My car stereo has only one 'Aux' input, but I have several gadgets that I like to hear on a trip without swapping cables: MP3 Player, talking GPS, Blackberry, XM player, Kindle, etc.

I couldn't find a commercial solution, so I came up with this little passive audio mixer that takes 1/8" headphone outputs from up to four devices and safely mixes the signals into one line that can feed a car stereo Aux input, powered speaker or any other amplifier that can take 1/8" stereo output.

No batteries are required, it's sonically transparent and most audio gadgets have their own volume control so the mixer can be done very simply and cheaply.

This little mixer also works great for connecting multiple computers to one set of amplified speakers and has many other possible uses. Note that this device is NOT RECOMMENDED FOR HEADPHONES!!!! Whatever you plug into the output needs to have its own amplification, or the volume will probably be too low.

Note: Soldering is required. If you don't know how, please search for Instructables on how to solder, as that's beyond the scope of this project.

Step 1: Materials

Qty 1    ---       Altoids Tin. See Note 1.
Qty 5    ---     1/8" (3.5mm) Stereo input Jacks, Radio Shack part 274-249 or equiv.
Qty 8    ---      1k ohm Resistors, Radio Shack part 271-004 (5-pack) or equiv. see Note 2.
1 foot   ---      22-30 gauge solid hookup wire, stripped bare

Note 1: A normal Altoids Tin can take 4 or more inputs, this example assumes 4 inputs. The Altoids Gum tins can fit 3 inputs comfortably. Adjust the number of jacks according to:

       # of Jacks = 1+ # of inputs

and number of resistors by:

       # of Resistors = 2 * # of inputs

Note 2: All resistors (2 per input channel) should have the same value, which can be anything between 1k  and 10k ohms. Higher values result in a higher volume drop. Also, the resistor power rating can be 1/8 or higher. This circuit runs no power through it, so there's no need for bigger than 1/8 w, but use whatever you have handy. Bigger power rating = bigger size.

