I finally got the Velleman FM radio kit working satisfactorily this week. My initial problems had been excessive noise and lack of sufficient time to debug. The radio specs show an input sensitivity of 5-10mV. They recommend (in broken english) supplying a signal with double the impedance of the signal generator you’re using (i.e. the mic or amplifier). Since I want to use a microprocessor to generate the sound, this presented problems.

The BX24 has no info on its internal impedence, and it outputs at a whopping 5 volts. By the time you bring this down to 5mV, you’re current is at virtually nothing. I hooked the BX24 up to the radio input through a bunch of resistors to bring it down to about 1 volt, and then tried it straight in (yes, supplying 1000 times the recommened voltage), and the radio didn’t seem to care which way it was plugged in, so I’m sticking with the straight input.

The kit also came with a diagram of how to wire up an electret microphone, and this works very well, although it’s not what I’m interested in right now. The circuit in the kit also includes a pot to vary the input level, and this seems a bit buggy and erratic, but does help attenuate the signal somewhat.

As a tip to anyone using a radio kit, don’t use the same power supply for the radio transmitter and the microprocessor and the radio receiver. Any combination of these two will lead to confusion and a bad signal. Each one must have its own power supply.

In other news, in our Spatial Design class last week, I designed a piece that uses joints and mechanical motions to kick a leg repetitively. The piece was a kind of illustration of kinematics of the leg with each joint following the path of a groove cut into foam. This was referrential to post-WWII artists who fetishized dismembered body parts, as well as a memory of various dissections (including frog legs and assorted brains) I was subjected to as an undergrad. I hope to do more work involving mechanical motions. As a tip to anyone interested in mechanical movements: the book “507 Mechanical Movements” is an amazing collection of the most common mechanisms you’ll find inside just about everything.

For this week, I prepared the “Nasty Bird”. Two weeks ago, my parrot was stolen from my apartment. The burglar was able to get into the apartment (must have had keys), and was not interested in the many thousand dollars worth of electronic equipment I had sitting in the same room as the bird. It’s true that “Paprika”, the cockatiel was cute, but in this capitalist system, she wasn’t worth more than $60.

My girlfriend has been depressed ever since, so in the spirit of healing, I created “Nasty Bird”. Nasty Bird looks soft and plush, but when you reach your hands at her, she starts chirping her nasty low-fidelity chirp. Maybe she just wants to look into her mirror. The eyes are infra-red phototransistors that I use to trigger the sound when the ambient IR level drops below a pre-set point.

As a continuation of my experiments on reactions to various user interface designs, this piece was very very well accepted. The venerable Tom Igoe insisted on knowing what the bird did, and other students were bemused at the mere sight of it, regardless of what it did. Since you really have to shove your hand right up to the bird’s face before it chirps, it’s somewhat surprising that nobody mentioned such limitations. Presented as it was as a self-contained unit, it was generally acknowledged as a finished piece whether or not that was my intention. Maybe because my intentions were hidden or because I told a sad story with general emotional appeal as a part of the presentation, or both.

For this week, I prepared a sort of mini theremin. Looking around the room, I noticed that everyone’s projects were neatly configured with a minimal amount of wire, and a very clean appearance. Mine had ratty remnants of past projects hanging up to a foot off the breadboard and wires wrapping all over the place to dubious-looking electronics components. You would have no idea what to do with it if I didn’t expain.

I think the lesson is that both appearances have a use – on the one hand, the incomprehensible mess intimidates and awes the viewer and leads naturally to wild gesticulations and exaggerations, on the other hand, a neat appearance garners esteem from your peers and seems to open the room to honest conversation about the inner-workings of the piece. Both approaches focus on the technology, and preoccupy the viewer with technical details. If I have time, I will try to completely enclose my next project to focus on the meaning and intention of the piece.

The mini theremin had two modes. The first used a potentiometer to vary the pitch, the other used a photoresistor to detect hand proximity (IR or sonar would have been better, of course). A switch changed between the two. The BX-24 has a FreqOut command that allows you to vary the pitch, but it’s rather limited. Also, a pre-amp was necessary but missing since the volume was almost inaudible on my tiny radio-shack speaker (Jamie Allen built a great-sounding pre-amp for his project which he intends to share on his site). But considering I built the thing that morning, I find it reassuring that here at ITP building a theramin prototype in a few hours is no big deal – because it’s really not – the emphasis is on ideas which are more difficult. People outside of this field don’t understand how easy and banal it is to follow directions and wire a couple of sensors up to a little chip.

I experimented with correlating knob and sensor movement with actual pitch change, and did not come up with a good solution to correlate the two. The knob only changed the pitch until it was turned 180 degrees, not the full 360 as you’d expect.. I’m interested in controlling and playing with the sensitivity and range of the sensors, and hope to update this page with my findings on how exactly to do that.

A “High-Q” FM radio kit by Velleman arrived today from Jameco – it only took 2 days to get here. It looks very simple and easy to set up. I’m going to solder all the parts together tomorrow and get started hooking it up to the BX-24 to trigger remote control events using some kind of coded signal.

Also got a servo motor to start playing around with – my Spatial Design class will be showing projects hanging from the ceiling during the Winter Show – the only first year class to be showing stuff – so I’m hoping to have an installation with some moving parts whispering personalized profanities to whoever stands underneath. It would be great to combine the 3 interesting classes this semester – Physical Computing, Spatial Design, and the Digital Sound Workshop – into the show piece.

In class, we’re learning digital input circuits and data types as understood by the BX-24. Somewhat dry, so I won’t reiterate it here, but very basic to everything that will happen from here on out. I was surprised that boolean values take up 1 byte of memory. A boolean is a true or false, so I was surprised that they use up more than 1 bit. In other words, don’t bother using them in the BX-24.

Viewing the flow of electricity as a set of voltage stages is a useful way to break down circuits. For example instead of the usual circuit loop diagram, break it up something like this:

+5V

I blew the serial port on my BX-24 already. I started using them more than 6 months ago, and should have known better than to cavalierly plug it in without making sure everything was grounded. Always keep receipts – it’s too late for me.

For this week, I wanted to experiment with piezo buzzers. The idea was to vary the pitch by configuring a 3-pronged potentiometer into a voltage divider – using the voltage change to create a pitch change. Unfortunately piezos don’t work this way. The crystals resonate at a fixed pitch. Crystals are interesting in that you can put mechanical strain on them to create a voltage difference, or you can apply a voltage difference and get a mechanical movement.

There seems to be a lot of reciprocity on. I’ve been reading about Stirling engines, which use a heat source to provide mechanical movement. By enclosing a gas in something like a tube, the Stirling engine heats one side of the gas. The hot gas exerts pressure onto a lever, which moves, opening a valve that lets the hot gas enter a cooling chamber. Once cooled, the gas contracts again and the lever returns to its starting position. By shuffling around the hot and cold gas between the heating and cooling chambers, the lever is cranked like in a gas engine, but without the noxious fumes. The heat source is irrelevant – it could be dung, fermenting beer, your sweaty palms, my mom’s breath, etc.

On a different note about reciprocity, a simple DC motor can be used backwards to let spinning motion generate electricity instead of vice-versa – something I experimented with in a windmill sculpture, but never got enough current.