# What is the simplest sound-reacting led circuit?

A few years ago (around 3 years) I've made a very basic circuit to make a led blink to the sound.

The circuit a single regulator connected to a USB port to extract 5V, with a 3.3V led, conected to a simple 3.5MM cable connected to the sound card, using the left channel.

This circuit worked great, but it was too weak and the sound would be very bad.
This is probably because the electrivity was being mixed with the audio signal.

I don't remember which regulator I've used, but it was one from an old TV, which would lose 0.5V below 12V, and worked with a minimum of 3V.

Posing this, what is the simplest sound-reacting led circuit?

To be a simple circuit, it must require the minimum amount of parts and must be easily assembled without a breadbord or prototype board.

My budget is incredibly low, so, I can't afford to buy and learn how to use a soldering iron, but if it is required I will try to find one.
If it isn't required, I would be really happy.

Optionally, this circuit may be made to work with a Molex connector, on the 12V rail, which I can easily find power supplies and cables to test it.

• What kind of sound do you want it to react to? battery+resistor+led easily reacts to excessive sound level of 180dB+ ... Feb 20 '15 at 15:47
• Just a regular MP3 player, TV, audio player, cellphone or something that the average user uses to listen to music. I would love to actually connect it to a HD Audio connector on my motherboard and make one with an audio connection to connect to connect the cable and then connect the headphones/speakers on another end. So, it must be well below 100dB. Feb 20 '15 at 15:47
• Buy a soldering iron! Turn the sound into an electric signal (with a microphone), Gain up the signal level (with an opamp) so that it's large enough to turn on an LED (a few volts) and send into LED.. but you'll be disappointed by the results. You most likely want the LED to blink with the beat, which means some sort of filter in the signal chain.. and maybe a rectifier. Feb 20 '15 at 15:53
• @IsmaelMiguel Nothing simpler is possible, in fact those are too simple. Perhaps you should just buy something and have it shipped to you- there's probably something on eBay etc. for \$5 or less. Feb 20 '15 at 16:19
• Hmm, well, a pound shop? Lojo de euro? My point is that a soldering iron can be found for very cheap. Feb 20 '15 at 17:36

OK since you already have it as an electrical signal you might try something like this.

simulate this circuit – Schematic created using CircuitLab

• I don't understand much, but I can search the symbols. Is the sine 1kHz the audio source? Feb 20 '15 at 16:33
• Yeah, the sine wave is your source.... If you don't understand LED's and resistors. Well there is a whole wide world of electronics out there waiting for you. (The D's are your LED's) Feb 20 '15 at 16:35
• I assume the R is a resistor. But now, where is the power source to power the leds? Feb 20 '15 at 16:37
• Hmm. The words "good", "free", and "physical" usually don't go together. But as long as your internet stays on, you can wander around here: sound.westhost.com/index2 He only wants the front page to be shared directly so that people can find the entire site, but if you go to Articles and scroll down, you'll find the beginners' section in the middle. (Why it's not on top, I don't know, but there it is.) Feb 20 '15 at 18:24
• @IsmaelMiguel you could try this website for electronics information. It has a lot of handy information and schematics. talkingelectronics.com/te_interactive_index.html Feb 20 '15 at 18:45

Because you want the LED to react to audio from a MP3 player or similar, I suggest that you have a look at a recent thread on this forum: 155914. I showed a circuit that uses 2 transistors: one transistor is wired as a diode and biases the 2nd transistor just below its' turn ON point. It should work well for you.

• It isn't properly simple, but it looks really good. I still have to learn what everything means, but it does look good. I'm not worried about lower volumes. If it could be made simples, I would stand up and clap. But it brings me one problem: power source. If it is only 3V, I can't use with a USB conector or inside my computer connected to the 5V or 12V of the Molex connector. I could try with a SATA connector, but not every single power supply has those. But I like your solution. But just one question: Won't the voltage leak though the capacitor to the input and cause crappy audio quality? Feb 20 '15 at 17:46
• No. Two things happening here: the capacitor keeps the DC where it belongs and the series resistor provides much higher input impedance compared to the very low output impedance of your headphone jack. Feb 20 '15 at 20:37
• I can always crank up the amplifier in my card to provide more power through the sound cable. My card has a special setting for headsets with 60ohms. It should do it. Feb 20 '15 at 20:50
• If it is closed, it says [on hold] after the title. Feb 20 '15 at 21:18
• @tcrosley You answered on the wrong place (I think). But it isn't saying anymore what I shown you on the comment. Feb 20 '15 at 21:45