

    3
    0
    Nov 9, 2014
    :oops:Valid response KJ6EAD - I forgot to mention that the power will be coming in via the connector block SL1 (12v 1A)

    Hi Hevans1944, Yes the power will be coming in via SL1 (12v 1A). The position of R3 is taken directly from the design of the original so can't answer that one. What would be your recommendation with regard to a bypass capacitor for T1?

    Thanks for the responses guys.
     
  5. hevans1944

    hevans1944 Hop - AC8NS

    4,534
    2,110
    Jun 21, 2012
    10 to 20 μF aluminum electrolytic rated for at least 16 WVDC, if needed, would be okay. Positive terminal to collector of T2, negative terminal to ground.

    On electret microphones: some (most) of these have a FET inside the case to convert the very high impedance of the electret element to a more reasonable impedance, on the order of a few thousand ohms. The current requirements are modest, a milliampere or less, and the voltage is quite small, only one to three volts. This is not an electret poling potential, it is only to supply current to the FET. Your capacitor, C2, connected to the base of T1 will block this DC "bias" while passing the audio.

    Do you have a part number for the electret microphone element? There are lots of datasheets available at DigiKey, for example this one.

    BTW, awesome SIM build you have going on there!
     
    Last edited: Nov 10, 2014
  6. RobHedley

    RobHedley

    3
    0
    Nov 9, 2014
    Hi Hevans1944,

    Thanks for your reply. Yes the simulator has turned in a labour of love and also saves me £1600 a day from my race budget which is what I was paying.

    I have just spoken to the supplier of the helmet kit about the specification of the Microphone. Unfortunately it look I need to start again with the design as it is a 150 Ohm 50mv Dynamic Microphone not an Electret as I had originally thought. Is there anyway I can use the majority of the designed circuit and amend to handle a dynamic mic i.e no power. Would placing a Capacitor in series with the microphone line?

    There was mention, by the engineer that I spoke to, of needing a compression circuit, I have found one that is designed as a single knob system that is designed for a rally intercom.

    CompressionCircuit.gif

    You can't see it from the image but it is based on the THAT 4301P. Would it be a case of simply inserting this between the mic pre-amp and the amp circuit?

    Once again thanks for all your help.

    Rob
     
    Last edited: Nov 10, 2014
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