Fig. 09 · Robotics Lab · A gyro and an accelerometer on two wires

MPU-6050 tilt alarm

Arduino Nano

Loading the figure…

Bench

No reading yet

Roll

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 chip, the regulator and the small parts first, then the header.
2
Pick and place the SMD parts, then reflow
3
Fit the 8-pin header
4
Solder the header
5
Wire the Nano
6
Wire the buzzer
7
Power the bench
8
Upload Raw readings
9
Upload Tilt alarm
10
Upload Never woken
11
Upload I2C scan
12
Wire VCC and GND the wrong way roundBreak it
13
Power VCC from a 2S LiPoBreak it
14
Leave out C14Break it
15
Leave out R6Break it
16
Leave a cold joint on SDABreak it
17
Fit C10 backwardsBreak it
18
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 LED stays dark and the regulator gets hot; afterwards the board may read nothing.
Nothing on the GY-521 blocks a reversed supply. Current flows backwards through the regulator's and the chip's protection diodes, limited only by the Nano's USB. A few seconds can kill the 662K and sometimes the MPU-6050.
Fix: Unplug at once, wire VCC to 5V and GND to GND, and replace the regulator if it got hot.
7.4 V on VCC
wrong voltage
Why it happens
Symptom: The regulator burns: smell, a discoloured package. Then nothing answers on the bus.
The 662K (XC6206) is rated for 6 V in. A 2S LiPo gives 7.4-8.4 V. If the regulator fails short, the battery voltage reaches the MPU-6050 too, which takes 3.46 V.
Fix: Power VCC from the Nano's 5V, never from the robot's battery or VIN.
No CPOUT capacitor
missing part
Why it happens
Symptom: gx jumps by tens of °/s while the board lies still. The accelerometer is fine.
The gyro's charge pump needs its capacitor to make the voltage that drives the sensing masses. Without it the gyro's output is noise. The Arduino Playground warns about this: a wrong capacitor gives a high noise level.
Fix: Fit C14 (2.2 nF) and reflow it.
No AD0 pull-down
floating input
Why it happens
Symptom: The buzzer sounds on a robot standing still: the sketch prints roll -135° and ALARM.
With AD0 floating high the chip answers at 0x69. The sketch asks 0x68, nobody answers, and Wire.read() returns -1 for every byte. Every value reads -1 count, and atan2(-1, -1) is -135°.
Fix: Fit R6, or set MPU to 0x69 in the sketch. Better: check that requestFrom() got its 14 bytes before using them.
Cold joint on SDA
cold solder joint
Why it happens
Symptom: Short false alarms: now and then one read comes back as roll -135°.
Through a joint that makes and breaks, some reads get no answer and come back as -1 everywhere. An alarm that trusts a single read beeps on every one.
Fix: Reheat the joint with fresh solder until it flows into a shiny cone. In the sketch, ignore a read that failed.
C10 in backwards
part in backwards
Why it happens
Symptom: The tantalum gets hot the moment the board is powered, and may crack or smoke.
A tantalum capacitor's oxide layer only blocks current one way. Reversed, it conducts, heats up and can fail short.
Fix: Unplug, fit a new one with the stripe (+) on the 3.3 V side, as printed.

Meet the bench

build the GY-521, wake it, then make it warn when the robot tips

A robot needs to know which way is down. The MPU-6050 on this small blue GY-521 board measures acceleration on three axes, gravity included, and rotation rate on three more, and hands all of it over I2C at address 0x68. Lying flat it reads 1 g on Z; tipped, part of that 1 g moves to Y, and atan2(ay, az) gives the angle. Build the board, read it from a Nano with plain Wire calls, then make a buzzer sound when the robot tips past 20°. Then break it: power it backwards or from a LiPo, leave out the capacitor the gyro needs, lose the address pull-down, leave a cold SDA joint and fit the tantalum backwards.

Sources

On the bench

Selected

Sketch uses 4,598 bytes of program storage

Code: Raw readings

// MPU-6050: raw I2C reads, no library
#include <Wire.h>

const int MPU = 0x68;   // AD0 low: address 0x68
int16_t ax, ay, az, gx;

void wake() {
  Wire.beginTransmission(MPU);
  Wire.write(0x6B);     // PWR_MGMT_1: write 0 to wake the chip (it powers up asleep)
  Wire.write(0);
  Wire.endTransmission();
}

