How to use a transistor as a switch.

Discussion in 'General Electronics Chat' started by n9103h, May 10, 2014.

  1. n9103h

    Thread Starter New Member

    May 10, 2014
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    I have a newtek video switcher. Camera 1- 8 buttons. I need to make a tally light system. I kind of got it working with a 123ap. I do not know what I am doing to say the least.

    When I press for camera 1 the signal for camera one goes LOW, I was able to hook up the 123ap to turn on an LED when the signal goes low, but I need to do it with an outside power source, like a 9volt battery.

    The unit supplies 3.4 volts and if I place a LED from the 3.4 to camera one, when camera 1 goes low the led comes on, I am very afraid I will draw to much if I run out 100 feet of wire. I want to put the 3.4 and low signal on one side of a switch and a higher separate battery voltage on the other side to run the LEDs' attached to the camera.

    Can some one point me to a beginners project that would help.... I am not understanding how to have two different voltages on a 123ap, the 3.4 volts to ground triggering and something higher from a separate supply to run the LED's. I hope I am making a little sense.
    thanks
    george
     
  2. winnipeg7777

    New Member

    May 8, 2014
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    0
    can you diagram that for me
     
  3. inwo

    Well-Known Member

    Nov 7, 2013
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    Do you have a PNP?
     
  4. ScottWang

    Moderator

    Aug 23, 2012
    4,854
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    Using your title to search on google then you will get the tutorials.

    When you know the method then draw the circuit and attach it.
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,361
    If the LED works with 100' of wire then that is safe.

    A longer wire draws less current, not more.
     
  6. Little Ghostman

    Member

    Jan 1, 2014
    294
    97
    http://www.allaboutcircuits.com/vol_3/chpt_4/2.html

    That should get you started, as others have stated you need to do a look and search for pnp transistor switch, pnp is normally used when you need to switch on with a low (0) level, NPN is normally used for a high on level, also pnp sources the current (normally) and NPN is for sinking the current (normally blah blah blah........insert normal caveats here), but to keep it nice and simple for you.

    Google PNP Transistor switch, you should find all the information you need, you will need to have a rough idea of how much current you want to put through the LED (15-20mA should be plenty), from that you can work out the base resistor for the transistor, this will depend on the transistor itself but, general rule of thumb back of a napkin jotting, go for around 10X less current at the base (depends on transistor gain(beta)).
    Again dodging all the maths and working on thin air figures, 9V supply at emitter of transistors so
    9V-3.0V (rough guess at led fV) = 6V
    you want 15 mA at led so led resistor =

    6V/0.015 =400R Now this will give you lower than 15mA at the led because of the wire length, but it dosnt matter this is napkin project.
    So
    9V to emitter of transistor, 400R (Ohm) resistor for the Led
    this leaves the base transistor.
    Total guess but lets say bog standard everyday pnp then aim for around 1.5mA at the base of the transstor

    3.3V at the base
    3.3/0.0015 n=2.2KR which just so happens to be another standard resistor size ;)

    Recap
    9V to transistor Emitter
    400R resistor to Led
    2.2K resistor and the 3.3V to transistor base

    It's not exact and holes could be picked in it all day, BUT it's about right and should under most situations work. If it dosnt work then dont blame me, I was working with the information given :D
     
  7. Little Ghostman

    Member

    Jan 1, 2014
    294
    97
    Also try 5K resistor at the base, that should also work (maybe better), like I said depends on the transistor you have, but in practice any resistor between 2.2K and 5K should be ok
    LG (covering my bum thats all)
     
  8. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    LG,
    That was my thought at first and why I asked about a pnp.

    If he only has 123 npn's.

    How about this?

    If you critique it, I'll redraw it.:)
    Or delete it.:(

    I thought it might clarify his "2 sources" question.
     
  9. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Or this.....................................
     
  10. Little Ghostman

    Member

    Jan 1, 2014
    294
    97
    I was wondering why 9V? he could use 3X AA batteries. I was assuming he had to go buy a transistor. Maybe he could give some feedback.
    I only posted because as a newbie myself, I know how hard it is to know the information you need to give, I dont like to say go google. Alot of beginners dont google because it brings up alot of stuff that can be misleading, better to give them a direction first then suggest what to google. It seems to be the fashion to tell newbies they should google the answer, the funny thing is, if people dont post the answers here then google becomes less helpful!
    Not moaning just observing.
     
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