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
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
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
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
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.
The difference between these two voltages, Vx – Vb,
determines if a touch (or release) event is generated.
Once a touch event is detected, it is published over an I2C
■ FIGURE A. Output on one of the MPR121
electrodes with no touch.
■ FIGURE B. Output on one of the MPR121
electrodes when touched. Notice the drop in peak
voltage compared to Figure 8.