This MidiBox is in beetween Sequencer (PC) and Midi-Keyboard. With one finger, you can adjust volume, pan and pitch of 8 individual channel. The board contains 8 channels with 3 "digital potentiometers" on each (Vol/Pan/Pitch).
The hardes designing point was "How to use PWM (Pulse Width Modulate) to control 634 LED:s?" Yes, normally 8 LEDs are being controlled with one PIC, but now we needed to control the intensity of emitted light for 634 LEDs.
"Digital Potentiometer" concept was choosed, becouse the Midi Sequencer can send changes in volume. The Sequencer want's to turn the potentiometer's knob or move the slider position, which is inpossible to work fast. The only idea we got was to create a potentiometer, which position is being read by LEDs surrounded it, and the potentiometer itself is only a knob with rotation calculator.
In future, we will post here a very good picture, explaining the idea.
My part was design of the electrical user interface. The controlling PIC will tell to Knob the position of the knob, like "Volume is now in 128 (full)". Knob-uC sets the rightmost LED on lit, and the three 7-Segment indicators below knob tells "128".
When user turns the knob, lets say about 10 'clics', the 7-SEG indicator calculates realtime "-10", and the rightmost LED is going darker on each click, as on same time the LED next to it is coming brighter. When the Volume is about "120", the second-from-right LED is brightest (lit) and rightmot is dark (off).
The brightness is created with Pulse Width Modulation (PWM). Becouse human eye isn't a R.P.M (real power meter) component, it is nearer a peak-device, we can pulse the LED once in a 20 milliseconds, and the eye will see it as it would be bright.
PWM solves couple problems in LEDs: Non-similar intensity (millicandelas) beetween two LEDs in same plastic bag from manufacturer, the "20mA +/- 10mA is bright" -rule. The non-similar intensity becomes from non-equal number of avaible free electrons in NP-border of two similar diodes. Now we always lit the LED in full brightness for about 1 microsecond, and human eye will not see the difference beetween DC-connected LED and PWM-controlled LED. The killer of led is the heat, which comes when too much power is created in PN-border of the diode.
From LED's datasheets we can see, that the peak current can be much greater than 20mA, as long as the peak is narrow. Well, the peak must at least 200ns to get the led from black to >90% brightness (300ns from >90% to <10%). Maximal current through the LED can be almost 1A, if the time beetween two peaks is long. The formula "D=t/T", where t is time of PWM-peak, T is duration beetween two PWM-peaks, and D is the Duty cycle. If the D is 0.005, we can use 100mA with 10microsecond wide PWM-peak, as long as the T is at least 2millisecond (we have 12,5 milliseconds with refresh rate of 80Hz)
If we use 80Hz as refresh rate, we also eliminate the blinking effect with fluorecent lights. This blinking appears with refresh rate similar to the Base Frequency of the Power Network (In US 60Hz, Europe 50Hz) or any harmonic of it:
--> Europe: 50,150,200,250,300...
--> US: 60,120,180,240,300,360...