Help with an edge detector please!

Discussion in 'The Projects Forum' started by canalla, Mar 29, 2013.

  1. canalla

    Thread Starter New Member

    Mar 29, 2013
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    Hey guys, I think its finally time I admit defeat and asked for help...

    Basically I'm after a circuit that will convert a mute signal from a Parrot in-car bluetooth system into a 1 second pulse that I can feed into a pre-existing interface that converts resistive buttons on a car steering wheel into keyboard presses to be fed into a computer.

    The Parrot bluetooth system has a mute line consisting of a switched ground, which is normally disconnected and becomes grounded when it requests the stereo to mute.

    What I believe I need is some form of edge detection circuit capable of converting this mute signal (ranging in length from seconds to minutes) to a 1 second or so pulse at the beginning and end of it that can be used to mimic a keypress on a computer keyboard. (ie Play / Pause).


    Mute signal ____|¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯|______

    Req. Output ____|¯¯|________________|¯¯|___

    What I'm looking for is some assistance with the edge detector, I've scoured the internet looking for solutions and have tried recreating them but seems I'm unable to get anything that works at all! (Probably due to my total ignorance when it comes to digital circuits!)
     
    Last edited: Mar 29, 2013
  2. MrChips

    Moderator

    Oct 2, 2009
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    Use a 2-input XOR gate. Signal goes to one input. Signal goes to an inverter before going to second input. Output of the XOR gate goes to a 1-sec monostable multivibrator.
     
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  3. canalla

    Thread Starter New Member

    Mar 29, 2013
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    Ok you'll have to bear with me a little bit, like I said pleading a lot of ignorance when it comes to digital circuits etc. Think this is what you mean right? (excuse the amazing MS paint techniques).

    What IC would you recommend for the XOR and Inverter, can only seem to find a 4030 or 74HC86 Quad XOR listed.

    And for inverter again can find a 4069 or 74HC04 or 74LS04 Hex Inverters

    I've tried my best to work out the RC values for as close to 1 second as possible, is this right?

    As a slight offtopic, whats the difference between 7400 series and 4000 series, as far as I can see 7400 is strictly 5v and the 4000 series is a lot more voltage tolerant?

    circuit.png
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,449
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    Not bad. The trigger input to the 555 timer needs to be active-low.
    Take the output of the XOR gate, feed it into an inverter before connecting to pin-2 of the 555.
    74HC04, 74HC86 or 74HCT04 and 74HCT86 should work fine. For the timer I recommend TLC555 or LMC555 (both CMOS 555).

    4000 series are all CMOS. These are not compatible with 7400 or 74LS00 series.
    74C00, 74HC00 and 74HCT00 series are CMOS and are somewhat compatible with 4000 series.

    The reason for the 74xx CMOS series is to reuse the same part numbers is in the 7400 non-CMOS family.
     
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  5. canalla

    Thread Starter New Member

    Mar 29, 2013
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    Ok, thanks again for the help, it's been much appreciated! I'm sure its probably such a simple thing for you, but trying to get my head round it has taken ages!
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    Oops. I think I made a mistake. The output of the XOR is already inverted. You don't need the extra inverter after all.
     
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  7. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,805
    And you don't need to buy a separate inverter IC. If you have unused XOR gates, simply turn one of them into an inverter by tying one of the pins HI.

    How are you powering this? Keep in mind that automative electrical systems are historically a pretty harsh environment. That isn't as much the case today because of all of the electronics that are in the car stock. Still, you may need to do some filtering to get it to work reliably.
     
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  8. canalla

    Thread Starter New Member

    Mar 29, 2013
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    Thanks for the input there WBahn. Would be using a 5v line from my Carnetix P2140 power supply to provide a clean supply voltage.
     
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