Getting line-level audio into an iphone

Now that more musicians are live streaming music during Covid-19 stay home orders, I thought this design might come in handy. If you are looking to get stereo line-level audio directly into your iphone (and your iphone still has a headphone jack), this is a pretty straight forward DIY hack that can get you there for under $20 if you have a soldering iron. Last year I wanted to do some live streaming of our local synthesizer collective and didn’t want the audio to be degraded by the iphone’s microphone, so I put this device together using parts I had in the shop.

If I knew I was going to be putting this project online, I would have called it something better than “STLN2IPOO” – which stands for “Stereo Line-level audio to iPhone”, and that’s exactly what this does. By cutting some traces and adding a few resistors on a cheap off-the-shelf D.I. Box and attaching a 3.5mm TRRS plug onto a microphone cable, you can feed a stereo or mono line-level audio signal from a mixer or other sources directly into your iphone without any signal loss. A volume knob is also included in the design so the level can be set to avoid clipping.


Full size instructions here (open in new tab to get out of modal window)


Open the PDC21, cut the traces shown and test to make sure they are cut with a multimeter if you can. Wire up the resistors and potentiometers, drill a hole in the chassis for the potentiometer. Reassemble and attach knob. For the cable, you can either attach the adafruit 3.5mm TRRS plug onto a regular microphone cable by cutting off the female end and wiring as shown, or you can cut the female end off of a TRRS extension cable and add the XLR Male connector.

how it works

This hack rewires the “input” and “output” 1/4″ jacks of the DI for our stereo inputs, the line-level signal gets summed into a mono signal through the two resistors before going into the potentiometer. The potentiometer acts as an attenuator, so you can turn down a loud signal to avoid clipping. The signal then goes into the DI’s transformer that converts it to microphone level and impedance. The 3k resistor lets the iphone know a microphone is plugged in and our special cable gets the microphone signal and ground to the right pins on the 3.5mm TRRS jack.

using it

Take line-level audio out from a mixer or musical instrument into your STLN2IPOO into one (mono) or two (stereo) 1/4″ inputs and run the mic cord into your iphone. Run the “voice memo” program and start recording to get a visual representation of the signal levels, if it is clipping turn the volume knob down. The DI box’s ground lift switch still functions if you find that you have a ground loop somehow between your gear and phone. That’s it! Now stream some video with good audio!

“35 Hz” at W.A.V.E.S. 2016

Last month I was asked if I had any ideas for an illuminated art piece to temporarily install in the sculpture garden. I had no ideas, so I declined. A week later I submitted an idea and with the help of Sarah from Windsor’s HackForge, I was able to get some grant money to make it.

Lots of Bike Lights!

The design required 100 rear flashing bicycle lights. They were supplied ‘at cost’ by local bike shop City Cyclery and it took almost 3 hours to open each package and install the batteries. My idea was to attach the lights onto the riverfront fence in a sine wave pattern that accurately represented the wave length of a 35 Hz sound. The frequency was chosen because it was the same as the “Mysterious hum that is supposedly originating from Detroit’s Zug Island” (More Info).

Lots of rain!

I installed the sculpture during record rainfall that flooded the city. Unfortunately I didn’t have time to prepare a spreadsheet that would outline each light’s location on the fence, so I had to use a tape measure and calculator. I got pretty fast at using the SIN() button, but the installation did take over 3 hours.

Less Dead Cyclists!

Now that the sculpture has been taken down and an art grant paid for 100 bicycle lights, we will be donating all of the lights to places that can properly distribute these lights to cyclists who need them. I’m sure it’s not just a Windsor problem, but there are a lot of folks on bicycles at night that have no lights. Having recently started driving, I found it very startling the first time I came across a bicyclist on the road at night without lights. Hopefully these lights find their way onto the seatposts of some of these bicycles.

DIY Bedroom Workbench and Shelves

Over the last few months I’ve been working on a small electronics workbench and shelving system that can fit into my bedroom. I started out with a floor plan in inkscape until I rearranged all of the furniture to maximize space and then designed everything in CAD. Everything is made out of plywood and assembled with a brad nailer and glue, the supports for the shelves are made from 1.5″ aluminum angle stock with holes drilled every 3″.

The desk height adjustable and appropriate for standing and has a self-closing drawer for hand tool storage. There is an integrated shelf for test equipment and this also holds a rackmount power bar that powers everything on the bench. The surface of the desk is covered with an ESD mat with attachments for grounding straps.

Underneath the desk fits a storage cart that holds 6 divider bins and also has a drawer for more hand tools. The cart is on industrial locking casters to be pulled out as a second work surface. After my first late-night stubbed toe I added glow-in-the-dark markers to the corners as well. The shelves are 1/2″ plywood and held together with t-nuts and bolts. I also used 1/16″ stainless steel aircraft cable and turnbuckles for stability and casters for easy moving.

So far I’ve been very happy with the setup and so has nin and bubs.

Stuffomatic – manual “pick and place”

I finally got a chance to test out this little invention I call the “Stuffomatic”. It is used for stuffing circuit boards with components by lighting up the correct part bin. It is activated by a footswich, and uses a Teensy 2.0, buzzer, OLED 2×16 character display and a MAX7219.

While manually stuffing and hand-soldering Therevox circuit boards, I wanted something that would speed up the process and cut down on errors. On first test, the stuffomatic seems to speed up the “stuffing” process by close to 30%. Grabbing a component directly out of the lit bin is also a lot less prone to errors, compared to the normal way (reading the reference off the board, cross referencing it with the Bill of Materials to get the part value, find the right value part bin, grab the part).

The stuffomatic advances to the next part when a footswitch is pressed. I used a buzzer to make a small “click” noise when the footswitch is pressed for some audio feedback. There is also an “up” and “down” switch on the stuffomatic to quickly go through the list of parts.

The only problem with this project was that the Teensy couldn’t store the entire BOM (bill of materials) in memory. Instead, larger amounts of data need to be stored in PROGMEM which requires very messy code to declare the variables. To get around this, I wrote a PHP script that reads a CSV file and outputs the PROGMEM variable declarations as a 2 dimensional array. This also makes it easier to recompile the stuffomatic code for different projects.

On the front of the stuffomatic is a “note” LED to draw attention to the display if there is a special note or instruction being displayed. The stuffomatic can show messages like “flip board over” or “solder”, or a specific component might have a note with it.

So far I’m happy with the stuffomatic, it did take a lot longer to finish than I expected, but building it was more fun than stuffing circuit boards.

Garage sale speakers, counter top and dust

Nice luck = finding a pair of studio monitors for $10 across the street at a garage sale several days before starting to mix an album!

This is a counter top I made out of some maple. It is finished naturally, but I probably should have stained it a bit “colder” to remove the pink-ishness that maple seems to have under cfl.

From the winter, very dusty! I gotta say that a full face mask for wood working is pretty awesome. It makes you feel like an astronaut and you can never forget your safety glasses – Norm would be proud.