<p>Made one!!<br>It was my first time soldering something.<br>I kinda screwed up half way and now the wires are crossing each other... but it works!!<br>If you have any constructive criticism, please shot it at me!!</p>
Hey, it looks great! I like they way you mixed 1/4&quot; and 1/8&quot; connectors. No real constructive criticisms, if it works, that's the real test. I'm not sure Jimi Hendrix' head was that square but otherwise looks great (jk ;^) -- Rich
Made one and it works great. Thanks.
Wow, really nice job! Good idea to use resistor networks!<br /> <br /> Would you care to share the artwork?<br />
Jacks are Mouser 161-3508-E. Resistor networks are Mouser 652-4608X-1LF-1K 1k ohm. Other resistor values are available. Board was fabricated by ExpressPCB.com.
Can this image be used to have a board made or do we need the ExpressPCB file?
This is a 2 sided board and I don't know if the image is scaled correctly once you download it. I do have the ExpressPCB file if you want it. Email me at steve&quot;at&quot;PowerSwitchTail&quot;dot&quot;com.
<p>I noticed the connection to ground on both resistor arrays (last but one pin). The &quot;rats' nest&quot; desing seems to omit that. Could you explain why it matters?</p>
<p>The circuit board layout is chipmonger's and while I see what you mean, I don't know why he choose to tie those 2 resistors to ground -- I can't see that it serves any necessary function (also doesn't hurt anything). The resistor network he's using contains 8 resistors with a value of 1k Ohm each, all tied together on one side and connected to pin 1 (output signal in this case). Pins 2-9 are the other side of each resistor, which chipmonger is using as mixer inputs. He could've done 8 inputs, but only did 4, and needed one network for each stereo channel (L + R). So again, no idea why he choose to do that, but maybe he'll reply. Guaranteed it's not necessary though.</p>
<p>Thanks a lot for the answer, I could not figure out its role either. Current would flow that way (i think) when an input is present, but there's nothing plugged into the output? I guess I'll build it as is and then make some measurments with a multimeter.</p>
Looks like a 2-sided board with the silkscreen mask drawn in so you would either need the file or just reproduce it on your own. It isn't mine and I don't have it. Maybe chipmonger would nice enough to provide if you asked.
Thanks for the artwork and again, really nice job!
<p>why did you put the resistors , can we do it with out them ?</p>
Please read FAQ on step 15. You can do anything you like, but unless you want to use this as a signal splitter, leaving out the resistors is a bad idea.
<p>It works!!!</p>
Hah! that's a crazy-looking tin, I like it! Good job, glad it worked.<br><br>Rich
I made a new passive mixer based on your ible. Thank you so much for the awesome instructions !
<p>that I can use my PC headphone output as input 1 &amp; a low level source as input 2? I know PC output is amplified, but thought you may have any suggestion. I know I can use car audio HI-Low adapter, but like to use your idea for a compact box, Thanks </p>
I would suggest just trying it first, if there is too big a difference in the input levels, you may need to lower the resistor values for the low level signal. You can do this either by replacing the resistors or by soldering resistors in parallel. The formula for the resulting resistance is (r1 x r2)/(r1 + r2)<br><br> Good luck
Great box! Thanks for sharing the pictures!<br> <br> Rich
<p>This was exactly what I was looking for. Thank you for making the instructions clear and actually including a circuit diagram that wasn't made in MS Paint. It works perfectly for what I need, and every input comes out the other end crystal clear, even when multiple sources are playing at once.</p><p>I ended up using a Radioshack project box instead of an Altoids tin, mostly because I just didn't have one sitting around and the box was actually cheaper than a tin of Altoids. This also makes the drilling significantly easier and prevents any unintentional shorts. </p><p>Thank you again for taking the time to make this useful, clear and concise guide.</p>
Hey, nice job! Glad you were successful and thanks for posting the pictures!
<p>works pretty nice! great idea!!!</p>
<p>Nice job! I like the jacks you used, where did you get them?</p><p>Rich</p>
<p>hi from Germany. I found the Jacks at www.conrad.de but I will make an other one with 6,2mm jacks :-)</p>
<p>thanks for the great idea! I did a mixer/splitter for an experimental music project</p>
<p>Hey Vanderaalle, great job!! Looks like you're using 1/4&quot; jacks and I *love* the tin (although not a smoker myself). Hope this works well for you.</p><p>-Rich</p>
I made mine. Not as pretty as yours but sure gets the job done! Stuck with the 1k resistors. Thanks for the great 'structable!
Thanks for posting. Here is my version,&nbsp;it works perfectly.<br> <br> C3-PO is an old ball mouse. After removing the guts there was plenty of room for the wiring. For the triple input audio jack I used parts from an old motherboard's sound card. They have five leads on each; 1) ground, 2) closed tip, 3) open tip, 4) closed ring, 5) open ring. When a plug is inserted, the part that touches the tip moves from the open tip lead to the closed tip lead. This must be how a computer knows when a plug is inserted. I used the closed tip/ring leads, leaving the open/tip leads alone. I wonder if the the open leads could be used for anything.
Nice! Good recycling!
Thank you. I used pulled parts from trashed sound card and a &quot;Smalls&quot; tin.... I love Old school point-to-point wiring...and Hot glue.
Here's my version of the mixer, using a Lucky Stars Candy tin. I plan to add a &quot;floor&quot; above the wiring so that I can store a short 1/8&quot; patch cord inside as well.
Nice! I particularly like the 'Hello Kitty' motif ;^) Thanks for sharing the picture.<br><br>Rich
