The MagPi - February 2020

the connections to the ribbon cable were made –

shown in Figure 11, along with where they connect

to a 20-pin header which fits onto the first pins

of Raspberry Pi’s GPIO. Note: pass the ribbon

connector through the slot in the base before

soldering up the 20-pin header socket. A small

wooden bar with foam glued on the back of it stops

the ribbon cable being pulled out.

13

Testing software

The simplesounds.py listing shows

some simple code that allows you to test all the

LEDs and trigger some simple sounds. There

is a sound for each of the six note triggers, as

well as the two control bars. The sounds were

taken from the Sonic Pi sample files, found at

/usr/share/sonic-pi/samples, and converted into

WAV files using Audacity. We will go more into

the details of what is needed next month, but this

software requires the installation of Adafruit’s

CircuitPython framework along with the MPR121

driver. We had no trouble installing this under

the Raspbian Buster operating system, but found

when we used earlier versions of the OS it did not

install correctly, throwing up errors.

Next month, we’ll look at software to drive the

touch sensor chip in a more reliable way and also

a fully fledged MIDI / sound sample keypad with

effects control, sequence recording, and playback.

11

Starting the circuit

The physical layout of this circuit is shown

in Figure 8. Start with the copper foil backing for

each sensor, which provides a good ground for the

sensors. Figure 9 shows a photograph of the foil,

with each strip connected by a small solder blob.

The foil was also kept in place using some blobs of

transparent Gorilla glue. Note the cut-out for the

touch sensor board screws; this was fitted on M2.5

screws and 5 mm M3 nylon spacers. Note that the

board was fitted with right-angle pins to keep a low

profile. The LEDs are pushed through the holes into

the cover, leaving them flush with the top surface.

12

Building the circuit

The resistors were mounted flat on the

board to the LEDs and, when all were wired up,

were held down in place by spots of hot-melt glue

in the LED holes. Then the touch sensor wires

were connected as shown in Figure 10. Finally,

3V3 (1)

Gnd (9)

SDA (3)

SCL (5)

GPIO 4 (7)

GPIO 22 (15)

GPIO 23 (16)

GPIO 24 (18)

GPIO 17 (11)

GPIO 18 (12)

GPIO 27 (13)

3V3 (1)

Gnd (9)

SDA (3)

SCL (5)

GPIO 4 (7)

GPIO 23 (16) GPIO 22 (15)

GPIO 24 (18)

GPIO 17 (11)

GPIO 18 (12)

GPIO 27 (13)

Hex-A-Pad

Raspberry Pi

Top Tip

Installing CircuitPython

In a command-line Terminal, type the following:

sudo apt update

sudo pip3 install --upgrade setuptools

sudo pip3 install adafruit-blinka

sudo pip3 install adafruit-circuitpython-mpr121

A small wooden bar with

foam glued on the back of

it stops the ribbon cable

being pulled out

Figure 10

Figure 11

Figure 10

Photograph

of wiring

Figure 11 Ribbon

connections to

Raspberry Pi’s

GPIO header

TUTORIAL

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