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Discussion in 'General Electronics Discussion' started by Angus, Jul 25, 2014.

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  1. Angus


    Jul 4, 2014

    For the circuit attached - when I close the contact (on the left of the diagram) 50 milli-amps flows. I wanted to use these 50 milli-amps as the base current for a transistor - so as to open up a 12V circuit to power a 12V LED - so that when the contact is closed - it lights up the LED ie. use the transistor as an ON/ OFF switch.

    Is this circuit logical - should it work?

    If so what would be the resistor value and transistor type/ model.

    I've mucked around with various transistors that I thought should work - no luck.

    Any help guidance appreciated.


    Attached Files:

  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Your circuit is not clear. Specifically it's not clear where and how the current flows in the switch.

    Normally, a switch is connected between a voltage source and a load of some kind. When the switch is open, no voltage is applied across the load. When the switch closes, the circuit is complete, and the voltage souce is connected to the load. Current flows from the voltage source, through the switch, through the load, and back to the voltage source.

    You need to show this part of the switch. We need to know what part of the existing circuit has the voltage applied across it when the switch closes. Then, adding an LED may be as simple as connecting the LED, with a "current limiting" resistor in series with it, across the load that receives power when the switch closes.

    Can you redraw the circuit showing the connections to and from the switch, and how they relate to the "EARTH" and "+VE 12V" connection pointss you've drawn.

    Attached Files:

  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    In addition to that:
    • There is no indication that anything is limiting the current through the LED (apart from base current and transistor gain) See here for more information.
    • You normally place the load connected to the collector in switching applications. See here for more information.
  4. Angus


    Jul 4, 2014
    Thanks Kris - your advice is very appreciated.

    I'm actually rethinking the circuit and will get back to you soon.

  5. Angus


    Jul 4, 2014
    Hello - after a rethink - do you think the attached circuit would work?

    If so - what would be the transistor and resistor values be?

    The idea is - a manual momentary switch is pushed - this triggers a 12V LED to light up and also merely to provide continuity over the 'button' contacts that would trigger the LED controller into the next color or mode (or whatever).

    Sorry if I'm being a pest.

    Thanks - Angus


    Attached Files:

  6. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    Your switch is shorting out your supply. Re-draw the circuit, would it also be possible to change the background colour to white.
    KrisBlueNZ likes this.
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    In addition to that, see here. (really this time, I'm serious)

    And in addition to when I made these comments the first time, your transistor just sitting out there connected to a button will likely do nothing at all.

    Is the power supply for this the same as the power supply to that controller with the button? If so, that's a good start, but you need to determine (and show us) how that switch is connected first before we can advise how to actuate it using a transistor.
  8. BobK


    Jan 5, 2010
    Which has no effect since the power supply is already shorted by the second vertical wire from the left.

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