The MagPi - February 2020

09

Polish the brass

First, the brass inserts need filing and

sanding to remove sharp edges and corners. Run

your finger over them to check all is smooth. The

brass only needs to be polished up to the mounting

hole. We used a P600 wet and dry grit paper – first

dry and then wet – to remove any scratches, and

finished off with Brasso polish. When installing all

the brass for the final time, make sure you check

continuity with a meter between the brass and the

screw. If it is open circuit, then put a very small

amount of conducting paint in the hole and try

again. Similarly, conducting paint is used to fix the

rectangular control fill pieces to the control strip to

bring them closer to the surface of the cover plate.

10

The schematic.

The schematic of our circuit is shown in

Figure 7. Basically, it is the MPR121 touch sensor

breakout board and a bunch of LEDs. We used

3 mm blue LEDs for the key triggers controlled

by GPIO outputs, and 3 mm red/green common

cathode LEDs for the control strip indicators, all

in a diffused package. You might have to enlarge

the two holes for these in the top plate to allow

you to push them through. These were driven

from four of the unused touch sensor connectors.

Special registers inside this complex chip were

used to make these four touch inputs into LED

driving outputs.

Figure 6, and placed the top plate over them and

tightened them up. Note: if the edge of the strips

overlaps a portion of the LED hole, a few strokes

with a rat-tail file above the hole makes sure the

insert will not foul the hole. Each trapezoidal brass

trigger was positioned to be at the edge of the

hexagon and slotted underneath the cover, and the

hole positions marked. They were then removed,

drilled, and numbered with an engineer’s scriber in

a place that would eventually be covered up. Do not

enlarge the holes too much, as we need the edge of

the screw to make contact with the brass.

The brass inserts need filing and sanding

to remove sharp edges and corners

Red Green

SCK

SDA

0 1 2 3 4 5

3V3

Gnd Gnd (9)

MPR121

Touch sensor

6 7

0

1 2

3

(^54)
3V3 (1)
SCK GPIO 3 (5)
SDA GPIO 2 (3)
7
Blue^3 Blue^4 Blue^5
GPIO 22 (15)
GPIO 23 (16)
GPIO 24 (18)
11
10
Red Green
9
8
Blue^0 Blue^1 Blue^2
GPIO 17 (11)
GPIO 18 (12)
GPIO 27 (13)
Touch Pins
I2C Pins
IRQ GPIO 4 (7)
Gnd
150R 150R
150R 150R
6
240R
240R
240R
240R
240R
240R
G^0
12
34
56
78
(^109)
11
0
2 1
3
4 5
7 6
MPR121
Vin3Vo
GndAddr
SDASCL
IRQ
Raspberry Pi GPIO number
(GPIO connector number)
3V3 (1)
Gnd (9)
SDA (3)SCL (5)
150R 150R 150R
150R
240R
240R
240R 240R
240R 240R
GPIO 4 (7)
GPIO 22 (15)
GPIO 23 (16)
GPIO 24 (18)
GPIO 17 (11)
GPIO 27 (13) GPIO 18 (12)
Figure 7
Figure 8
Figure 9
Figure 7 Schematic
of the Hex-A-Pad
Figure 8 Physical
layout of the
Hex-A-Pad
Figure 9 Grounding copper foil
TUTORIAL
50 magpi.cc Make beats in style with Hex-A-Pad