void readImu() {
  Wire.beginTransmission(MPU);
  Wire.write(0x3B);     // ACCEL_XOUT_H: 14 bytes from here are accel X, Y, Z, temperature, gyro X, Y, Z
  Wire.endTransmission(false);
  Wire.requestFrom(MPU, 14);
  ax = Wire.read() << 8 | Wire.read();
  ay = Wire.read() << 8 | Wire.read();
  az = Wire.read() << 8 | Wire.read();
  Wire.read(); Wire.read();   // the temperature: skipped
  gx = Wire.read() << 8 | Wire.read();
  Wire.read(); Wire.read(); Wire.read(); Wire.read();
}

void setup() {
  Serial.begin(9600);
  Wire.begin();
  wake();
  Serial.println("mpu6050: awake, +-2 g, +-250 deg/s");
}

void loop() {
  readImu();
  Serial.print("ax ");
  Serial.print(ax / 16384.0, 2);   // 16384 counts = 1 g at ±2 g
  Serial.print(" ay ");
  Serial.print(ay / 16384.0, 2);
  Serial.print(" az ");
  Serial.print(az / 16384.0, 2);
  Serial.print(" g | gx ");
  Serial.print(gx / 131.0, 1);     // 131 counts = 1 °/s at ±250 °/s
  Serial.println(" deg/s");
  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.

Sound the alarm past 20°

Arduino C++

The starter reads the tilt and prints it, but nothing warns anyone. When the roll is past 20° either way, set D13 HIGH and play a 2 kHz tone on the buzzer (D8); otherwise D13 LOW and noTone(). The board is level at first, tipped 35° at 2 s, back to -10° at 3.5 s and tipped -40° at 5 s.

Checks

Not run yetRuns
Not run yetLevel: D13 LOW
Not run yetTipped 35°: D13 HIGH
Not run yetTipped 35°: the buzzer sounds at about 2 kHz
Not run yetBack to -10°: D13 LOW again
Not run yetTipped -40°: D13 HIGH
Not run yetTipped -40°: the buzzer sounds

Hints

abs(roll) > 20 catches both directions.
tone(BUZZER, 2000) keeps playing until noTone(BUZZER).
digitalWrite(13, alarm ? HIGH : LOW); sets the LED in one line.

Code for Sound the alarm past 20°

// Warn with D13 and the buzzer when the robot tips past 20°
#include <Wire.h>

const int MPU = 0x68;   // AD0 low: address 0x68
int16_t ax, ay, az, gx;

void wake() {
  Wire.beginTransmission(MPU);
  Wire.write(0x6B);     // PWR_MGMT_1: write 0 to wake the chip (it powers up asleep)
  Wire.write(0);
  Wire.endTransmission();
}

void readImu() {
  Wire.beginTransmission(MPU);
  Wire.write(0x3B);     // ACCEL_XOUT_H: 14 bytes from here are accel X, Y, Z, temperature, gyro X, Y, Z
  Wire.endTransmission(false);
  Wire.requestFrom(MPU, 14);
  ax = Wire.read() << 8 | Wire.read();
  ay = Wire.read() << 8 | Wire.read();
  az = Wire.read() << 8 | Wire.read();
  Wire.read(); Wire.read();   // the temperature: skipped
  gx = Wire.read() << 8 | Wire.read();
  Wire.read(); Wire.read(); Wire.read(); Wire.read();
}

const int BUZZER = 8;
unsigned long lastPrint = 0;

void setup() {
  Serial.begin(9600);
  Wire.begin();
  wake();
  pinMode(13, OUTPUT);
  Serial.println("tilt alarm: beeps past 20 deg");
}

void loop() {
  readImu();
  float roll = atan2(ay, az) * 180.0 / PI;
  // past 20° either way: D13 HIGH and a 2 kHz tone; otherwise D13 LOW and noTone()
  if (millis() - lastPrint >= 1000) {
    lastPrint = millis();
    Serial.print("roll ");
    Serial.print(roll, 0);
    Serial.print(" deg");
    if (abs(roll) > 20) Serial.print(" ALARM");
    Serial.println();
  }
  delay(100);
}

Calibrate the gyro

Arduino C++

The board lies still, yet gx reads about 0.6 °/s: the gyro's zero-rate offset, up to ±20 °/s on this chip. Integrate that and the angle drifts away. In setup(), read gx 200 times, average the counts into offset and print it as "calibrated: offset N counts". loop() already subtracts it.

Checks

Not run yetRuns
Not run yetPrints the offset it measured, about 79 counts
Not run yetAfter calibrating, gx reads 0.00 °/s at rest

Hints

long sum = 0; for (int i = 0; i < 200; i++) { sum += readGx(); delay(2); }
offset = sum / 200; keeps it in counts, like the reads.
Keep the board still while it calibrates: any turn becomes part of the offset.

