I am controlling an LED strip based on audio input from my phone. I am having some good results, but not exactly what I want, and my ignorance of transistors is holding me back. (I'm a mechanial engineering major rather than electrical.)
I have my phone connected to an amplifier which then outputs an AC audio signal through a bridge rectifier and then that DC signal is sent through an NPN transistor (TIP 121) acting as a switch to control an LED strip. I've got the lights to choreograph to music and it looks great, however, it only works when the volume is very loud. As I increase the volume, the LED strip gets brighter, and as I decrease it, the LED dims.
My first question is why does this happen? I have the strip connected to 6V so shouldn't the input from the amplifier to the transistor only switch on and off the 6V supply?
And if I want the lights to work at low volume (when there is too of low current from the amp to switch the transistor,) can I add a second transistor to act as an amplifier (based on a common collector/emitter follower circuit?) Should that second transistor go before or after the bridge rectifier?