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N8200 Theory of Operation
The N8200 contains an on-board precision 5-volt DC regulator with schottky barrier diode protection against accidental reverse polarity power connection. The board has a wire cable and connector for a standard 9-volt battery power source.
The regulated 5-volts provides power for the Planner’s LEDs and switch decoding and buffer isolation circuitry. Once the power switch is turned on, LEDs are activated by choosing (switching) either the left bank (left side of board) or right bank, and rotating the appropriate LED selector switch to the desired LED. Once selected the LED’s intensity level can be adjusted to any of 7 different brightness levels using the two 4-position slide switches on the left side of the board.
The rotary switches are coded in BCD (binary-coded-decimal) format, which provides 10 switch positions using 4 output control lines. These control lines are decoded and buffered using the on-board components to provide ground (–DC) isolation of an active (chosen) LED so no current leakage can occur between adjacent LEDs sharing power.
Each LED’s current is controlled using a series-pair connection of two resistors. Intensity setting “1” is for full LED brightness (20ma of current). This setting’s resistor is chosen based on providing 20ma to the highest device voltage LED of the 20 that are on-board (the N1021 at 3.6-volts).
The remaining 19 LEDs have lower device voltages and therefore, each require additional resistance to ensure only 20ma of current are provided. “Trimming” resistors are connected in series with each of these 19 LEDs to adjust their current at setting “1” to equal 20ma. The N1021 has a jumper in place of its trimming resistor. As intensity levels are adjusted (reduced), higher resistance values are added to a chosen LED’s trimmer value causing less current to flow reducing the LED’s brightness. As these larger resistances are connected the small resistance values of the trimmer resistors become insignificant and can be ignored for LED brightness planning purposes.
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