Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I am currently working on LED to music sync circuit. The schematic is displayed below:


simulate this circuit – Schematic created using CircuitLab

Mostly I am using this schematic image: ledMusicSync. Because I was unable to find TIP31 transistor I used quite similar - BD243C. What I found out, I also needed an amplifier for my headphones signal, because the voltage is to low to activate transistor so I bought TDA2822M and build directly from documentation the amplifier: http://pdf.datasheetcatalog.com/datasheet/SGSThomsonMicroelectronics/mXqwxzy.pdf

After all of this unfortunately LED is barely blinking. To be honest I am not an expert in electric circuits and I have no idea what is wrong with it. Can someone please give me an advice what am I doing wrong.


I've edited my initial circuit due to the Spehro Pefhany answer. Now the outcome is quite interesting. When I plug in jack to the mp3 player the diode is turning off. When it is not connected it is turned on.


I suppose I might have just found an error. The amplifier isn't working. All the time I've been connecting my wire to the mp3 player and now I connected it to pc jack. With PC connected everything is working even without an amplifier. Even it is working better, because from amplifier I have outputs with resistors.

Now I need to check somehow my amplifier and probably get the new one. I'll update this questions when I'll know if my bet is correct.

share|improve this question
What is your audio source? – Vladimir Cravero Jun 11 '14 at 9:37
It is 3.5mm jack cable from headphones connected to the headphones input in my mp3 player. – sebap123 Jun 11 '14 at 9:43
well that's odd, your circuit should work fine... you are missing the base resistor for the transistor, maybe you fried it – Vladimir Cravero Jun 11 '14 at 9:46
Maybe OUT1 and OUT2 don't like being shorted together (after the caps) even though they may have virtually the same voltage on them? – Andy aka Jun 11 '14 at 11:02
@VladimirCravero I din't add any base resistor because I didn't found it in any schematic I've been looking at. – sebap123 Jun 11 '14 at 12:09

Either the LED circuit is faulty or the audio circuit. To help you hone in on the problem I would suggest the following:

Disconnect the base of the transistor from audio+. Rewire the base pin via a 10kOhm resistor to +9V. If the LED lights, then the problem has something to do with the amplifier. If it doesn't light, then you know something is wrong with the LED driver circuit.

share|improve this answer
Conversely, replace the led driver with a speaker to test the amp. – Passerby Jun 11 '14 at 16:50

Aha! Your problem is that the transistor base acts as a diode, and the output capacitors charge, meaning once the capacitors charge there is no base current except to replace that lost from leakage.

You need to connect a diode from the transistor base to emitter, reverse biased, to allow reverse current flow.

A series base resistor would not be a bad idea, maybe 100 ohms 1/2W.


simulate this circuit – Schematic created using CircuitLab

share|improve this answer
but I still need amplifier am I right? Or just as Brian Drummonds suggested add extra 0.5V from source? – sebap123 Jun 11 '14 at 12:21
My comments refer to the top circuit with amplifier. The bottom one is too dependent on the source to guess whether it will work okay or not. The top one will work for sure, and can drive very high LED currents if desired. – Spehro Pefhany Jun 11 '14 at 12:23
Ok, thank you. I'll check this configuration. – sebap123 Jun 11 '14 at 12:24
I've edited my circuit according to your suggestion. I didn't have 100ohm resistor so I used the smallest and closest I have 220ohm. But now it is working very strange. – sebap123 Jun 11 '14 at 13:20
The gain is about 100 in the amplifier, so you may have to reduce the input voltage. – Spehro Pefhany Jun 11 '14 at 13:52

Most likely the voltage from the audio source doesn't quite reach 0.6V peak, leaving the transistor turned off.

Add about 0.5V to it, for example with two resistors, 180K from base to +9V, and 10K from base to ground. If the audio source is DC coupled, you would also need to AC couple it (use a 100nf capacitor between base and audio input)

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.