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On/Off switch using MOSFET idea needed

Discussion in 'Electronic Basics' started by elfa, Nov 9, 2003.

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  1. elfa

    elfa Guest

    I'm building a standard LM386 low power amplifier for use with small portable
    radios to amplify the sound via the earphone connection. What I need is a way
    to turn on the low power amplifier JUST by using the voltage it receives from
    the input (earphone jack). The idea is that the speakers circuitry will
    automatically turn ON when the radio is turned on and turned OFF when the radio
    is turned off. Portable, amplified, speakers like this used to be made but
    apparently not anymore (I have an old one from Radio prized

    I'm thinking that a MOSFET would be the likely transistor for a job like this.
    Anyone with any idea of the best way of doing this?

    Any ideas or webpage link will be appreciated.


  2. Lord Garth

    Lord Garth Guest

    For decades, phono jacks have been made with a switch built in. These
    break the speaker connection when the earphone plug is inserted. You can
    that switch to activate your battery powered amp.
  3. And when no switching jack is available a one-transistor-amp with a reed
    relay may help.
  4. elfa

    elfa Guest

    That's not what I'm looking for. I want it turned on 'electronically' when the
    radio is turned on.

  5. Luhan Monat

    Luhan Monat Guest


    Technically, what you are describing is a 'retriggerable, one-shot,
    multivibrator' (if memory serves). You want audio to trigger a timer
    and the timer to only turn back off after a specified time with no audio.

    This envolves amplifying the audio to a sufficient level to convert it
    to a set of digital pulses that keep resetting some timer circuit. Lots
    of ways to do all this with a bit of prototyping and experimentation.
  6. Lord Garth

    Lord Garth Guest

  7. You may be able to find a schematic of such a circuit by googling for
    clap switch. Here's one.

    This one needs some work. B1 should be a wall wart. The relay should
    have a 1N4002 across the coil, cathode to positive. The R10 should be
    much lower, 2.2k or 1k. As for performance, don't expect much out of
    it. It doesn't have any bandpass filters at all. So noise at any
    freq can trigger it.

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  8. elfa

    elfa Guest

    Actually...I've already got an idea that works! In my simple test, I used 2
    transistors in an arrangement similiar to a Darlington Array. Except that Q2 is
    a MOSFET (IRF510), not a second NPN (904). The test was to input just the
    signal from the earphone jack of a radio into the base of Q1 which had a
    separate power supply and make its way from Q1's emitter into the MOSFET's gate
    and light up an LED which was connected to Drain; then, when the radio is turned
    off, the LED goes out, which would tell me the power is turned off. If it
    worked with an LED, then it could do the same with LM386.

    It works...but for one radio only (a real junker). It won't work with any other
    radio I've tried. figure out why.

  9. Luhan Monat

    Luhan Monat Guest

    elfa wrote:


    If you get steady light from the LED, you are probably getting some DC
    offset from your radio. Serendipitous solution at best.
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