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HELP ! (Please !) - Very basic switch / LED question

Discussion in 'Electronic Repair' started by [email protected], Jan 25, 2005.

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  1. Guest


    I am hoping to modify an audio mixing desk so that when a latching
    pushbutton switch is depressed, a miniature LED will illuminate to
    indicate activation (as it is currently very difficult to see with
    black (non indicating) pushbuttons on a black background used in dim
    lighting conditions !!).

    The problem I have is that the switches are DPDT latching, and BOTH
    poles are currently used to route audio signals. I presume that it is
    not possible to connect an LED to one of these poles as it will
    introduce voltage / current where the original design wasn't expecting

    I had thought about miniature DPDT relays... re-route one 'pole' of
    the audio signal off the existing switch onto one pole of the relay,
    use the freed-up pole on the existing switch to drive the relay and
    use the spare pole on the relay to switch the LED. However, with
    thirty switches to deal with, this could be expensive and cause quite
    a power drain.

    Is there any easier way (transistor ?) to switch an LED when you only
    have a DPDT switch (with both poles used for other things !) ??

    Many thanks for any help you can give.


  2. Paul Burke

    Paul Burke Guest

    Does the switch have a bit that sticks out the back when it is in the
    depressed position? If so, you could use that to operate a microswitch
    to turn the LED on/off.

    Paul Burke
  3. Andy

    Andy Guest

    If you're looking for a pure electric/electronic method (i.e. not
    involving mecanical fixtures around the switches), it will of course
    depend on the caracteristics of the audio signals that are routed by the
    switches and the circuit stages before and after.

    Can you caracterize them?

    For example, if both the preceding and the following stages were
    AC-coupled (net very probable, I think), you could try to inject a small
    current on one side of the switch and sense for it on the other side.
    The disadvantage would be some 'click' during the commutation.

    Another, more complex approach would be to inject an sine wave of a
    given frequency not within the working range of the desk's circuits, and
    sense for it on the other side of the switch.

    -- Andy
  4. Luhan Monat

    Luhan Monat Guest


    How about just replacing the switch with one that...

    a) fits in the same hole.

    b) is a toggle/handle type that is easy to see which way it thrown.
  5. Mark Jones

    Mark Jones Guest

    c) is a 3PDT switch.
  6. NSM

    NSM Guest

    Have you tried a booklight with doublesided tape? Sometimes easy works well.
    Otherwise you are looking at replacing all the switches.
  7. Guest

    OK, I think I can replace the switches with multi-pole changeover
    types, now that I've found more suppliers than RS and Maplin !

    I'm a bit confused though.. according to this site : , near the bottom,
    there is a pic of the kind of switch I need - labelled "Multi-Pole
    Switch". However, the text says the pic is of a SIX pole changeover
    switch. It's got twelve tags... !?? Huh ??

    My logic says that if the switch is a 'Changeover' that means it's
    double-throw, right ?

    So if it's laid out like this :-

    1 2 3 4 5 6
    1 2 3 4 5 6

    Then pins 2 and 5 in each row are the common connections and depending
    on the switch position will connect 2-1 and 5-4 OR 2-3 and 5-6 in each

    Am I misunderstanding this ? ....or have they just got the wrong
    picture there and are actually showing a FOUR pole changeover (or
    possibly a SIX pole 'on-off' type).


  8. Rich Grise

    Rich Grise Guest

    That pic doesn't show it all - it's a PCB switch, and the lugs on the top
    seem to be for convenience:

    Good Luck!
  9. Guest

    I don't see what you're getting at... I know it's a PCB switch -
    that's what I need. I was questioning my understanding of which pins
    connect to which when operated. The pic in my url shows a switch with
    twelve connectors (forget the lugs). If my understanding of how these
    work is correct then the one shown is a FOUR pole switch - common and
    two alternate connections per pole = 3 x 4 = 12. OR it's a six pole
    single throw (i.e. on-off) switch - 2 connections per pole = 2 x 6 = 12.
  10. Rich Grise

    Rich Grise Guest

    Well, you could always ask the guys who are selling them.

    Good Luck!
  11. Gary J. Tait

    Gary J. Tait Guest

    That is the way that sort of switch usually works.
    It is indeed a 4 pole double throw latching pushbutton switch
  12. Bill Jeffrey

    Bill Jeffrey Guest

    Methinks it is a typo. What I see is exactly what you think it is - a
    FOUR-pole double-throw switch. There are several minor errors like this
    in the rest of the page.

    Bill Jeffrey
  13. Guest

    Thankyou both for clearing that up ! I thought I was getting senile
    for a moment then !
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