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How it's possible to turn on/off a single LED with the left (or right) sound channel of an audio jack and a pnp transistor.

Everywhere npn transistors are used. Like here:

  1. Driving LEDs from audio signal
  2. LED Color Organ Triple Deluxe

I'm very new to this topic and need the led control for a simple project of my computer science master thesis. My uncle gave me some old pnp transistors and I built something like this: enter image description here and this: enter image description here.

When I use my fingers the LED glow. But when I connect my audo jack and play a song, nothings happens or the LED glows very very (not visible) low. I thought, that the 5V will give the power although the audio jack has not enough power.

Are npn transitor needed or is there a very simple way to use pnp transistor to let the LED glow, when a smarthone plays a sound throuh the audio jack? A simple circuit diagram would be helpful.

(image source: http://www.talkingelectronics.com/projects/TheTransistorAmplifier/TheTransistorAmplifier-P1.html)

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  • \$\begingroup\$ Careful with that! The only reason the first one didn't blow up is that your fingers and the audio source both had high enough resistance (or sufficient output protection) to keep the base current down. Keep that away from line drivers! \$\endgroup\$ – AaronD Jan 7 '15 at 22:19
  • \$\begingroup\$ Is it acceptable for your circuit to load and distort the audio signal? Minimally? A little? Or as much as needed (don't care about loading/distortion)? \$\endgroup\$ – mkeith Jan 7 '15 at 23:01
  • \$\begingroup\$ Try outputting a loud moderate frequency square wave. \$\endgroup\$ – Chris Stratton Jan 7 '15 at 23:26
  • \$\begingroup\$ @mkeith It is acceptable to distort the audio signal. What matters is when a sound on left channel exists so light up the left LED. \$\endgroup\$ – tm297 Jan 8 '15 at 8:28
  • \$\begingroup\$ @ChrisStratton I will try it. So far I was using the output from old tv. \$\endgroup\$ – tm297 Jan 8 '15 at 8:33
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If the output voltage is high enough you could consider an emitter follower which has a high input impedance β*Zl and therefore hardly loads the jack.

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  1. If you're running from batteries, or an isolated power supply with nothing else attached, then there's no inherent ground connection. This means that you can choose any point you want to be ground.
  2. PNP and NPN are mirrors of each other, as are P- and N-channel FET's. If you mirror all transistors and swap the supplies, it usually works the same way.

Put those two together, and you can adapt most circuits to use the opposite type.

It doesn't always work because of slight physical differences, so in some cases you might need a different size package to get the same performance, but that's more for power applications than small signals.

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Your problem is not the voltage, it's how you are connected. Transistors are driven ("turned on") by voltage across the base and emitter. That's the flat side and the arrow in the schematics. So, instead of trying to connect from base to -6, connect to base and +6. It will also be a very good idea to connect a 1k resistor from the base to the input.

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  • \$\begingroup\$ The poster did say it was a pnp transistor. In general the base resistor is a good idea, but appropriate capacitor size should work instead. \$\endgroup\$ – Chris Stratton Jan 7 '15 at 23:23

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