Recently, as part of an iPhone development project I’ve been working on with the Institute of Zoology and Birkbeck, I had a need for an audio line in adapter so that we’d be able to record stereo audio from an audio sensor. The application tags audio data with GPS information as part of a citizen-scientist data collection project.

Strangely (or perhaps I didn’t dig around the interweb long enough) I didn’t find many resources for a home-brew line-in adapter and the ones I found were pretty vague and hard to follow. So Peter and I put our heads together and rolled our own. It’s a fairly straightforward circuit, but has a little twist, because the iPhone OS is smart enough to detect what’s plugged into the stereo jack. (The diagram above is a lot more clear than anything I found.) This particular project only required a left channel audio input to the monaural iPhone mic, but if you wanted to route both left and right channels to the mic, that’s represented by the dashed line.

The resistor+capacitor network provides a pull-up that the iPhone is looking for to detect whether you’ve got a standard stereo headset plugged in or whether you’ve got a microphone (i.e., iPhone) headset and can take a phone call with it. This particular circuit is tuned for the audio sensor we’ve been using, but is about the right spec for most audio recording purposes and works fine with the audio recording app that ships with the iPhone. The parts cost about £3 and I whipped one up in about 10 minutes!

Sometimes, the ideal little package comes along and you don’t have to do all the hard work fabricating a physical user interace. While working on the Spot-On project at LKL, I found a very nify little “finger torch” that solves the problem of powering up an LED in a small form factor. This little guy’s is perfect for retrofitting with an LD271 infrared emitter, for Wiimote hacking, IR Motion tracking, and generally mucking about moving around infrared sources, for you Johnny Lee fans. This package combines 3 button cells, a switch and an LED into a tight little case with a hook-and-loop strap. Not sure if it’s emitting continuously or in pulses (PWM) but there’s a microcontroller under that resin blob (pic 2), so it seems likely. A little de/re-soldering and, you’ve got yourself a perfect finger-mounted IR pointer!

Last year at Dorkbot London I gave a demonstration of the Brockenspiel and fielded a few questions afterwards. Some clever guy in the audience asked if I could hook up a bar code reader - a thought I’d had but not implemented. When I said it would probaly work just fine, as long as the serial protocol is the same, someone in the audience called out “Hang on a minute, I’ve got a barcode reader around here!” (Only at Dorkbot!) Hidden in the depths of Limehouse Town Hall, he hauled out the reader, and by a stroke of luck, it worked. We were playing the music on beer bottles in a matter of minutes. Later, I made a quick video to demonstrate the concept.

Brockenspiel Bar Codes from Brock Craft on Vimeo.

Thought it would be good to post the simple PIR (passive infrared) circuit that I used to create the interactive sensors for the installation.