Thursday 29 March 2012

A heart beat to midi converter

EDIT: This project actually finished up sometime in 2013. There is now a project page at, complete with a link to video of the device in action. You may want to go there directly.

Some while ago, David Usher approached Foulab about creating a device to convert heartbeats into midi signals.  Fortunately for us, he had already done a large part of the work and had got a hold of the optical pulse sensor recently featured in Make.

The pulse sensor shines a green light on the skin which is partially reflected onto a photosensor.  This outputs an analog signal, that, when conditions are right, indicates the intensity of blood flow beneath the skin, and thus to the heartbeat.  First tests were promising - modifying the Arduino code provided with the sensor to play a midi note was straightforward.  In the video below you can hear a note played for 50ms every detected heart beat.
First build is a bit ghetto, didn't even have a midi cable.  Also - note to self: dust the piano, sheesh.

One catch was that David wanted to take this on tour in a couple days.  Gapzap and I spent a crazy Tuesday evening putting together a robust project that would hopefully hold up in stage use.  In the end, the construction quality was pretty good (for which Gapzap should get most of the credit, I was up to my usual sloppiness, note the broccoli elastics).

Alas, it was for naught.  The sharp eyed readers will have noted the key phrase "when conditions are right" above.  They have to be exactly right.  In playing with it, David found sometimes it would work, sometimes not.  Some people it just never worked on.  And in all cases, the heartbeat was not as regular as you would expect - which might be a real phenomenon, the heart is not a clock.  You can hear this in the video, it seems alternately jumpy, then halting.  But in any case, it couldn't be played along with.

Still, not bad for a Tuesday night.  So that iteration was put aside, until its time for take 2.

Saturday 24 March 2012

Nixie tube goodness

When shopping for the VFD, LegionLabs also brought me back some Nixies!  Nixies require a high voltage supply... ok, given certain other experiments on this blog, a medium voltage supply - usually said to be 200V or so.  In my laziness, I tried the simplest possible supply - rectifying the 110V wall voltage and smoothing it with a capacitor.   Works a peach.

This ends up at 155V or so (110V x 0.707).  Not that a voltage doubler is hard in the end, i just had a bridge rectifier right there.  I'm using a 47K resistor in this picture - although in other tests a 36K seemed to give a slightly better brightness.  The tube uses around a millamp.
In an example of total overkill, i simulated the circuit using CircuitLab.  It was a really just a way to try out Circuitlab on something easy, and in the end, i'm a fan.
Simulated on Circuitlab

Sunday 18 March 2012

Vacuum Fluorescent Display

In his world wide travels, LegionLabs picked up a little vacuum fluorescent display showing micro, amps and volts.  Definitely a unique souvenir!  I'd never worked with VFDs before, but in the end, they are quite simple.  A hot filament emits electrons, which are evened out by a grid and then impact on fluorescent surfaces.  They run at relatively low voltages, as compared to, say, Nixie tubes.

The leads are pretty easy to figure out just by examination.  I followed the directions here and pretty quickly had it working.

Heres a zoom out on the setup.  On the right is a slapped together bench supply, using an LM315.  Its running the filament at 1.24 V, with no limiting resistor.  This is pulling 35mA, on start up it pulls quite a bit more (~50), but it quickly reaches temperature and stabilizes there.   In real use, I may try to reduce that current a little.

I have no idea why there are so many leads on the tube.  Theres only the three elements inside (micro, V and A),  each element is connected to several leads, but they dont seem to be lighting up different parts - they are just redundant.  Go figure.


Thursday 1 March 2012

Golem: One bit to rule them all

It was quite a long time ago I first heard LegionLabs talking about the one-bit processor he was designing.  It was one of those ideas that was either deep, or crazy.  Turns out it was deep.  It took my little brain a while to catch on, but eventually I was able to help out a bit.  We finally put together a write up on the Foulab site.

The write up doesn't permit posting of comments, so go ahead and post em here.  Note the challenge - if you manage to write some code for this thing, we'd love to hear about it.