Fig. 10 · Robotics Lab · Distance from an echo you can time

HC-SR04 parking sensor

Arduino Nano

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Bench

No reading yet

Distance

No reading yet

Current draw

0 mA

Build and break

1BuildNext up
Strip to the bare PCB
Take every part off the blue board. You will put it back the way it is made: the chips on the back first, then the crystal and the header, then the two cans.
2
Pick and place the SMD parts, then reflow
3
Fit the crystal and the header
4
Fit the two cans
5
Solder the through-hole parts
6
Wire the Nano
7
Wire the buzzer
8
Power the bench
9
Upload Ping and print
10
Upload Parking beeper
11
Upload Ping a cushion
12
Upload Nothing in range
13
Wire Vcc and Gnd the wrong way roundBreak it
14
Power Vcc from the Nano's 3V3 pinBreak it
15
Pull the receiverBreak it
16
Leave a cold joint on EchoBreak it
17
Forget pulseIn()'s timeoutBreak it
18
Short Vcc to GndBreak it
19
Exploded view

Break it on purpose

Each fault is one that catches real builders. Trigger it, read the symptom on the bench, then fix it.

Vcc reversed
reverse polarity
Why it happens
Symptom: The chips get hot within seconds and the Nano's USB may drop out. Afterwards the board often reads nothing.
Nothing on the HC-SR04 blocks a reversed supply: current flows backwards through the protection diodes inside every chip. The MAX3232 and the MCU usually die.
Fix: Unplug at once, wire Vcc to 5V and Gnd to GND. Connect Gnd first, as the datasheets say.
Only 3.3 V on Vcc
wrong voltage
Why it happens
Symptom: Every ping reads no echo. The burst rings go out faint and nothing comes back.
The MCU runs on 3.3 V, but the MAX3232's drive and the LM324's gain need 4.5-5.5 V. The burst is too weak and the receiver too deaf to hear it come back.
Fix: Power Vcc from 5V. On a 3.3 V board use the HC-SR04P, which runs from 3.0 V.
No receiver
missing part
Why it happens
Symptom: Every ping reads no echo, although the burst goes out.
With R gone nothing hears the echo, so the MCU waits its 38 ms and drops Echo. A cold joint on a can lead does the same.
Fix: Fit R and solder both leads.
Cold joint on Echo
cold solder joint
Why it happens
Symptom: Readings come and go: a good distance, then no echo, then a good one again. Touching the board changes it.
Through a joint that makes and breaks, the Echo pulse sometimes never reaches the Nano, and pulseIn() times out.
Fix: Reheat the joint with fresh solder until it flows into a shiny cone.
pulseIn() with no timeout
wrong setting
Why it happens
Symptom: An empty room reads 655 cm, beyond the sensor's 4 m, and each ping takes longer.
With nothing to echo, the board holds Echo HIGH for its own timeout, about 38 ms, and drops it. pulseIn() with no timeout waits up to a second, measures that 38 ms pulse and the sketch prints it as 38000 / 58 = 655 cm.
Fix: Pass a timeout, pulseIn(ECHO, HIGH, 30000UL), and treat 0 as "no echo", not as a distance.
Vcc shorted to Gnd
short circuit
Why it happens
Symptom: Everything goes dark: the Nano resets or its USB drops out, and its polyfuse gets warm.
The board has no fuse of its own. The short pulls the Nano's 5 V down until its USB polyfuse limits the current.
Fix: Unplug, remove the stray wire and check the header pins for a solder bridge.

Meet the bench

build the sonar board, ping it, then make a buzzer beep faster as a hand comes closer

A robot that drives into walls needs to know how far away they are. The HC-SR04 shouts a burst of 40 kHz sound from one can and listens for the echo with the other; its Echo pin stays HIGH for the round trip. Sound covers a centimetre and back in 58 µs, so a 1740 µs pulse means a wall 30 cm away. Build the board, ping it from a Nano, then make a buzzer beep faster as a hand comes closer, like a car's parking sensor. Then break it: power it backwards or from 3.3 V, pull the receiver, leave a cold joint on Echo, forget pulseIn()'s timeout and short its supply.

Sources

On the bench

Selected

Sketch uses 2,340 bytes of program storage

Code: Ping and print

// Ping and print: a wall 30 cm in front
const int TRIG = 9;
const int ECHO = 10;

float ping() {
  digitalWrite(TRIG, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG, HIGH);   // a HIGH of at least 10 µs starts one ping
  delayMicroseconds(10);
  digitalWrite(TRIG, LOW);
  unsigned long us = pulseIn(ECHO, HIGH, 30000UL);   // give up after 30 ms (about 5 m): 0 means no echo
  if (us == 0) return -1;
  return us / 58.0;   // 58 µs of round trip per cm
}

void setup() {
  Serial.begin(9600);
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);
  Serial.println("hc-sr04: one ping a second");
}

void loop() {
  float cm = ping();
  if (cm < 0) {
    Serial.println("no echo");   // 0 is not "very close": nothing came back
  } else {
    Serial.print(cm, 2);
    Serial.println(" cm");
  }
  delay(1000);
}

Practice tasks

0/2 tasks

Write the code, then run the checks. The run uses a virtual clock and a scripted world, so the same code always gives the same result.

