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Light sensitive relay switch alternative?

Discussion in 'Electronic Design' started by Andy Turner, Jul 24, 2005.

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  1. Andy Turner

    Andy Turner Guest

    Hi, a few months back I posted about a crackpot plan I had to install
    a kind of laser harp in my house by shining a dozen lasers into light
    sensitive switches, which in turn were connected to the switches on a
    cheap MIDI keyboard (non-velocity sensitive), to provide the MIDI out,
    and then on to a decent sound module. You guys gave me some good
    advice back then, but I've only just got to the point of trying it.

    I bought these two items:

    Then I soldered it all together and tried it. I'm shining the laser
    onto the LDR from over a metre away and... it works!

    Well sorta.. the problem is with the switch implementing a "delay
    circuit to avoid cycling", which I guess I should have spotted when
    ordering it.. So this means that it's simply not fast enough for the
    application. I presume the delay is implemented in the chip so there'd
    be nothing I could do about removing it? Or does the capacitor do the
    delay (smaller capacitor, smaller delay?). Alternatively, is there a
    different switch I could buy that doesn't have this feature? Ideally
    it could do with it coming complete, I didn't realise the switch board
    would be a kit! It's OK doing one, but doing another 11 would be


  2. Yes, make the capacitor at + input of opamp smaller
    for a smaller delay.
  3. Andy Turner

    Andy Turner Guest

    Cheers! I've noticed that it takes longer to switch one way than back
    again. Also, I notice that there's two capacitors, one 100uf and one
    470uf. Is this implementing the two different delays? What would
    happen if I took the capacitors off and just wired direct? Would that
    give me no delay or would it stop working or blow something?

    I apologise for not knowing too much about what must be GCSE level
    electronics. I do very much regret opting for a different subject at

  4. Andy Turner

    Andy Turner Guest

    I've just noticed that my problem is almost the exact opposite of the
    one that Nikki has in the post "Time delay using relays". Spooky! And
    that seems to be a similarly crackpot scheme too!

  5. Fred Bloggs

    Fred Bloggs Guest

    Well it sure as hell wasn't accounting- you're using about $350 worth of
    needless stuff....
  6. Andy Turner

    Andy Turner Guest

    heh, well I obviously don't know how to design it more efficiently. If
    you've got suggestions, I'm all ears!

  7. Cheers! I've noticed that it takes longer to switch one way than back
    again. Also, I notice that there's two capacitors, one 100uf and one
    470uf. Is this implementing the two different delays? What would
    happen if I took the capacitors off and just wired direct? Would that
    give me no delay or would it stop working or blow something?[/QUOTE]


    The other capacitors, according to circuit diagram at
    the webpage selling the circuit, are just for powersupply.
    Schools are just for realizing that by reading books
    you can find out ANY information. For learning electronics,
    find a book on electronics, read it and start doing stuff.
    You can test circuits with free switcher cad 3 download:
    Most simple electronics, like this circuit, are very easy
    to design - it is just resistive voltage dividers and basic
    stuff like that.

    One resistive divider, 100k potentiometer, gives the
    comparator switch level to - input.

    Another resistive divider 150k, VDR, 15k gives the
    other voltage. There is, in addition, a "delay" circuit
    by the 17k and 180k resistors, diode and 100uF capasitor.

    When LDR has high resistance (dark), voltage is high and the
    100uF capacitor charges slowly to this high voltage (and some
    current goes through 100k but to simplify forget this now).
    When LDR resistance drops (light), diode prevents the lower
    voltage from charging the capasitor, and capasitor discharges
    through the 100k resistor until eventually voltage is
    lower than the voltage set by the 100k potentiometer, and
    the state of the relay changes.

    Now, when LDR again has high resistance (dark), the 100uF
    cap is starting to charge and voltage eventually becomes
    higher than the voltage from 100k pot, and the state of
    relay changes again.

    The opamp (the triangle part with inputs marked "-" and
    "+" is a comparator - it compares whether - or + input
    has a higher voltage, and changes output accordingly.

    LDR = light dependant resistor

    I hope this helps you understand how the circuit works.
    You can enter the circuit to switcher cad 3, and replace
    LDR with a switch.. See how it works!

    To answer your original question: Only the 100uF capacitor
    matters for the delay. Take it away and you have no delay.
    I'd suggest setting a little delay. Simulate the circuit
    with Switcher cad 3, and find out correct size for cap!!
  8. Fred Bloggs

    Fred Bloggs Guest

    How is this "laser harp" supposed to work activation-wise? Do you play
    it or does wind blow through it or something else? How do you simulate
    pressing a key on the MIDI kbd? Is the key grounded or how is that working?
  9. Andy Turner

    Andy Turner Guest

    It's just light triggers, so there's a bunch of beams shining above,
    down to a row of light sensitive switches and as soon as your hand
    breaks the beam and stops the laser reaching the LDR, it triggers the
    key. When you move your hand to allow the beam through, the key is

    That I haven't tried yet (today's experiment was simply wired to a
    multimeter set to buzz on connection). The idea is to buy a very
    simple MIDI controller keyboard (one that isn't velocity sensitive),
    which hopefully will have simple contact switches for key down/up.
    I'll hook each relay into a key on the keyboard. The keyboard will
    then think the hay has been pressed or released and send out the
    appropriate MIDI signal to a sound module.

    This is perhaps the design of someone who doesn't actually know
    electronics hence the use of an actual keyboard when perhaps some
    latch/MIDI convertor board would do..! However, I figure a keyboard
    will give me octave shifting (small keyboards often feature this),
    which might be useful when I set the sound module to a drum machine
    (which usually starts at key C1), or to a piano, where I would want a
    higher octave.

  10. Andy Turner

    Andy Turner Guest

    Ah! Ok, cheers.

    OK, I'm gonna play with that. I do want to learn this stuff and with
    an actual project to push me into learning it, it should be the
    motivation I need.

    Thanks for that. I kinda understand it, but only in that way where you
    understand what someone's saying, but you couldn't write it yourself
    (if you see what I mean!). I'll play with that software and perhaps
    figure out for myself what changes I want to make. At least any
    mistakes won't result in real damage or mishaps!

    I'll do that, thanks. And I appreciate your help with the bigger
    picture, rather than just telling me what to do, but helping me figure
    it out for myself. Teach a man to fish and all that...

    Thanks again.

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