Saturday, 2 April 2011

MIDI Trigger

This box translates footswitch inputs to MIDI note on/off messages. We're using this to trigger a sampler with some special Zaardy noises on. It has three banks of three notes. The notes are played using the three footswitches on the left. The bank is selected using either the red button on top of the box or the single footswitch on the right.

It's based on an Arduino Uno, a microcontroller with handy digital inputs for the footswitches and serial out for MIDI.

The electronics is straightforward - mostly LEDs, resistors, connectors and wires:

The Veroboard sits on top of the Arduino but has to be in two parts because the pin spacing is non-standard.

Plastic boxes are not very robust for live use, but they're much easier to make holes in than metal:


The LEDs are power (red), triggered (green), bank (yellow).


To make the Arduino read the footswitch states and send the appropriate MIDI messages requires some bespoke software. The Arduino comes with a framework to help with this, so that all you have to do to read a digital input (footswitch) is:

state = digitalRead(pin);

...and state will be either HIGH or LOW.

Playing MIDI notes is also easy:

Serial.print(0x90 | (channel-1), BYTE);
Serial.print(note, BYTE);
Serial.print(velocity, BYTE);

To put this together into a complete application takes quite a lot more code, and this is what I ended up with:

The .h & .cpp files on the left are C++ classes, each performing some discrete function. They all get put together in the code on the right. The final application is downloaded to the Arduino through USB.

Future developments - Three notes are OK, but what we really need is thirteen:

This is the next project...

Friday, 1 April 2011

The Electric Violeg

Category: Rustic Homebrew

What does it do? - this is a two-stringed electric fiddle made from the amputated leg of an old table and some old guitar parts. And some Meccano. It's made entirely of things I had lying around the house and garden, the only thing I had to buy was the bow, which was the cheapest factory second I could lay my hands on. It sounds vaguely violin-ish, but in what I think is quite a good way. It's seen a good bit of usage on stage and in the studio.

This is the first attempt. The neck was much too fat to play at this stage. 

Strings anchored by strips of Meccano.

3-ply plastic strips are shielded with foil and connected to ground via the Meccano to reduce hum from the single-coil pickup. With no body to enclose the output jack and volume control, an old die-cast box was fitted to the side.

Front view of the one-a-side headstock.

Addition of crudely-carved chin-rest and corderouy-upholstered shoulder pad. 

Rear view of head, tuners mounted upside-down to conserve space.

The slimmed-down neck makes the top notes accessible to people with normal fingers.

Originally this was to be played under the chin, but it was found to be easier to have it mounted vertically on a microphone stand like a tiny 'cello. Parts of a mic clip are now permanently attached to the rear for ease of mount and dismount. It was also found to be useful to mark the notes faintly on the fingerboard with pencil, and wear a head-mounted torch when playing the instrument under stage lighting. Mark can be seen demonstrating this in the picture at the top of the article.

Future Developments? - I quite like the idea of having a quartet of these, one for each leg of the table.

Thursday, 31 March 2011

Casio MT-40 Sequential Voice Switch

Category - Modification

What is it? - a circuit added to a Casio MT-40 keyboard to rapidly switch between four voices. It could also be adapted to switch audio or control voltage sources.

How does it work? - I've lost the circuit diagram (I made this back in 2005), but it's pretty basic. The MT-40, in common with many Casiotone keyboards of the early '80s has two keyboard modes; "Play", where the keys function in the standard musical fashion, and; "Set", where the keys are used to select the voice, or Tone as Casio call it. Each white key selects a Tone, the names of which can be seen just above the keys.
The slider switch which toggles between these modes is just out of shot to the right. To the right of that is another switch labelled "Tone Memory", which allows you to change between four pre-selected tones without changing to the "Set" mode. I found that rapidly moving this switch whilst playing made a cool sound, but I could only play one-handed. So I decided to automate the rapid switching.

The circuit I built to do this is a standard sequential switch that I found many variations of online. The larger IC is a 4017 decade counter, triggered by a 555 timer, the speed of which is determined by the value of the capacitor and the potentiometer. Four of the 4017 outputs are connected to the Base pin of each of the transistors (by way of indicator LEDs, and buffered by 1M resistors, as the unit runs at 7.5v). The Collector and Emitter of each transistor are connected to the four pairs of contacts in the "Tone Memory" switch. The 5th output of the 4017 is connected to its reset pin so it counts to 4 rather than 10.

The picture below shows the circuit installed into the MT-40 enclosure. To the far right are the controls - an on/off switch and a rate knob. To the left you may be able to make out the empty slot where the Tone Memory switch used to be.

Further Developments? - I set to building a variation on this which was to have +/- 5v power and transmission gates in the place of the transistors, which would allow me to cycle rapidly through up to 10 audio signal paths (or more if a second 4017 was added). My intention was to build something like a step-wah, but it turned out that somebody was already making one, so it quite literally never got off the drawing board.

Another use that this might be put to is sequentially linking bend-points in a circuit-bent device to create a rhythmic pattern. An external trigger input could be substituted for the 555 so that this could be synched to a multitrack by sending the device an audio pulse e.g. an isolated click.

Credits: Big thanks are due to Imogen on this one!