Basic rule: the smallest amount of elements…

- five potentiometers for manipulation of midi data (velocity, sustain, pitch,…, controllers), rhythms/ events - with some presets / including various random options that prove interesting (on the level of pitch, velocity, rhytmic event, tonality).

- eight push switches, eight-channel analog multiplexer/demultiplexer CMOS chip 4051 (74HCF4051, CD4051) for activation/ deactivation of individual sequence steps and eight LED for visualisation of active steps

- the 4051 has internal on-resistance of 50 to 150 Ohms, so we don\'t use any limiting resistors for eight LEDs

- to sense/read the pushed switches we use a 10 Kohms resistor as the lower part of voltage divider – the upper part is the so-called pullup resistor in atmel microcontroler on arduino board

- there is a silicon diode on the common cathode of rotary display LEDs - this is needed to increase the voltage drop on the combination of LED and diode to over the threshold defined for logic one (HIGH) - in order for the push on switches bring it  to the logic zero (LOW).

- two push switches are used to select different steps for two additional midi channels in sequencer mode or for some notes\' transposition in controller mode.

- three LED and limiting resistors 100 to 220 ohms show midi channels activity (or a note on and note transposition in controller mode)

- there is a (too large...) number of flying wires, sometimes flying resistors - because of single sided PCB.

Read and write (4051):

HEF4051 is eight channel analog switch which receives binary encoded data on three pins to select one of eight channels. The chip is just opening and closing internal transistor switches to the common pin - it provides no power for the LEDs.

Reading of switches and writing of LEDs are performed in interval interrupts in the following sequence:

- every 4000 microseconds (or so) there is a call for interrupt routine where we check for the current states of all switches and also send current to rotary display LEDs

- we select the current multiplexer channel to read or write (with three binary encoded pins from Arduino board we open/close/scan individuaslly the eight  inputs/ outputs of 4051 multiplexer)

- we first define LED not to glow – arduino pin 5
analogWrite(5, 0); // switch of LED with PWM write or
digitalWrite(5, LOW); // switch it off with faster digital write

- then we redefine arduino pin 5 as input, activate the internal pull up 20 Kohmov
pinMode(5, INPUT);
digitalWrite(5, HIGH); // pull up activated

- read pin 5
digitalRead(5);

- if the selected switch is pushed (to ground) the voltage on pin 5 is just below the threshold of LOW, or it is above (0.6 * Vcc) – so the logic \"one\" (1, HIGH)

- compare and remember the new value

- next deactivate the pull-up resistor on pin 5
digitalWrite(5, LOW);

- redefine arduino pin 5 as output to prepare to write to LED
pinMode(5, OUTPUT);

- during each interrupt we increment by one the current sequence step

- each time we check for the state of the current step - it can be 0 or 1

- we also show the circulation of steps. We do this with full brightness of the current step (override the previous)

- so, the LED glows:
analogWrite(5, 30); // active current step
analogWrite(5, 255); // full glow - current step is also in-syncro with the step of circulation display

Problems:

Main problem in this \"common pipeline\" for reading and writing is that the corresponding LED is always there - going to the ground. There is the necessary change on reading when the switch on the same channel is pressed, but different types of LEDs have different resistances/ threshold voltages (2.2 to 2.6 V for red, green, yellow - and 3.2 to 3.5 V for white and blue).

At first I used analogRead() arduino function where the setting of threshold programatic, but the reading was at some steps too slow and the step "hanged".

Using digitalRead() - as described above - limits the eight display LEDs to white or blue color. The threshold voltage is around 3.5V - for other colors it is around 2.5V and less.

But in all cases we have to use some additional threshold thingy - a forward biased silicon diode adding 0.6V to the cathode side. For white and blue a small signal Si diode seems to be ok (1N914, 1N4148). In some cases a 1A rectifier diode did the trick and also b-e pn junction of bipolar Si transistor is ok.

For other colors we would need around 1.5V added - three diodes in series - or a Zener-diode 1.8V. I tested it some time ago but it was at that time dissapointing.

Solution: use medium to high brightness white LEDs for the rotary display!