I'm currently using this circuit, but without the rectifying portion,a audio source from mp3 player instead of a mic, and a 4V LED instead of a bulb. My power source is 12v. However, the voltage across the LED is only around 1-1.5V. How can i increase the voltage across the LED? Here is what i want to do. I'm trying to have the LED light up only when the input voltage from audio source is above a certain voltage. When the input is above a certain voltage, the LED lights up fully, and when the input drops below that certain voltage, the LED switches off completely. The LED is either at its maximum brightness, or not emitting any light at all. Hopefully, I can replace the LED with another load and have the voltage across the load to be either 12V or 0V, or either 4V or 0V. How can i achieve this?