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Transistor selection

Discussion in 'Beginner Electronics' started by Coyoteboy, Jun 16, 2004.

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  1. Coyoteboy

    Coyoteboy Guest


    I'm trying to monitor the 12v(13.Xv) outputs from a cars ECU. Obviously i
    dont want to disturb the signals, but i do want to light two LEDs, one for
    'output on' and one for 'output off'.
    Id considered a whole logic circuit to do it but then thought - can i just
    connect up a set of standard transistors - an npn (bc108) and a pnp
    (BC214)to each ECU pin, and when the output is low one will light, when high
    the other will?
    Can anyone see a problem with this? How do i calculate the resistor needed
    at the base leg on each, assuming they are running 20mA/3v leds.

  2. scada

    scada Guest

    Not a good idea, unless your sure what the input / outputs are! Many of the
    inputs are from sensors and are high impeadance, your circuit could load
    that down and change the engine settings. You may not even notice it at
    first, it may be a subtle change that will effect engine performance or
  3. Coyoteboy

    Coyoteboy Guest

    After some more in depth investigation ive found the ECU actually grounds
    these inputs - they are basically just switches BTW. But this makes the
    whole LED lighting issue even harder. I am pretty sure I can whap more
    current thru it - it operates a solenoid which probably has more resistance
    variation in manufacture than that of an LED.

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