Nuts and Volts - July/August 2018

try to get the other ends of the strings into the neck block.

This is surprisingly difficult and very, very fiddly because the strings are already fixed at one end!

The easy way to string the lyre is as follows:

1. Loosen all the nuts on the neck terminal block so the holes are completely open.

2. Slide each string through the block to about its halfway point.

3. Offer the end of each string up in turn to the bridge terminal block and tighten in place.

4. Slide the neck terminal block up to the opposite end of the strings and tighten everything.

This is a very quick and easy method because you can move the strings independently.

If you need to replace a string (highly unlikely), loosen all the bolts holding it, slide it out of the top of the neck block, and slide the new string in. It’s interesting to note that most string instruments have a preferred — and sometimes necessary — way of being strung, and the MIDI lyre happens to be no exception.

I remember spending an afternoon stringing a lute and getting halfway before I realized that I wasn’t going to get any further doing it that way.

A word of warning: Be careful when you tighten the nuts in the terminal blocks. You need to just catch each carbon fiber rod so that if you give it a little pull, it won’t come out. Although carbon fiber rod is immensely strong, it can be quite friable if crushed.

Once you have fastened the strings in place, you need to mark some of the strings to give you an indication of

30 July/August 2018

About the MPR121 Capacitive Touch Sensor

The MIDI lyre uses the MPR121 touch sensor chip in a convenient breakout board by Adafruit.

The MPR121 is a sophisticated device that has 12 independent electrode inputs for capacitive touch sensing. It measures the total capacitance to ground, C, on each of its 12 electrode sensing channels.

On each channel, this capacitance comprises the background capacitance, Cb (usually about 10 pF or so) and the finger touch capacitance, Cx (usually about 1 pF).

When the electrode is not touched, Cx is zero; when it is touched, it’s about 1 pF or so.

C = Cb + Cx The chip measures Cb and establishes the baseline capacitance which corresponds to no touch. This baseline changes, so the chip dynamically recalibrates to compensate for this. When an electrode is touched and Cx is large enough, the chip registers a touch. When the capacitance falls to the baseline again, it registers a release.

It works by sequentially charging each electrode to a peak voltage, V, by applying a constant current, I, for a given time, T. It then measures V using a built-in 10-bit ADC (analog-to-digital converter), then shorts the electrode to ground ready for the next cycle.

Both I and T can be changed by altering the values of internal registers, but for this application, the default values seem to work well. We know that capacitance is defined as: C = Q/V where Q = I T is the amount of charge. Therefore: V = I T / C So, the peak voltage, V, is inversely proportional to the total capacitance of the electrode. You can see this clearly in the oscilloscope traces in Figure A and Figure B.

Figure A shows the peak voltage, Vb, on an electrode when C = Cb, and Figure B shows the smaller voltage, Vx, when the electrode is touched and C = Cb + Cx.