ESP32-S3 board
The brain that runs your uploaded sketch.
01 First, know the pieces
What you need
Six things, nothing more. Tap Define on any part you haven't met — the answer opens as a field note you can read and dismiss without losing your place.
GPIO extension board
Spreads the pins into rows you can reach and label.
HC-SR04 sensor
The distance sensor with two round transducers.
4 jumper wires
Temporary, solder-free connections.
Arduino IDE
Uploads code and opens Serial Monitor.
Official sketch
Sketch_19.1_Ultrasonic_Ranging.ino
02 Make the physical circuit
Chart the circuit
The official Freenove diagram is your chart. Click it to enlarge. Each connection below tells you where the wire goes and why it's there — hover a row to light it up.
VCC
→5V
Powers the ultrasonic sensor.
Trig
→GPIO 13
The board sends a short "start" pulse here.
Echo
→GPIO 14
The sensor returns a timed pulse here.
GND
→GND
Gives board and sensor the same zero point.
Check before power. Keep the sensor pins in the order printed on the module — VCC, Trig, Echo, GND — and unplug USB before you move any wire.
03 One action at a time
Build it
This is the main path — you can finish the day without opening a single field note. Tap each step as you go to keep your place.
0 / 7 done
Place the HC-SR04 so the two round transducers face outward, away from loose wires.
Connect
VCC → 5V,Trig → GPIO 13,Echo → GPIO 14, andGND → GND.Pause and compare your wiring to the chart before powering or uploading.
Open
Sketch_19.1_Ultrasonic_Ranging.inoin Arduino IDE.Upload the sketch to the ESP32-S3.
Open Serial Monitor and set the baud rate to
115200.Hold a flat object 10–30 cm in front of the sensor and move it slowly closer and farther.
04 Read just enough code
Read the code
You don't need the whole sketch today — just four lines that carry the whole idea: name the two pins, time the echo, then turn that time into centimetres. Switch to MicroPython if you'd rather see the same idea in Python; the wiring stays the same.
#define trigPin 13
#define echoPin 14
pingTime = pulseIn(echoPin, HIGH, timeOut);
distance = (float)pingTime * soundVelocity / 2 / 10000;#define trigPin 13Gives GPIO 13 the name trigPin, so the pin's job stays readable wherever the sketch drives it. pulseIn(echoPin, HIGH, timeOut)Starts a stopwatch when Echo rises and stops it when Echo falls, returning the sound's round-trip time — the one measurement everything else is built from. … / 2 / 10000Halves the round trip to get the one-way distance, then scales the microsecond timing down into centimetres. Optional side path · same circuit
trigPin = Pin(13, Pin.OUT, 0)
echoPin = Pin(14, Pin.IN, 0)Pin(13, Pin.OUT, 0)Sets GPIO 13 as the trigger output — the board's side of the ping.Pin(14, Pin.IN, 0)Sets GPIO 14 as the echo input — where the return pulse arrives.Same pins, same behaviour. Run it in Thonny and compare the printed distance with the Arduino version. If MicroPython isn't set up yet, skip this — it should never block the Arduino-first day.
05 Understand, don't memorise
Measuring distance by timing an echo
The trick behind the whole day is that the HC-SR04 measures time, not distance. It fires a sound pulse, then reports exactly how long that pulse takes to reach an object and return. Because sound moves at a known, steady speed, the board can turn that time into centimetres. It's the same idea as counting the seconds between lightning and thunder to judge how far off a storm is — you're timing something at a known speed and reading off a distance.
Send the pulse
The board holds Trig HIGH for about 10 microseconds. That short command tells the HC-SR04 to emit a burst of ultrasonic clicks, a pitch far above what you can hear.
Echo marks the flight
The instant the burst leaves, the sensor raises the Echo pin and holds it HIGH. It keeps Echo HIGH for exactly as long as the sound is in flight — out to the object and back again.
pulseIn times it
pulseIn(echoPin, HIGH, timeOut) starts a stopwatch when Echo rises and stops it when Echo falls. What it hands back is the round-trip time, measured in microseconds.
Speed turns time into length
Sound travels at a fixed speed in room air, about 343 metres per second. Multiply that speed by the round-trip time and you get the total path the sound covered.
Halve the round trip
The sound went to the object and came straight back, so it covered the gap twice. Divide the total path by two, and you're left with the one-way distance to the object.
The model
distance = speed of sound × echo time ÷ 2
Why you divide by two
The pulse makes a round trip — sensor to object, then object back to sensor. The raw echo time covers both legs, so halving it leaves the single one-way distance you actually want.
Why the speed of sound matters
The whole conversion leans on sound moving at a known speed. That speed shifts slightly with air temperature, which is one reason a reading can drift by a little even when nothing has moved.
Why flat targets read cleaner
A flat surface square to the sensor bounces most of the sound straight back, giving Echo a strong, sharp pulse to time. Curved or soft targets scatter the sound, so the return is weaker and the timing less certain.
06 Know it worked
Test & debug
Success and recovery sit side by side, so you never have to go hunting when something looks off.
What you should see
Distance: 24.31cmDistance: 23.92cmDistance: 24.08cm
A good reading changes when you move the object. Small wobble is fine.
If it doesn't
- 0 cm or frozen? Check Trig, Echo, GND, and the target's shape.
- Blank monitor? Confirm 115200 baud, then the board and port.
- Upload fails? Swap in a data-capable USB cable.
- Readings jump? Aim straight at a flat object.
07 Make the idea yours
Try this: check the sensor against a ruler
You've got a distance sensor and a claim to test — that timing an echo really does measure length. Hold it to a ruler and see. It fits inside today's 30 minutes.
Measure it, then check it
Stand a book at a ruler-measured 15 cm from the sensor face, read what Serial Monitor reports, then repeat at 30 cm. Close agreement means the time-to-distance math is sound. A steady offset — the reading always a centimetre or two off in the same direction — tells you where the sensor's real zero point sits: the transducer face, not the board edge you measured from.
Break the echo on purpose
Aim at a flat wall and note a steady reading. Now swap in a folded towel, or tilt the target to about 45 degrees, and watch the number get weaker, jumpier, or drop to zero. You're seeing the mechanism from the outside: a clean reading needs a strong echo, and soft or angled surfaces scatter the sound away instead of bouncing it back.
08 Learn it with a hand on the tiller
Coach me through it
Every lesson ships with a code and a machine-readable packet, so an agent can guide you with full context.
Lesson code
TSK-DAY26-ULTRASONIC
Context packet
How the agent should behave: guide one physical step at a time and wait for the learner to confirm. Once the circuit is reading, teach the real method — the sensor times an echo and the board converts that time into distance — and make sure the divide-by-two lands. Always check wiring, board, port, and USB before changing code.
Keep your place
Finished Day 26?
Mark it complete — it shows on your course map, and your place is saved on this device.