Introduction: How to Make Led's Pulse on Bass

In this tutorial i am going to show you how to easily make a cool light setup that would be music sensitive.

We are going to build a small circuit in order to drive those led's. The way i deed it was using an atx power supply witch some time ago i converted into a lab power supply ( in witch i integrated the led controller but you can make a small case for it as well.

First of all i need to that opening the power supply case it's dangerous!!! Remember to disconnect it from the wall plug and even after that let it sit for a couple of minutes because the internal capacitors can store a dangerous charge even after that.

The circuit we are going to make is going to work even at low volume because i' ve seen others doing it with a simple transistor but the volume has to be at almost the highest volume in order to work and we are also going to put a small low pass RC filter in order for the led' s to flash only on the bass because otherwise the will stay pretty much on all the time.

Too much talking, let's actually build something!

Step 1: Required Parts

Picture of Required Parts

Needed parts:

1 x pc power supply

1 x lm741 or lm324 or equivalent operational amplifier

1 x prototyping board

1 x switch

1 x Tip31 or equivalent transistor

1 x uF at 25 V or so electrolytic capacitor

3 x 1 kohm resistors

1 x 10 kohm resistor

some copper wire

speaker connector

1 x 3.5 mm female audio jack


- 3.5 mm audio splitter

- a cable with 3.5 mm audio jack a both end's

the type of cables you need depends if you connect it along side the aux or from the amplifier' s out line which usually is an RCA connector so you will need an RCA to 3.5 jack cable


- soldering iron

- screwdriver' s

- patience :))

Step 2: Now Let's F**king Build It!!!

Picture of Now Let's F**king Build It!!!

First off all we need power, so we are going to get it. You don't actually need to open the power supply, you can use the wires that are attached to it already an cut the ones you don't need. If you read my other instructable regarding the lab power supply you'l find out that the green wire need's to be connected to a black wire (ground) in order for the power supply to start. You will need three more wires which are +12 rail (yellow) ground (black) and -12 rail (blue) so make sure to leave them intact in case you cut the rest.

Look at the circuit diagram and understand it. The circuit composes of three main blocks: filtering stage, amplifying stage and led's driving stage. The low pass filter is made using a 1 kohm resistor and a 1 uF capacitor and it's role is to let only the lower frequencies (the bass) to be amplified. The filtered audio signal then goes into the non-inverting input of the op-amp ic, the inverting input is connected between two resistor, 1 kohm resistor connected to ground and 1 10 kohm resistor connected between output and inverting input; those set the gain, in our case the signal will be amplified 11 times. Gain formula is this gain=1+R2/R1. Last stage is just a transistor which drives the led's. You connect the op-amp output to the base of the transistor using a 1 kohm resistor (the transistor must be capable to handle the current passing through the led's so it is a good idea to first measure the current with led's on and refer to transistor' s datasheet to find the collector maximum current "Ic", if the current is higher than that you should mount a small heat-sink on the transistor or get one with much higher current rating). And finally the led's "+" pin is connected on the +12v rail if you have 12v led strip like i have and the "-" pin is connected to the collector of transistor which will open the path to ground, emitter being connected to ground. The way I did it was using a speaker connector like one in the pictures above, so I can easily connect and disconnect the led' s or ad multiple strip's of led's.

Now you can start putting everything together and solder all connections.

After done soldering take a multimeeter and check connectivity, power.

A small tip if using lm741: power it from +12v to -12v or it won't work because it needs at least 18 V to function. Newer ones like lm324 might work even at 12 V but it is best to power it at a higher voltage to get best results.

Step 3: Testing

Connect all the cables and test the beast.


PallabD (author)2016-10-22

I have a serious question , actually I am trying to make this circuit, and by calculating the resistor watt I found its 144mW for 1k ohm and 14.4mW for 10k ohm, so what watt did you use or can I use 1/4watt for both, please answer. And also in the parts list there is a switch but in the circuit diagram its not there , where should I place it. Again Can I use TIP 122 instead of TIP31 as 122 collector current is 5A continous and 8A peak. And how can I send you PM. Thanks for reading such long post

caisa16 (author)PallabD2016-10-22

1/4 w is more than ok since those are high value resistors, so the current will be small

transistor is ok too and the switch is just for powering the amplifier board assembly, it can work just as well without it.

this is my e- mail adress

PallabD (author)2016-10-21

Excelllent project, and details are easy to undersyand. Thanksa lot for sharing, I was searching for something to secure. This is it.

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