IR Modulator

  1. Hey all. I am building an IR modulator but seem to have a small kink in the design. Here is my schematic:

    [​IMG]

    Here is a picture of the oscilloscope, connected between ground and the output of the TL555 timer and the output of the second transistor (the output pulse going into the LED's).

    [​IMG]

    The problem is that the transistor looks like it IS switching at 38kHz, but the off voltage is only in the low mV range. How could I make it so that this is actually modulating on/off?

    Thanks for any help,

    John
     
  2. jcsd
  3. The transistor is hooked in a common collector configuration which doesn't have any voltage gain.

    You could replace the second transistor with a PNP, or move the LEDs up across Vcc and collector while grounding the emitter.
     
  4. So instead of putting the LED's between the second transistors emitter and ground, I would connect it from +5V to the collector, and connect the emitter to ground?

    Thanks, I just want to be sure. Also, do you think this would increase the maximum current (Should I be careful to not burn out my LED's when switching)?

    John
     
  5. yes

    Yeah it would. One may place a current limiting resistor between 5V and the LEDs.
     
  6. vk6kro

    vk6kro 4,059
    Science Advisor

    You probably need a circuit like the one in this post:


    https://www.physicsforums.com/showpost.php?p=2612341&postcount=35

    As a general rule, never put semiconductors directly across a source of power either on their own or in series. They are quite capable of destroying themselves if you do that.
    You always need a resistor in series with them to limit the possible current.
     
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