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Radar App

Prerequisites​

  • Raspberry Pi Pico W with MicroPython
  • Servo
  • Ultrasonic distance sensor
  • Android phone (App Inventor Companion app installed)
  • Basic familiarity with the previous Wi-Fi lesson (connecting Pico W & HTTP)

Wiring​

  • Servo:

    • Brown/Black (GND) -> GND
    • Red (V+) -> 3V3
    • Orange/Yellow (signal) -> GP3 (you can choose another PWM-capable pin)

  • Ultrasonic Distance Sensor:

    • TRIG -> GP4
    • ECHO -> GP5
    • VCC -> 3V3
    • GND -> GND

Part 1 - Pico W Firmware (HTTP API)​

1.1 Servo helper​

servo

from machine import Pin, PWM

class Servo:
    def __init__(self, pin=3, min_us=500, max_us=2500, freq=50):
        self.pwm = PWM(Pin(pin))
        self.pwm.freq(freq)
        self.min_us = min_us
        self.max_us = max_us
        self.us_range = max_us - min_us

    def angle(self, deg):
        deg = max(0, min(180, int(deg)))
        us = self.min_us + self.us_range * deg // 180
        # duty_u16 for RP2040: duty = us / period * 65535
        period_us = 1_000_000 // self.pwm.freq()   # 20,000 us at 50 Hz
        duty = int(us * 65535 // period_us)
        self.pwm.duty_u16(duty)

1.2 Ultrasonic distance helper​

ultrasonic

from machine import Pin, time_pulse_us
import time

class Ultrasonic:
    def __init__(self, trig_pin=4, echo_pin=5, timeout_us=30000):
        self.trig = Pin(trig_pin, Pin.OUT)
        self.echo = Pin(echo_pin, Pin.IN)
        self.timeout_us = timeout_us
        self.trig.value(0)
        time.sleep_ms(50)

    def measure_cm(self):
        # 10 us trigger pulse
        self.trig.value(0)
        time.sleep_us(2)
        self.trig.value(1)
        time.sleep_us(10)
        self.trig.value(0)

        # measure echo high pulse (us); -2/-1 indicate timeout
        t = time_pulse_us(self.echo, 1, self.timeout_us)
        if t < 0:
            return None
        # distance = speed of sound * time / 2
        return 343 * t * 1e-4 / 2

1.3 HTTP server​

Using the helpers and what you learned last lesson, write a HTTP server for controlling the servo and getting distance data.

Part 2 - Build the MIT App Inventor App​

appinventor

2.1 Designer (UI components)​

Open appinventor.mit.edu -> Create Apps -> new project.

Add these components:

  • User Interface
    • Label (Title: Moving Radar")
    • TextBox (Hint: Pico IP, Text default your Pico IP, e.g. 192.168.1.50)
    • Slider (Min=0, Max=180, ThumbPosition=90) -> for angle
    • Label (Name: LabelAngle, Text: Angle: 90)
    • Button (Text="Set Angle")
    • Button (Text="Scan")
    • Label (Name: LabelDistance, Text: Distance: -- cm)
    • Canvas (Width=Fill parent, Height=200) -> simple chart area
  • Connectivity
    • Web (Name: Web1)
  • Sensors (optional)
    • Clock (TimerInterval=500, Enabled=true) -> poll distance every 0.5s

Optional: use the built-in Charts component (Category "Experimental") instead of Canvas if available to you. Below we'll use Canvas for broad compatibility.

2.2 Blocks - Configuration​

  • Create a helper block to build a base URL:
    procedure BaseURL -> returns "http://" + TextBoxPicoIP.Text

2.3 Blocks - Set angle​

  • Slider.PositionChanged -> set LabelAngle.Text to "Angle: " + round(position)
  • ButtonSetAngle.Click:
    • Web1.Url = BaseURL() + "/servo?angle=" + round(Slider.ThumbPosition)
    • call Web1.Get

2.4 Blocks - Poll distance (Clock)​

  • Clock.Timer:
    • Web1.Url = BaseURL() + "/distance"
    • call Web1.Get

2.5 Blocks - Handle responses​

  • Web1.GotText (url, responseCode, responseType, responseContent)
    • If responseCode = 200 and responseType contains "application/json":
      • local obj = call JsonTextDecode(responseContent)
      • If URL contains "/distance":
        • LabelDistance.Text = "Distance: " + round(obj["cm"]) + " cm"
        • Plot this value on the Canvas (see next).
      • If URL contains "/scan":
        • Clear Canvas, iterate obj["points"] list, draw a polyline (angle on X, cm on Y).
      • Optionally, if URL contains "/servo": update angle label.

2.6 Drawing on Canvas (simple chart)​

Create procedures:

  • CanvasClear: call Canvas.Clear, draw axes:
    • X axis: angle 0->180 mapped to canvas width
    • Y axis: 10->80 cm mapped to canvas height (invert y so 10 cm is near bottom or top as you prefer)
  • PlotPoint(angle, cm):
    • x = angle / 180 * Canvas.Width
    • y = map(cm, 10..80 -> Canvas.Height..0) (linear map)
    • call Canvas.DrawCircle(x, y, 2)
  • PlotPolylineFromList(points):
    • loop i=1..len-1: draw line from previous point to current (x1,y1) -> (x2,y2)

Use these in:

  • Web1.GotText when handling /distance or /scan
  • ButtonScan.Click:
    • Web1.Url = BaseURL() + "/scan?n=19"
    • CanvasClear()
    • Web1.Get

Part 3 - Try It​

  1. Flash the Pico code, note its IP in the REPL.
  2. In App Inventor, connect the AI2 Companion on your Android phone.
  3. Enter the Pico's IP in the TextBox.
  4. Move the slider -> tap Set Angle -> servo should move.
  5. Watch Distance label update every ~0.5s.
  6. Tap Scan -> servo sweeps; a profile appears on the Canvas.

Exercise​

  • Turn the Pico into a remote music player, with a play/pause button on the phone.

Optional: Using Flask as a Relay (Bonus)​

If you built the previous Flask lesson, you can point the app to a Flask relay on your laptop, which then proxies to the Pico.