Simple transistor circuit help

Discussion in 'The Projects Forum' started by ScorpFire, Oct 23, 2009.

  1. ScorpFire

    Thread Starter Member

    Jun 4, 2009
    13
    0
    Hi im building a simple circuit that uses a Digital Output pin from a modem to turn on an LED.

    [​IMG]

    The modem output pin when high is 2 - 2.8v, when low 0.1 - 0.9v.

    The problem is even a LOW Signal turns on the Transistor. Is there any way i could make it such that only a high signal can turn on the transistor? Thanks in advance
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Try it more like this:

    [​IMG]

    I changed the configuration around so that the transistor would be saturated instead of an emitter follower.
    The addition of the 1N4004 diode keeps the transistor from turning on until the input voltage exceeds about 1.3v.

    R1 limits the current through the LED (Vf=2v) to 20mA.
    R2 limits the base current.
    R3 keeps some current flowing through D1 so that it's Vf stays in the desired range.
     
  3. ScorpFire

    Thread Starter Member

    Jun 4, 2009
    13
    0
    Both high and low outputs from the modem didnt turn on the LED...im guessing theres too much current going into the ground??
     
  4. flat5

    Active Member

    Nov 13, 2008
    403
    17
    Is the diode connected the right way?
    Have you tried a higher value for R3?
     
  5. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    If this Modem interfaces to RS232 then there is no need for a transistor Driver. RS232 is more than capable of driving a Led with a 470R to 1K limiting resistor. When RS232 was first introduced it had a typical voltage swing of -30V to +30V. Over the years this standard was lowered to -15V to +15V but RS232 will recognize much lower swings like -5V to +5V.
     
  6. ScorpFire

    Thread Starter Member

    Jun 4, 2009
    13
    0
    Nope its not RS232. Its a standard General Purpose Input Output 2v8 voltage level.

    Thanks for your suggestions will try ASAP.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Looking again at the simulation of the circuit I posted, the transistor should be off if the input from the modem is less than 1.5v. Once the modem input reaches 1.95v, the LED should be near it's maximum brightness.

    Changing D2 to a 1N4148/1N914 diode and R3 to 2k will reduce the thresholds; below 1.36v the transistor will be off (D1 off) and above 1.7v the transistor will be on (D1 on).
     
    Last edited: Oct 28, 2009
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