SPDT Switches & Relays

Discussion in 'The Projects Forum' started by ThievingSix, Jul 6, 2012.

  1. ThievingSix

    Thread Starter New Member

    Jun 16, 2012
    I have a circuit with several Illuminated SPDT switches.


    The switches have 4 legs at the bottom, the positive and negative leg's of the LED and the two sides of the switch. The LED and switch aren't connected in any way. When i wire the LED up it is always on. However i only want the LED to turn on when the switch is pressed down. Is there any way i can do this without bypassing the switch?

    I realise that its a single pole, but i guess i'm wondering if anyone can think of a nifty circuit that could turn the LED on only when the switch is depressed. Ideally converting the SPDT into a DPDT. I've heard about relays but I'm not entirely sure how to wire one up or choose the correct one or if it would even work.

    My circuit runs off a 9V battery and i'm using the switch listed above.(the LED runs at 8v). I'm trying the build a very basic 8-bit adder/computer that adds two 8-bit binary numbers together and the switches are the binary input.
  2. evilclem

    Active Member

    Dec 20, 2011
    Your link says that the LED needs 2V, not 8V.

    You would simply power the LED off the switched side of the switch (with appropriate voltage applied), not the battery side.
  3. ThievingSix

    Thread Starter New Member

    Jun 16, 2012
    Ah my mistake i meant its an SPST switch not SPDT. I've tested the LED and it runs bright at around 8V i believe 2v is the minimum, as its barely visible at 2v.


    Sorry for the mistake. That's the circuit i have at the moment for clarification. The switch controls another part of the logic circuit running at 9V.

    If i connect the LED to the + and - it always stays on(obviously) but i want it to only turn on when the switch is depressed. This would be easy with a DPST switch, however i couldn't find any that were like what i needed.

    Thanks for the help!
  4. SgtWookie


    Jul 17, 2007
    Here, like this:


    R1 limits the current through your switches' LED. If it has an internal resistor, you won't use R1.

    Q1 switches the ground side (cathode) of the LED on and off.

    R2 limits the base current to Q1.
    R3 keeps Q1 turned off when S1 opens.
  5. strantor

    AAC Fanatic!

    Oct 3, 2010
    SGT, I know you have a reason, but what is the reason? Why so complicated with the transistor?

    It seems to me that this would work just as well, and be less complicated:
  6. ThievingSix

    Thread Starter New Member

    Jun 16, 2012
    Thanks for all the help guys, I can't believe I didn't see that :D. I ended up using strantor's suggestion and it worked perfectly.