Basic Electronics 10, Chapter 10: Challenge 3 solution (level shifting, safe motors and choosing a board)

Formulas: V(out) = V(in) x R2 / (R1 + R2)          I = V / (R1 + R2)          R(source) = R1 || R2          rise time = 2.2 R(source) C
          sense voltage = I x R(sense)          counts = V / 3.3 x 4096          duty = DEAD + (100 - DEAD) x knob          V(avg) = D x V

(a) HC-SR04 echo to a Raspberry Pi pin, R1 = 1.8 k (series), R2 = 3.3 k (to ground)
    V(out) = 5 x 3300 / (1800 + 3300) = 5 x 3.3 / 5.1 = 3.235 V       (above the 2.3 V threshold, below the 3.6 V limit)
    Divider current = 5 / 5100 = 0.980 mA
    Source impedance = 1800 || 3300 = 1800 x 3300 / 5100 = 1165 ohm
    Time constant with 30 pF:  1165 x 30e-12 = 34.9 ns        rise time = 2.2 x 34.9 ns = 76.9 ns
    Why not 3.3 V up to 5 V: a divider's output is always lower than its input (k is below 1), so it can only shift a signal DOWN.
    Going up needs a gain stage: a MOSFET shifter module, a 74HCT buffer powered from 5 V, or just an Uno input, whose threshold of about 3.0 V accepts 3.3 V directly.

(b) Current sense, R(sense) = 0.22 ohm in the motor's ground return (motor M1: stall 1.09 A, running 0.25 A)
    Running 0.25 A:   V = 0.25 x 0.22 = 0.055 V       counts = 0.055 / 3.3 x 4096 = 68.3     -> 68
    Stall 1.09 A:     V = 1.09 x 0.22 = 0.2398 V      counts = 0.2398 / 3.3 x 4096 = 297.6   -> 297
    Trip at 0.8 A:    V = 0.8 x 0.22 = 0.176 V        counts = 0.176 / 3.3 x 4096 = 218.5    -> 218
    One count = 0.806 mV / 0.22 ohm = 3.66 mA          (so the 0.8 A trip is about 218 counts and 1 count is under half a per cent of it)
    Power in the resistor at stall:   I^2 R = 1.09^2 x 0.22 = 0.261 W      -> use a 0.5 W resistor (a 0.25 W one would run hot)
    The resistor also takes up to 0.24 V of the motor's supply, and lifts the motor ground by that amount: acceptable for a 4.8 V pack, a poor trade at 1.5 V.
    Program logic: if the count stays above 218 for 25 loops (25 x 20 ms = 0.5 s), set the duty to 0 and stop; the brief start-up peak does not count.

(c) Ramp and dead-zone mapping from Circuit 2 (DEAD = 18, STEP = 2 per loop, loop = 20 ms)
    Knob 0.5:   duty = 18 + 82 x 0.5 = 59 %
    Average on a 4.8 V pack:   0.59 x 4.8 = 2.832 V     (the driver drops a little more)
    Ramp from 0 to 100 %:      100 / 2 = 50 steps x 20 ms = 1.0 s
    Ramp from 0 to 59 %:       59 / 2 = 29.5, so 30 steps = 0.6 s
    The motor does not move until about 18 % (0.86 V on 4.8 V), which is the first step of the dead-zone map, so the knob has no dead region.

(d) Choosing a board
    (i)   Line-following robot with five 5 V analogue sensors: Arduino Uno. 5 V logic and ADC range match the sensors; six ADC pins; simple, forgiving and well simulated.
          (A Pico would need its inputs protected or divided to 3.3 V; a Raspberry Pi has no ADC at all.)
    (ii)  Robot driven by a phone over Wi-Fi: ESP32. Wi-Fi and Bluetooth are on the chip, with many PWM and ADC channels.
          (An Uno or a Pico has no radio; a Raspberry Pi has one but is heavier, costlier and slower to start.)
    (iii) Robot identifying objects with a camera: Raspberry Pi. It runs Linux, Python and the vision libraries and has a camera connector.
          Add a Pico or an Arduino for the motors and sensors, since the Pi has no ADC and loose timing.
    (iv)  Motor controller at 20 kHz with a TB6612 and a battery monitor: Raspberry Pi Pico. 3.3 V logic matches the TB6612, hardware PWM at 20 kHz on any pin,
          a 12-bit ADC for the divider, a watchdog, and MicroPython for fast testing.

Expected readings
    (a) 3.23 V at the pin with a meter when the echo is high (the 5 V pulse is brief, so a meter sees an average: use a scope or a steady 5 V test)
    (b) with a meter on the sense resistor: 55 mV running, 240 mV stalled for a second at most
    (c) printed duty 59 %, motor average about 2.8 V after the ramp

Wiring checklist
  - Divider on the echo: 1.8 k in series, 3.3 k to GND, pin at the junction; never connect the echo straight to the pin.
  - Sense resistor between the driver's GND (the motor's ground return) and the common ground; the ADC reads the voltage across it, so the sense lead goes to the resistor's driver side.
  - Fuse in the pack lead rated above the running current and below the wire rating (a 2 A fast-blow fuse for M1).
  - 100 k pull-downs on every driver input so the motor is off while the board boots; a switch in the pack lead as the hard stop.