I am done a google search on this and have come up blank. What I have is a signal/sound being fed through a wire that ranges from (0-9V). When the volume is low, and i need 1 or 2 leds to light up, when it's high, more leds should light up. How would I solve this problem?
Since my signal fluctuates between 0 and 9V, I could use a BJT/Mosfet to set at what voltage my LED will light up yes? Basically, create a whole array of them.
By the way, I can only use discrete components for this project. No special IC's are allowed.
Best Answer
An electronic VU meter is show below so you have an idea how to "rectify" the audio before it goes into the chip.
I have circled the input where you connect your audio signal and the output is circled where you feed the rectified audio in the next circuit. Please note that I would have suggested this type of VU meter using the chip shown but because you can only use op-amps or simple components you'll have to use comparator circuits as mentioned elsewhere in this question. Alternatively this is a simple idea using diodes and transistors: -
Hopefully this is simple enough to mafe an effective LED sound meter. You can omit the opamp circuit - go straight in with the output from the rectifier circuit via a 1k resistor. As for power, the LED circuit shows it running from 13.5V nominal - it should be fine to run it at 12V or possibly a little lower.