dhaulagiri/io_panel_PCB at master · jdeitmerg/dhaulagiri · GitHub

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Main Connector 'CN11'
=====================
Pinout (top to bottom)
----------------------
1   DIS1
2   DIS0
3   LEDC
4   KEY
5   +5V
6   GND
7   DAT
8   CLK

Shift register pin current
==========================
Measurements
------------
 -> when only one LED is on:
    LED1 on
        Voltage at Q_A (shift reg pin)       .26V
        Voltage 'behind' R20                3.00V
                                            -----
        Voltage over R20                    2.74V
        Value of R20                      330   R
                                          -------
        Current through Q_A                 8.3 mA

 -> when only one Segment of 7seg display is on:
    7SEG.8 (top element) on (LED1 glows a little)
        Voltage at Q_A (shift reg pin)       .26V
        Voltage 'behind' R20                2.98V
                                            -----
        Voltage over R20                    2.68V
        Value of R20                      330   R
                                          -------
        Current through Q_A                 8.1 mA

    7SEG.4 (bottom left element)
        Voltage at Q_E (shift reg pin)       .26V
        Voltage 'behind' R18                2.96V
                                            -----
        Voltage over R18                    2.66V
        Value of R18                      330   R
                                          -------
        Current through Q_E                 8.1 mA

Conclusions
-----------
1)  Though the LEDs glow a little when the corresponding 7seg element is
    on, it doesn't add much to the current drawn.

2)  It's save to power all 3 LEDs at the same time, as the combined output
    current by far doesn't exceed the 50mA through VCC or GND rating.
    The single output currents are also far below the specified 20mA maximum
    output clamp current.

3)  Powering all 8 7seg elements at the same time is a little risky, as the
    combined current would be around 65mA, which is above the maximum rating
    of current through VCC or GND (50mA).
    When continuously cycling through LEDs, DIS0, DIS1 (and KEYs, but the KEY
    cycle is going to be much shorter), the maximum rating would only be
    exceeded maximally ~65% of the time, so we don't need to worry
    too much. This is tested, works well.