Filter the light: the whole track on one board
Your CA-2 capstone built a light sensor. It has a flaw you can now name: indoor light isn't steady — lamps on Ontario's 60 Hz grid flicker at 120 Hz — so the LDR divider's output carries a 120 Hz wobble the ADC will faithfully record as noise. The fix is this unit's filter, sized by this track's every tool. This is the Circuit Analysis finale: all six units on one small board.
- Thevenin the sensor (CA-4): the LDR divider (LDR on top, 10 kΩ fixed on bottom, 3.3 V rail) seen from its output node. Compute R_Th in bright light (LDR ≈ 5 kΩ → R_Th = 5k ∥ 10k ≈ 3.33 kΩ) and in darkness (100 kΩ → R_Th ≈ 9.09 kΩ). Note that V_Th is just the CA-2 divider voltage you already know.
- Design the filter (CA-6): a capacitor from the output node to ground turns R_Th and C into an RC low-pass. Choose C = 10 µF and compute f_c in both lighting conditions (f_c = 1/(2πR_ThC): ≈ 4.8 Hz bright, ≈ 1.75 Hz dark). Verify both sit far below 120 Hz — the flicker is attenuated to a few percent — while slow, real light changes pass.
- Check the step response (CA-5): τ = R_Th·C gives ≈ 33 ms bright / ≈ 91 ms dark, so the reading settles within ~5τ ≈ 0.17–0.45 s of a light change. State the trade you just made: flicker rejection bought with response time — every filter makes this trade; f_c is the dial.
- Honour the laws (CA-1/CA-3): confirm the capacitor adds no DC load — at steady state it draws zero current (open), so the divider's DC output and power budget are untouched; KCL at the output node holds with the capacitor branch at zero.
- Build it (PCB Studio): rebuild the LDR + potentiometer board on ADC pins, generate, run PCB layout, and export the Gerbers. The filter capacitor itself is a one-part addition the parts library will meet when PS-1 formalises board specs — your task-1..4 numbers are its complete design file, ready and waiting.
- Track complete. You can now predict any linear circuit's DC values, transients, and frequency behaviour — which is everything a schematic promises. Microelectronics (ME-1) begins where ideal elements end: real silicon.