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Handsfree enquiry

  1. Jul 23, 2005 #1
    hi . I am trying to construct a circuit to light a white led.i want to use as a switch a transistor. i intend to take the voltage required between the base and the collector of the transistor from the handsfree slot of the mobile ericsson p800. i bought the hndsfree of the mobile, cut one of the soundbox in order to measure the voltage that reaches the soundbox. I play one sound from the mobile and i try to measure with a multimeter the voltage.the measurement is zero.please give an idea.is it the wrong way i try to measure the voltage.the slot of the handsfree accepts a plug that looks like a nail.thank you
  2. jcsd
  3. Jul 24, 2005 #2


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    Staff: Mentor

    Is your multimeter set on Vdc or Vac? Even on Vac, your DMM may not have much of a bandwidth for the measurement -- it may only be good up to 60Hz, for example. It's best to use an oscilloscope to make measurements like you are trying to do, so that you can see the full character of the waveform.

    If you want to light the LED when there is sound going to the headphones, you would normally tap into that signal using an AC tap (capacitor tie), amplify that signal, rectify it, and use that larger rectified signal to turn on your LED drive transistor. You should be able to do that with just a couple of transistors, a diode, a few resistors and two capacitors plus your LED.
  4. Jul 25, 2005 #3
    thank you berkeman.is it possible to draw may be a sketch of the circuit you described?thanks
  5. Jul 25, 2005 #4


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    Staff: Mentor

    What supply voltage do you have to work with? Were you able to measure the characteristics of the headphone waveform? What visual attack and decay time do you want on the LED?
  6. Jul 26, 2005 #5
    earphone characteristics

    thank you for your reply berkeman.the problem is that i do not know much about electronics.Decay time i guess you mean the time i want the led to be turned on.The perfect situation would be the led to be turned on with a sound and then turned off with another sound.The visual attack i do not understand what it is.about the voltage i will have around 3.8V at the collector.At the base i am not sure yet as i could not measure.I hope today i will be able to measure the voltage at the hands free slot.No i could not even measure the wave form of the earphones.Whatever info i get today i will post it.thank you very much again.
  7. Jul 27, 2005 #6
    transistors enquiry

    Hi. i played a sound on the handsfree of the mobile and i was able to take a waveform.the maximum value of voltage that i get is 80 mv.

    can you explain me how do i have to connect the transistor in order to function as a switch?i tried yesterday but it did not work.the transistor must be connected with common base ?common collector?or common emitter?in order to function as a switch.

    if i want it to function as an amplifier it has to be connected as common base?

    i was thinking to use a transistor to amplify the voltage i have(even though it varies)(maybe a darlington pair of transistors?) and then drive the amplified to a second transistor to work as a switch for my led.

    thank you
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