Made one in less than an two hours, including trip to radio shack for parts, and safeway for MINI altoids tin. Used 1k resistors. Very Small, works fine.
Nice job!<br> <br> Good idea to run the connectors out of the bottom of the mini altoids tin.<br> <br> Looks like you used heat-shrink tubing for insulation as well. Heat-shrink is one of the best things ever for building things and professional wiring repairs.<br> <br> I'm glad it worked for you and thanks for sharing the picture.<br> <br> - Rich
<p>I'd like to build this to connect my PC and my Focusrite Scarlett 6i6 audio interface to the same set of PC speakers. The Scarlett has two 1/4&quot; jacks (L &amp; R) as output, and the PC has a 3.5mm, and I want to mix all of these into a single 3.5mm jack.</p><p>So mainly I'm wondering how to wire the two mono 1/4&quot; plugs, and whether a different type of resistors are recommended in this case.</p><p>Here are the specs on the Scarlett's outputs if it helps at all. Any suggestions are appreciated!</p><p>Maximum Output Level (0 dBFS): +11 dBu, balanced</p><p>THD+N: -100 dB (1 kHz, &ndash;1dBFS, 20 kHz BW)</p>
I'd build it as-is, but replace one of the 3.5mm stereo input jacks with two mono 1/4&quot; jacks (L+R). On the two 1/4&quot; jacks, the ground/sleeves are tied together, the tip of one goes to the tip of the 3.5mm stereo jack, the other tip goes to the ring (middle) of the stereo jack. You shouldn't have to change anything else. Make sense?
<p>Thanks for the reply! That makes perfect sense. I'll give it a shot as soon as I can and post some photos of my &quot;masterpiece&quot;. :)</p>
Worked perfectly on the first try! The reduction in sound output is not bad at all. I went with the round design because I like the way the inputs are fanned out and because the hinged tins looked like they might block the top side of the tin when the top is open. Thanks for a well-documented, money-saving, fun, useful Instructable.
<p>I made this, tested each input individually and they all work great, but when I plug my four Volca synths into this, I can only hear two of them. Any thoughts?</p>
<p>Hi Marc, if everything is built correctly, the cables are all good and the outputs of all 4 synths are good then you should hear all 4. I would suggest the troubleshooting method of changing 1 variable at a time, for example: for each cable in turn, test each one with Synth #1 and input #1. If all 4 cables are good run through the inputs in turn, then the synths. If all of those single combinations are good, start working on pairs, etc, until you can reproduce the problem. Not much fun, but that's how I troubleshoot.. if you're patient and methodical, you'll eventually find the issue. Rich</p>
<p>I made it! Great thanks! It works just perfectly! Inside it's kinda sloppy and I haven't cut the wires yet but was my first time soldering something : ) I really really appreciate this instructable!</p>
<p>I need basically a stereo Channel and a MONO Channel to go in and everything to play out both L&amp;R side I have a XLR to 3.5mm cable for Mic(MONO) and output on laptop (stereo) to be able to go into amp 3.5mm </p><p>What's best way to do this? Would I need half the rating(resistor) on MONO or Double? </p>
<p>Hi, just a question, if I wanted to add a second output? maybe with a selector switch, do you think it could work? (basically I need to connect at least 3 PCs to speakers and/or headphone, yes I've read the part about amplify the headphone signal, that will come later)</p>
As long as the total impedance of the outputs is high, practically you can have as many outputs as you want. No need for a switch. Remember, the output loads are seen in parallel, meaning that 1 low impedance output load (like headphones) will kill the sound for all of them. But you could drive many pre-amp inputs without a problem.
Made one. Works a treat. I used two inputs only and a basic board.
<p>Looks like you have 4.7K resistors in the pictures and 1K resistors in the parts list. Which works better?</p>
Honestly, it's not that critical as long as they're all the same value. Lower resistor values will result in less signal loss, but at some point there will start to be some degree of distortion (when you start to get down below 10x the output impedance of the devices, for example), but that depends on a lot of factors. I think 1K ohm is plenty high enough and you could probably get away with half that -- say 470 ohms or higher, and be fine.
Made one in 5 min today. Looked online and I only found one that would have run me $75. I had most of the parts (missing another audio jack). Works great and i can add the jack later.
<p>Thank you Rich!</p><p> This solved sooo many issues I was trying to maneuver around.</p><p> My '07 E90 BMW (3 series) came without Bluetooth and USB features but thank goodness for the Aux input! This solution allows me to use my external navigation(which doubles as a USB input for music) and a separate external Bluetooth hands-free unit simultaneously through the audio system!</p><p> I chose a smaller tin which fits perfectly into the arm rest compartment, where the Aux input is. I placed the stereo jacks on the lid facing upwards for easy access and compact setup. </p><p> The internal wiring is conceptually the same as you've advised, just arranged differently. I also used 1mm heat shrink tubes to cover up the resistor leads. High quality 1% 1/8W metal film resistors with flame retardant coating were used. </p><p> An upgrade to an OEM <br>Bluetooth/USB upgrade head unit is not only costly, it would force me to <br>choose one among Bluetooth, USB and Aux inputs, whereas I want to use all sources/options as I choose, independently. For example, an OEM Bluetooth/USB upgrade head unit <br>would shut off all other audio when taking a call, whereas I want to be <br>able to hear the navigation (as well as music if I choose) while taking a call.</p><p> Again, thank you! Cheers!</p>

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