Driving one LED from audio signal only with pnp transistor

Question

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: and this: .

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)

Answer 1

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.

Answer 2

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.

Answer 3

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.