3 switched lights with only 3 wires

Discussion in 'The Projects Forum' started by Zimerhonzl, Dec 20, 2012.

  1. Zimerhonzl

    Thread Starter New Member

    Dec 20, 2012
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    I'm working on a three color signal cue light which can be operated remotely. To take advantage of commonly installed wires I am limited to 3 conductors between the control and the signal box. I've attached a graphic of what I could come up with using relays, but I'd much rather use transistors as they are much smaller and more reliable than a mechanical relay.

    Unfortunately CircuitLab doesn't have a SP3T switch so for the graphic I have used two SPDT. If there is a much simpler way to do this circuit I am welcome to any suggestions, though the completed solution will need to work without a circuit board.
     
  2. gerty

    AAC Fanatic!

    Aug 30, 2007
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    The relay coils are in series with the leds and resistors, won't work.
    What are the diodes for?
    Is there a common ground between leds and switches/source?
     
  3. Zimerhonzl

    Thread Starter New Member

    Dec 20, 2012
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    When I tested it on CircuitLab, everything worked out. I adjusted the resistors to account for the resistance of the relay coil, but as I mentioned I'm hoping to replace the relays with transistors but don't understand transistors well enough. The diodes are for the visual signal, which is the main part of the cuing system. The whole system is battery powered, so technically there is no ground.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    What are the relays for?

    What's D10 and D11 for?

    How come 4 wires are OK when you state you are limited to 3 wires?

    Try Goggling "rhetorical question" then look up "Charlieplexing" if your 3 wire requirement really does need to be met.
     
  5. Zimerhonzl

    Thread Starter New Member

    Dec 20, 2012
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    The sarcasm is unappreciated, I was hoping for a little help from this forum. Thanks at least for the 'charlieplexing' lead. I have no knowledge of microcontrollers, so it leads to a much more complex circuit than I had hoped.
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Ugh... replace "microcontroller pin" with "wire" and read that material again, and try try try to understand that you actually did get a little help from this forum, as with charlieplexing you can control up to 6 lights with 3 wires without a friggin' micro.

    It's just we're not gonna do the work for you. We'll show you where answers are then off you go.
     
  7. John P

    AAC Fanatic!

    Oct 14, 2008
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    You should make use of the fact that these are LEDs and current can only go one way. Forget about relays. How about this:
    https://dl.dropbox.com/u/28291527/switches.jpg

    With S1 and S2 both up as shown, D1 lights. With S1 down, the current reverses and D2 lights. With S1 up and S2 down, D3 lights. With both switches down, D3 receives voltage but it's going the wrong way, so D3 won't light and the other 2 diodes are out of the circuit, so nothing lights.
     
  8. Zimerhonzl

    Thread Starter New Member

    Dec 20, 2012
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    John, thanks so much! I went round and round with this for hours before posting anything and for some reason I never flipped my center LED. I've tweaked it a bit to use a DP3T switch so I can cut it down to one switch.

    Again, really appreciate the help!
     
  9. crutschow

    Expert

    Mar 14, 2008
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    John P has a simple solution. The only addition likely required would be a diode is series with D3, since the reverse voltage ratings of LEDs are typically low.
     
  10. John P

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  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    What about a red/green LED? Some of these have red and green devices in the same LED package, with 3 wires.

    That's your entire solution for a 3-stage or 3-colour indicator;
    wire1 on = green
    both wires on = amber
    wire2 on = red
    (the third wire is ground)
     
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