Read a finished product into subsystems, one silkscreen label at a time.
Map each subsystem to a library part and its pins.
Recover the pin map from the board itself, not from a file.
Build the component and trace ledger before writing any JSON.
This unit ends the track the way the platform began: with the Bitsy Board — the real, manufactured board this curriculum runs on. You will rebuild its spec from what you can SEE, the way an engineer reads any board that lands on the bench: subsystem by subsystem, pin by pin, until the whole thing is accounted for. The board is not a mystery box; it is a document printed in copper.
Decomposition is a discipline, not a glance. Walk the silkscreen: two labelled indicator LIGHTS (GP15, GP12). A four-LED BAR (GP18, GP19, GP20, GP21). Two BUTTONS, A on GP14 and B on GP17. A BUZZER on GP13. Eight RAINBOW PIXELS driven from one pin, GP16. A LIGHT SENSOR on GP26 — one of the three analog pins, and not by accident. Two PLUG-IN PORTS: the distance eyes on GP3 and GP2, the servo arm on GP10. Every label on the board is a line in the spec you have not written yet.
Subsystem (silkscreen)
Part
Pins
Ledger (components / traces)
lights x2
led_5mm x2
GP15, GP12
4 / 6
LED bar x4
led_5mm x4
GP18-GP21
8 / 12
buttons A, B
pushbutton x2
GP14, GP17
2 / 4
buzzer
buzzer_passive
GP13
2 / 3
rainbow pixels
ws2812_strip8
GP16
3 / 6
light sensor
ldr_divider
GP26
2 / 4
distance eyes
header_1x4
GP3, GP2
1 / 4
servo arm
header_1x3
GP10
1 / 3
MCU + rails
pico2w_socket
—
1 / 2
Subsystems to parts: what the silkscreen says, what the library calls it, what it costs in the ledger.
Sum the ledger: 24 components, 44 traces. You have not opened the Studio, and you already know what the engine will report — that is what decomposition buys. The board also whispers its design decisions: six plain LEDs but only one data pin for eight pixels; the sensor on an ADC pin; the moving parts on plug-in headers instead of solder. Each of those is a lesson in this unit.
WORKED EXAMPLE
Ledger drill
Cover the table and rebuild the components column from the parts alone.
An LED is always 2 (itself and its series resistor); a button is 1; the buzzer is 2 (itself and its resistor); the pixel strip is 3 (module, data resistor, bulk capacitor); the light divider is 2 (the LDR and the bottom resistor); each header is 1; the MCU is 1.