Switching a transistor with audio output

Question

I am wondering how can an audio signal be used to switch a transistor, if the voltage is too low? I am trying to switch some LEDs with the output of an amplifier, but it seems that the audio voltage is too low for the transistor to turn on.

I have the output of the amplifier going to the base of a BC549 transistor, the collector connected to the negative of the LEDs, and the emitter connected to ground. There's a 100K Ohm resistor each from the collector and emitter to base. The positive of the LEDs is connected to 5V, which the amplifier is also running off.

When I put some music through the amplifier, I have to turn the volume up to probably 90% of the amp's power for it to switch the LEDs.

So does anyone know how I can boost the voltage of the audio to the transistor?

Answer 1

1. Insert a 100 ohm or so resistor in series with the LED, to limit the current if you get the transistor fully turned on.

2. Disconnect the audio signal for now.

3. Instead of your BE and BC resistors, use a pair of resistors, one from B to +5, and the other from B to ground. The ratio should bias the base so that the LED is just dark. That should be about 10k to +5 and 1k to ground -- but adjust until the LED is slightly off.

4 Connect the audio to the base via a capacitor -- this is actually the crucial ingredient. A range of values will probably work -- say 10uF. That will be polarized -- the + end should go to the base.

That should be much more sensitive than what you've got. Your main issue is that the audio is likely at zero volts, and so you need about 0.5V to 0.7V peaks (on the positive side) to get the transistor to turn on. Using biasing resistors and AC coupling with the cap means you need much smaller +ve peaks to get some LED lighting.