# What is the cheapest way to make led sensitive to sound around?

There is always a way to do this using arduino, rasberry pi etc. However, in many cases in discussion in forums i've come across things where whole 'logic' can be uploaded to $0.50 chip. Instead of$50 dollar part. DRASTIC change. This defines a line between a one time thing that you made as a hobby, and something you can sell around.

So basically if i want led to get brightest at loud sound and almost off on silence. Or with button that switch to 100% on all the time.

I was working on a similar project a while ago. Assuming that you are using an AUX for audio input, the best thing that I have done that seemed to work was using a TIP-31c NPN Transistor. You could also use a TIP-31a/b transistor.

Here's an image on how you could connect the transistor and get the affect you are looking for: http://cdn.instructables.com/F7J/0GZT/HH2VYB6L/F7J0GZTHH2VYB6L.LARGE.gif

Summarizing the image: You want to connect the Base of the transistor to the left or right channel of the AUX, LEDs to the collector and ground the Emitter.

If the intention is only to have one LED glowing based on surrounding sound, use a condenser micro phone amplifier connect the output to LED, If you are doing some robotic project and already audio out put is available, you can connect LED directly with a voltage divider to avoid over voltage, also you can have an Op-Amp circuit to drive the LED.The solution will be based on where are you trying to use this? how do you capture the surrounding audio? what kind of surrounding audio are you expecting?