This post will show you how to make multiple strands of christmas lights flash to the music. We'll first begin with a video that demonstrates the circuit I have built and begin to build our knowledge from there. The circuit that I built was housed inside an exit sign. It can be housed in almost anything that it will fit into or could just not be housed at all. 

Step 1: Geting Started

The Importance of the Relay:
The key electrical component in this circuit is the relay. Simply put, a relay takes the first voltage signal and acts as an intermediate to relay a second signal of a different voltage. So for instance, a signal voltage of 1V is transmitted to the relay; this triggers the relay to then send 10v in order to power an electronic. The relay is only responsible for transmitting or relaying one signal to another, in the case above 1v to 10v. The relay DOES NOT amplify the signal from 1v to 10v, so there must be a power supply wired into the circuit that allows for the final 10v signal.
So with the use of a relay, we can trigger a new circuit every time an impulse is received. This is the basic understanding of how we can make multiple strands of Christmas lights (or any other electronic) turn off and on to the pulse of music.

What You’ll Need:
Soldering Iron
Heat Shrink (recommended)

Which Relay to Buy:
Unfortunately they don’t sell the kind of relay you need at the local radio shack. So you’ll have to resort to either an electronic components website or ebay to buy your relay. There are two main criteria that your relay must fit into.

The relay must be a solid state relay. Solid state relays are just that, they remain solid and consist of no mechanical or moving parts. The older style mechanical relays might work but will mostly like not be able to stand up to the constant demands of being triggered on and off multiple times per second and soon fail if not immediately.The relay also requires one set of the terminals to be DC and the other set to be AC. The DC side should be as low of a voltage you can find; the lowest I have been able to find is 3v DC. The lower the minimum amount of DC voltage required to turn on the relay means the quieter you will be able to play your music and have the relay working. The AC side of the relay should be either 120 or 125v (the same voltage as your house) and consist of as many amps as possible. The more amperage means the more stuff you can plug into the circuit and have it operate to the music.

Links to Relays For This Project :


This is the relay I used (I found one on eBay for about $10):


Step 2: The Schematic

Putting Everything Together:
So basically what we are doing is taking a small fluctuating DC voltage from the back of a stereo and converting those fluctuations into impulses along a 120 – 125v AC circuit (that powers most household lights), causing lights to illuminate at the same intensities of your music.

Understanding the Circuit/Schematic: The signal that is being sent from the back of your stereo and running through the speaker wire to your speakers actually contains a small amount of DC voltage. The DC voltage in the speaker wire fluctuates to the demands of the musical signal being sent. So for instance, every time there is a loud kick on a bass drum, the DC voltage spikes a little bit.
We can then use this fluctuation of DC voltage in the speaker wire to trigger a relay that will operate a much stronger voltage to power hundreds of lights that will appear to operate in perfect cadence to the music. (There is in actuality a small delay, but it is impossible for human sight to even come close to detecting it in real time.)

Schematics Included:
I included two schematics. The first one is the simple version which includes just the necessities of what is needed to build a working circuit. The second is a slightly more complex circuit which is the exact same circuit that i have wired in the back of my exit sign. 
<p>Works great. I mod-ed up up a notch. Instead of hooking the unit directly to the sound system, I picked up an old computer external sound unit (powered and with volume control), removed the speakers and wired the outputs to the audio in of the relay circuit. Used two relays and a split power receptacle to run both the left and right channels of the stereo. Not only do you get a far more dazzling effect, you can use the volume control on the old speaker system to get the desired light show without having to increase the volume on your actual sound system to outlandish levels. Not to mention the is far less of a chance of blowing your main (and expensive) amp. <strong>Great Instuctable! Easy, Inexpensive and superior results.. Thank You</strong></p>
<p>Which relays did you end up going with? Awesome job improving the original! </p>
<p>Sorry to take so long getting back to you.. just noticed the comment. I used </p><p>US Seller Satronix Solid State Relay -PCB Mount I/P:3-32VDC, O/P: 240VAC,3 Amps. Got them on E-bay</p>
<p> Use Crydom D2W203F works great, look at this Instructable, I did this works nice.</p><p>https://www.instructables.com/id/Audio-Controlled-Christmas-Light-Flasher-With-Indi/</p>
i built the simple circuit but my lights just stay on..... any suggestions?
How would that be a dead short?
$17.00 Shipping. Ouch!
A little confusion about the relay. mouser.com is charging $12.95 and wellgaineelectronics is charging $34.50. Question is can either be used with the same results?
how many lights can i power with a 25 amp relay
you may want to look at second switch...schematic not drawn properly....yes it will turn off, but not by switch, but by house breaker. Drop a wire across 120v positive to the neutral and what happens...dead short, but your first pic is good and must say what a brilliant idea!!
hey, ive been looking to do this for awhile. and id just like some explanation on the &quot;stereo output&quot; side of the project if you dont mind explaining
The stereo output is just simply the two wires (usually red + and black -) that extend from your stereo to the stereo's speakers. In reference to the picture you may use any postive and negative port. I just simply used the unused ports.
That is sooooo hot!!! Thanks for this instructable! I am setting up white lights for a party in a few weeks and this is going to take it over the top!

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