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The LT3459 boost converter is operated in a modified form as a current regulator. For this purpose, the current flows from VOUT (5) via VDD (16) of the 10 shift-and-store registers HEF4094 through the LEDs at their outputs QP0 to QP7 back via R5, R7, R6(LDR) to ground.
The boost converter regulates its output voltage via an internal comparator so that VFB (3) - ground is a constant 1.23 V. So there is always a drop of 1.23 V via the series-parallel circuit R5, R7, R6(LDR) which determines the current for the LEDs. There are three times two LEDs in series, which are connected together in parallel. The total current is therefore divided between three LEDs (neglecting the flashing second). The LDR has ~25 Mohm in the dark and ~500 Ohm in the light. The resulting current through the LEDs is ~ between 37 uA and 260 uA.
The PCB was produced as under "Dropletwatch / PCB". The red lines are insulation trenches, the black rectangles are pads for the LEDs.
The cathodes of the inner LEDs are divided into six segments of 10 LEDs each. These LEDs
1-10 are connected to the corresponding LEDs of all segments, which is why only one pair of LED can be active at any one time.
The segments are adressed by the MOS-FET's Q3-Q8.
The outer LEDs can be addressed independently, which is why several can be addressed simultaneously.