Code for Calibrate the gyro

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 chip, the regulator and the small parts first, then the header.

Pick and place the SMD parts, then reflow

Fit the 8-pin header

Solder the header

Wire the Nano

Wire the buzzer

Power the bench

Upload Raw readings

Upload Tilt alarm

Upload Never woken

Upload I2C scan

Wire VCC and GND the wrong way roundBreak it

Power VCC from a 2S LiPoBreak it

Leave out C14Break it

Leave out R6Break it

Leave a cold joint on SDABreak it

Fit C10 backwardsBreak it

Exploded view

Meet the bench

build the GY-521, wake it, then make it warn when the robot tips

On the bench

Selected

Sketch uses 4,598 bytes of program storage

Code: Raw readings

// MPU-6050: raw I2C reads, no library
#include <Wire.h>

const int MPU = 0x68;   // AD0 low: address 0x68
int16_t ax, ay, az, gx;

void wake() {
  Wire.beginTransmission(MPU);
  Wire.write(0x6B);     // PWR_MGMT_1: write 0 to wake the chip (it powers up asleep)
  Wire.write(0);
  Wire.endTransmission();
}

void readImu() {
  Wire.beginTransmission(MPU);
  Wire.write(0x3B);     // ACCEL_XOUT_H: 14 bytes from here are accel X, Y, Z, temperature, gyro X, Y, Z
  Wire.endTransmission(false);
  Wire.requestFrom(MPU, 14);
  ax = Wire.read() << 8 | Wire.read();
  ay = Wire.read() << 8 | Wire.read();
  az = Wire.read() << 8 | Wire.read();
  Wire.read(); Wire.read();   // the temperature: skipped
  gx = Wire.read() << 8 | Wire.read();
  Wire.read(); Wire.read(); Wire.read(); Wire.read();
}

void setup() {
  Serial.begin(9600);
  Wire.begin();
  wake();
  Serial.println("mpu6050: awake, +-2 g, +-250 deg/s");
}

void loop() {
  readImu();
  Serial.print("ax ");
  Serial.print(ax / 16384.0, 2);   // 16384 counts = 1 g at ±2 g
  Serial.print(" ay ");
  Serial.print(ay / 16384.0, 2);
  Serial.print(" az ");
  Serial.print(az / 16384.0, 2);
  Serial.print(" g | gx ");
  Serial.print(gx / 131.0, 1);     // 131 counts = 1 °/s at ±250 °/s
  Serial.println(" deg/s");
  delay(1000);
}

MPU-6050 tilt alarm

The bench

Build and break

Strip to the bare PCB

Pick and place the SMD parts, then reflow

Fit the 8-pin header

Solder the header

Wire the Nano

Wire the buzzer

Power the bench

Upload Raw readings

Upload Tilt alarm

Upload Never woken

Upload I2C scan

Wire VCC and GND the wrong way roundBreak it

Power VCC from a 2S LiPoBreak it

Leave out C14Break it

Leave out R6Break it

Leave a cold joint on SDABreak it

Fit C10 backwardsBreak it

Exploded view

Break it on purpose

VCC reversed

7.4 V on VCC

No CPOUT capacitor

No AD0 pull-down

Cold joint on SDA

C10 in backwards

Live status

Current draw

Check yourself

After: Solder the header

After: Upload Tilt alarm

After: Fit C10 backwards

End of bench quiz

Meet the bench

Sources

On the bench

Practice tasks

Sound the alarm past 20°

Checks

Calibrate the gyro

Checks

MPU-6050 tilt alarm

The bench

Build and break

Strip to the bare PCB

Pick and place the SMD parts, then reflow

Fit the 8-pin header

Solder the header

Wire the Nano

Wire the buzzer

Power the bench

Upload Raw readings

Upload Tilt alarm

Upload Never woken

Upload I2C scan

Wire VCC and GND the wrong way roundBreak it

Power VCC from a 2S LiPoBreak it

Leave out C14Break it

Leave out R6Break it

Leave a cold joint on SDABreak it

Fit C10 backwardsBreak it

Exploded view

Break it on purpose

VCC reversed

7.4 V on VCC

No CPOUT capacitor

No AD0 pull-down

Cold joint on SDA

C10 in backwards

Live status

Current draw

Check yourself

After: Solder the header

After: Upload Tilt alarm

After: Fit C10 backwards

End of bench quiz

Meet the bench

Sources