Nuts and Volts - July/August 2018

Check all the connections on the breadboard carefully. When you’re satisfied they’re okay, you can attach the Arduino.

For convenience, I first attached the Uno and breadboard to a small Perspex carrier plate. You can see this in Figure 4. You can often buy an Uno, half size breadboard, and plate as a kit.

Connect the Uno 5V pin to the 5V rail of the breadboard, and the Uno GND pin to the GND rail. Connect Uno pin 1 (TX0) to the

220R resistor that goes to pin 5 of the MIDI socket. This is the serial transmit output from the Uno, and it’s where we will output our Note On events.

I used Dupont male-to-male 10 cm jumper cables to connect the Uno to the breadboard.

Now, connect up the MPR121. This communicates with the Uno using the I2C serial protocol, so you only need two wires. Connect Uno pin A4 to SDA (the data line) and Uno pin 5 to SCL (the clock).

That’s all the connections needed, and the electronics are now complete. Now would be a good time to check all the connections again!

Mechanical Construction

The MIDI lyre has 12 strings made of 2 mm carbon fiber rod. To make the strings, buy four pieces of 1 m long/2 mm diameter extruded carbon fiber rod. This is readily available for a few dollars in hobby shops or online because it’s often used in RC planes, cars, and drones.

Cut each rod into three equal 333 mm lengths using a hacksaw or (if you have one) a bandsaw. I find it useful to tape all the rods together using masking tape and then mark and cut them all at once. The rod is surprisingly easy to cut even by hand.

The length of the strings is not critical, so if you want to make them a bit shorter or longer, it should be no problem. In fact, 25 mm is also a good length because you can get four strings from a 1 m rod.

In an acoustic lyre, the strings are stretched over a bridge to a neck where they are tensioned by some sort of tuning machines that are usually simple pegs. To create the bridge and the neck of the lyre, use two 12-way terminal barrier block strips with a 12 mm spacing between terminals (this is usually the 15A rated model).

Make sure you get the simple kind of strip that has brass tubes in which the wires are secured with small bolts. Other fastenings are available, but they may not accept the 2 mm carbon fiber rods.

String spacing is not critical. It’s largely a matter of personal preference and what terminal block spacings are readily available. If the strings are too close together, it’s hard to play individual notes. If the strings are too far apart, it’s hard to strum the instrument.

I find that 12 mm is a decent compromise that allows me to use off-the-shelf terminal blocks. Of course, if you break the terminal block up into individual terminals, you can have whatever string spacing you want! In fact, you can arrange the strings however you want, and make a whole range of other instruments! For the MIDI lyre, it’s convenient to keep the blocks in one piece.

Stringing the MIDI lyre involves fixing each carbon fiber string between the bridge and neck terminal blocks. There is an easy way to do this and a hard way.

The hard way is (sadly) the more obvious way, so we need to discuss it so you don’t waste your time on it. This is to fix each string in the bridge terminal block and then

July/August 2018 29 ■ FIGURE 5. Photo of a female five-pin DIN MIDI socket suitable for mounting on a breadboard.

■ FIGURE 6. Diagram to show correct wiring of a MIDI out socket. Note that the socket is viewed from the back so you can see exactly what pins to connect to.