Don't brake in an empty room

Arduino C++

This robot's bumper sketch lights the brake (D13) when something is closer than 20 cm. It works against a wall, but in an open room it brakes too: when nothing echoes, pulseIn() returns 0 and 0 cm is "closer than 20". Brake only when there really was an echo under 20 cm. The wall is 80 cm off, 15 cm at 1.5 s, gone at 3 s and 8 cm away at 4.5 s.

Checks

Not run yetRuns
Not run yetWall at 80 cm: no brake
Not run yetWall at 15 cm: brake
Not run yetEmpty room: no brake
Not run yetWall at 8 cm: brake

Hints

ping() returns 0 when nothing came back. 0 is not a distance.
Both must be true: there was an echo, and it was close.
bool brake = cm > 0 && cm < 20;

Code for Don't brake in an empty room

How fast is it coming?

Arduino C++

A robot needs to know not just how far but how fast. The sketch pings every 500 ms. Work out the closing speed from this distance, the last one and the time between them, and add ", approaching at N cm/s", ", moving away at N cm/s" or ", holding" (within 2 cm/s) to each line. A hand comes from 1 m to 30 cm at 20 cm/s, holds, then backs off.

Checks

Not run yetRuns
Not run yetSays it is approaching at about 20 cm/s
Not run yetSays holding when the hand stops at 30 cm
Not run yetSays moving away at about 20 cm/s

Hints

speed = (last - cm) / seconds between the pings: positive when it comes closer.
Keep last and lastAt from the previous ping; skip the speed when either reading is no echo.
Serial.print(speed, 0) prints it with no decimals.

Code for How fast is it coming?

Build and break

1BuildNext up

Strip to the bare PCB

Take every part off the blue board. You will put it back the way it is made: the chips on the back first, then the crystal and the header, then the two cans.

Pick and place the SMD parts, then reflow

Fit the crystal and the header

Fit the two cans

Solder the through-hole parts

Wire the Nano

Wire the buzzer

Power the bench

Upload Ping and print

Upload Parking beeper

Upload Ping a cushion

Upload Nothing in range

Wire Vcc and Gnd the wrong way roundBreak it

Power Vcc from the Nano's 3V3 pinBreak it

Pull the receiverBreak it

Leave a cold joint on EchoBreak it

Forget pulseIn()'s timeoutBreak it

Short Vcc to GndBreak it

Exploded view

Meet the bench

build the sonar board, ping it, then make a buzzer beep faster as a hand comes closer

On the bench

Selected

Sketch uses 2,340 bytes of program storage

Code: Ping and print

// Ping and print: a wall 30 cm in front
const int TRIG = 9;
const int ECHO = 10;

float ping() {
  digitalWrite(TRIG, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG, HIGH);   // a HIGH of at least 10 µs starts one ping
  delayMicroseconds(10);
  digitalWrite(TRIG, LOW);
  unsigned long us = pulseIn(ECHO, HIGH, 30000UL);   // give up after 30 ms (about 5 m): 0 means no echo
  if (us == 0) return -1;
  return us / 58.0;   // 58 µs of round trip per cm
}

void setup() {
  Serial.begin(9600);
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);
  Serial.println("hc-sr04: one ping a second");
}

void loop() {
  float cm = ping();
  if (cm < 0) {
    Serial.println("no echo");   // 0 is not "very close": nothing came back
  } else {
    Serial.print(cm, 2);
    Serial.println(" cm");
  }
  delay(1000);
}

HC-SR04 parking sensor

The bench

Build and break

Strip to the bare PCB

Pick and place the SMD parts, then reflow

Fit the crystal and the header

Fit the two cans

Solder the through-hole parts

Wire the Nano

Wire the buzzer

Power the bench

Upload Ping and print

Upload Parking beeper

Upload Ping a cushion

Upload Nothing in range

Wire Vcc and Gnd the wrong way roundBreak it

Power Vcc from the Nano's 3V3 pinBreak it

Pull the receiverBreak it

Leave a cold joint on EchoBreak it

Forget pulseIn()'s timeoutBreak it

Short Vcc to GndBreak it

Exploded view

Break it on purpose

Vcc reversed

Only 3.3 V on Vcc

No receiver

Cold joint on Echo

pulseIn() with no timeout

Vcc shorted to Gnd

Live status

Current draw

Check yourself

After: Solder the through-hole parts

After: Upload Parking beeper

After: Short Vcc to Gnd

End of bench quiz

Meet the bench

Sources

On the bench

Practice tasks

Don't brake in an empty room

Checks

How fast is it coming?

Checks

HC-SR04 parking sensor

The bench

Build and break

Strip to the bare PCB

Pick and place the SMD parts, then reflow

Fit the crystal and the header

Fit the two cans

Solder the through-hole parts

Wire the Nano

Wire the buzzer

Power the bench

Upload Ping and print

Upload Parking beeper

Upload Ping a cushion

Upload Nothing in range

Wire Vcc and Gnd the wrong way roundBreak it

Power Vcc from the Nano's 3V3 pinBreak it

Pull the receiverBreak it

Leave a cold joint on EchoBreak it

Forget pulseIn()'s timeoutBreak it

Short Vcc to GndBreak it

Exploded view

Break it on purpose

Vcc reversed

Only 3.3 V on Vcc

No receiver

Cold joint on Echo

pulseIn() with no timeout

Vcc shorted to Gnd

Live status

Current draw

Check yourself

After: Solder the through-hole parts

After: Upload Parking beeper

After: Short Vcc to Gnd

End of bench quiz