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Transistor as a switch

  1. Jul 12, 2005 #1
    Hi guys, i want to fix a circuit that will use a led and i want to use a transistor as the switch to the led. I would like the base current to be produced by a sound played throug ha mobile. A sound turning the light on and the same or other sound turning it off. No ideas how to do it.Thank you for your help.any ideas are welcomed
  2. jcsd
  3. Jul 13, 2005 #2


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    Staff: Mentor

    Check out this thread from a couple weeks ago:


    In my last post at the bottom, I show a circuit that turns on a red or green LED based on the battery voltage (as a low-voltage indication). The two cross-connected transistors at the right would be how you would normally make a small 1-bit memory cell that can turn on an LED in response to something, and then turn it off in response to something else. By turning on one transistor, that shuts off the other transistor.

    Now you just need to figure out what you want to do with your input signals to generate the base drives for the two transistors. For example, you could low-pass the music to generate one base drive signal, and high-pass the music to generate the other base drive signal. Then different tones in the music would turn the LED on or off. If you leave two LEDs in the circut like for the low-batt indicator, you'll get a mix of colors based on the music content.
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