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Help please ? - Really simple LED circuit

Discussion in 'Electronic Design' started by [email protected], Jan 17, 2006.

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  1. Guest

    Hi, I was wondering if anyone here could help me with designing a
    really simple circuit that would illuminate an LED when a signal was
    present on an audio input.

    Basically, I'm trying to create a "visual metronome" that I can feed
    with regular pulses from a minidisc / CD player. Ideally, the LED
    would remain lit as long as the signal (sine wave, anything really) was
    present and extinguish when the signal disappeared - I could then
    control the length of illumination by varying the length of the
    recorded audio pulses.

    Is this as simple electronics-wise as it is seems (to me anyway !) in
    principal ?

    Any help would be very gratefully received.



    Background Info - I'm a guitarist in a small band and certain songs we
    do could benefit with additional accompaniment (a bit of keyboards
    etc). I can record these parts myself onto MD/CD, but need to make
    sure our human (just !) drummer keeps in sync with the recorded
    accompaniment (which would have long gaps of silence). I could record
    the accompaniment on the Left channel and a 'click track' of audio
    pulses on the Right channel. My idea is then to feed this "visual
    metronome" with this signal, and hey-presto, the drummer (and therefore
    the rest of us !) can keep in sync with the accompaniment.
  2. Mochuelo

    Mochuelo Guest

    Yes, the circuit would be very simple, but I would recommend audio
    instead of light. This way, the drummer would not have to constantly
    look at one point, and with sound it would be easier to distinguish
    normal beats from those that start the beginning of a pattern (you
    could just use different frequencies). The drummer would wear a small
    earphone, or plain headphones (so that s/he could also listen to
    another audio track, on the other channel).

    If you want to go with the LED, you need a small amplifier (using an
    operational amplifier) plus an envelope detector (diode + R + C), or
    even just the amplifier alone.

  3. Derek Potter

    Derek Potter Guest

    node name components

    IN input resistor A 100R
    input signal

    base input resistor B
    NPN base
    bias resistor A 1K *
    divider resistor A 100R **

    GND input ground
    NPN emitter
    divider resistor B **

    collector NPN collector
    LED +ve

    LED-ve LED -ve
    LED resistor A 100R

    supply + +5V
    LED resistor B
    bias resistor B

    * nominal adjust to taste
    ** divider resistor only needed if input is AC coupled.
  4. Deefoo

    Deefoo Guest

    I used to play in a group that used backing tapes (stereo cassettes). I was
    the drummer and I made half of the backing tapes. On my tapes I used one
    track for metronome while the other contained the music. On the other tapes
    we had the music on both tracks. I then played along with headphones... It
    had to be pretty loud though.

  5. ehsjr

    ehsjr Guest

    Something like this should do it.
    Adjust the pot so the LED lights at the right times.

    +12 -----------+-------+------+------+
    | | | |
    | | | [470R]
    | | | |
    [1M] | | [LED]
    | | |\4 |
    Audio ---||---+-------|---2|-\ |
    .1uf | / | }1----+
    | 1M \<--3|+/ LM324
    [1M] / |/11
    | | |
    Gnd -----------+-------+------+

    Adapted from a post by John Fields - his stuff is always good.
    Modified a bit for this application.

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