Diodes

The LED you already built, explained

Your very first capstone, back in CA-1, computed a resistor for an LED using a forward drop of $2.0$ V that was simply handed to you. You now know where that number comes from, why it depends on colour, and why the resistor is not optional. This capstone closes that loop and hands the board a second job: a Zener-clamped input the microcontroller can trust.

1. Revisit CA-1's calculation with real physics: choose a red LED ($V_F = 2.0$ V) and a target of $10$ mA from the Pico's $3.3$ V rail, and compute $R_S$. Then redo it for a white LED ($V_F = 3.2$ V) and explain in two sentences why that design should be rejected on a 3.3 V rail.
2. Estimate the LED's power dissipation and the resistor's, and confirm both are comfortable for standard parts (a 5 mm LED handles about 100 mW; a $0805$ resistor about 125 mW).
3. Design a protective clamp: a sensor feeds a Pico ADC pin that must never exceed $3.3$ V, but the sensor can swing to $6$ V on fault. Using a $3.3$ V Zener and a series resistor, size the resistor so that under fault the Zener current stays under $20$ mA, and confirm the clamp does not disturb normal operation (below $3.3$ V the Zener is off — an open circuit — so the ADC sees the sensor through a series resistor only).
4. Open PCB Studio, build the Pico 2 W with one LED and the LDR divider, and generate the schematic. Read the LED's series resistor from the spec JSON and compare it against your task-1 number; account for any difference (target current, standard values).
5. Look at the schematic's LED symbol and confirm its orientation — anode toward the GPIO, cathode toward ground through the resistor (or the reverse if the design sinks current). Then run PCB layout and note that on the routed board, the diode's polarity is carried by the footprint's silkscreen marking. On a physical board, that marking is the only thing standing between a working LED and a backwards one.