25 sensors
5 rows × 5 columns
pins (`see SparkFun`_)::
14
23
1. phototransistor collector, to pull-up for row
2. phototransistor emitter, to ground for column
3. led anode, to limiter for row
4. led cathode, to ground for column
so…
::
aM aM aM aM aM cN cN cN cN cN
14 14 14 14 14 14 14 14 14 14 …
23 23 23 23 23 23 23 23 23 23 …
Mb Mb Mb Mb Mb Nd Nd Nd Nd Nd
/---------- input1
a -+-10kΩ--+- output1
b ---220Ω--/
/---------- input2
c -+-10kΩ--+- output2
d ---220Ω--/
…
…
M ------------ output6
N ------------ output7
amplifier
https://learn.adafruit.com/stereo-3-7w-class-d-audio-amplifier/assembly
https://forum.arduino.cc/index.php?topic=269972.0
https://www.hackster.io/julianfschroeter/stream-your-audio-on-the-esp32-2e4661