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Help with TIP 31 Sound Reactive LED's Answered

I am having trouble getting my led strip to blink to music.  I built this circuit:

I have tried this with a battery powered strip of led's and it worked perfectly, but when I connect it to the led's powered by the power supply, it either doesn't turn on at all or is extremely dim.
Could someone explain why it isn't working?



Best Answer 4 years ago

Here is the tip circuit u want

aa gain.GIF

Hi iceng, I guess this Circuit will work perfectly. But I need a
clarification regarding the image you posted in here. Since your Circuit
Design is showing to add the 100 Ohms resistor to the Positive side of
the LEDs, (what I know, Resistors must be connected at the Positive
Supply line),but the schematic you showed, there you connected the 100
Ohms at the negative side of the LEDs. Then which one is correct one?

it would be great if you can help me out, that if I connect a zener
diode (1N914) in reverse between the 1st and 3rd pin of Tip31, then that
would be okay or not?

Please clarify my two doubts.


The first pic shows how SERIES circuits do not care who is in line in what order.

For the second ask your own question and label the Base, Emitter and the zener voltage and why you want to hook this up.

This also gives someone a chance to earn a best answer from you.


Hi iceng, I just tested the circuit on a bread board and I connected everything as exactly you shown on the 1st pic (with two Tip31 Transistor). Only thing I did extra as, I connected the R channel wire from the 3.5 mm aux to the Base pin of the first Tip31 through 4.7 K resistor and the L channel wire to the Base pin of the 2nd Tip31 through another 4.7 K Resistor. And I started the same and LEDs started beating with the music but they seemed very dim and then time goes and the LEDs went more dim and after some time they are not glowing at all. I tested those LEDs individually and all of them are fine. So what's I am missing??? :( :(



Gain = 1000
Saturates 5A  @ 20ma Base Current

Good_One !


1 year ago

Sorry this is the REAL FIX a bit late.


First of all thanks to iceng for posting this. It does make a lot of sense. Since iceng cleared one of my doubts related to connecting of Resistor to Positive side or Negative side of the LEDs and iceng replied the same with reference image that it doesn't matter for a series circuit.

But my 2nd question is for all respected members in here: Please dig into this and help me out to understand the situation.

My 2nd question is: Should I hook up a zener diode named 1N914 between the Base and the Emitter? Please see the reference Image for better understanding.

I want to hook this up because what I understand about the Tip31 transistor, it needs to be protected. More precisely, I want to limit the input voltage swing to 5V to the transistor because the max allowed reverse voltage is only 5V for this transistor. (I could be wrong. Please make me correct if I am wrong...), and it could damage the transistor if the peak voltage is more than 5V. So, I want to hook up the above mentioned zener diode to protect the transistor.

Please look at the images I have attached. And clear my doubt.


4 years ago

Could you mean the 40V NPN TIP31with only hfe=25 @ 1A ?
as opposed to the TIP121
  •   60V NPN TIP120 hfe=1000 @ 3A
  •   80V NPN TIP121 hfe=1000 @ 3A
  • 100V NPN TIP122 hfe=1000 @ 3A

unless the transistor is turned well on, you'll struggle to light a 12volt strip, with 12volt supply and that circuit

Can you explain that a little more? My understanding is that the tip31 circuit simply acts as a switch that completes the circuit between the led's and the power supply whenever the music gets loud enough.

no, it's a kind of very weak switch, there is a few volts dropped across it. I don't think the tip31 is a good device for this. The tip121 would be better. You should be able to make the '121 turn on better, but you could do with slightly more volts possibly.

That's a better solution altogether, and it might get the lamps glowing well, if not at full brightness, close enough. Make sure you put a base resistor from the opamp to the transistor. The TIP 31 doesn't have much gain, and if the string takes say 1A, then the transistor base needs 1/20 A to drive it - your opamp might not be happy sourcing 50mA !

Here is a thought turn up the volume.


Not enough current or not enough voltage. what is the output of the power supply volts and amps?

What is the battery volts and amp hours.


the power supply's output is 12VDC and 2A, which match the led strip's requirements. Im pretty sure the problem is with the tip31 circuit. I wired the TIP31 circuit in line with the power adapter after the converter, so it should be DC current...but it isn't working. I tested the circuit with just one led and it worked fine

What is the battery it can be 12 volts and 12 amp hours, or 12 volts, 12 amps for 1 hour.

Can you see why a 2 amp supply won’t run a 12 amp load?


What power supply are you using?

It sounds like you are trying this with 2 different strips, one what is battery powered and a strip that uses a wall wart? Or are you talking about the same strip being plugged into batteries and working fine but won't work when plugged into a wall wart?

I connected the tip31 circuit in line with a battery pack powered led strip and it worked fine. When I connected it in line with a wall wart powered strip of led's, it does work, but it is veeery dim.

Not sure why the image didn't upload