Sensor validation

Time estimate

Scheduled time: 11:35 AM – 12:15 PM (40 minutes)

Your PiCar-X has an ultrasonic distance sensor, a 3-channel grayscale sensor, three servos, two DC drive motors, a camera, a battery pack, and a speaker. Before you write any code that depends on these, you need to prove every one of them works. The test suite walks you through each component one by one.

Running the test suite

SSH into your robot

Open a terminal and connect to your Pi over SSH. Use the hostname and username you set during imaging.

ssh car@picar.local

Launch the test script

Run the test suite with sudo (required for hardware access):

sudo python3 ~/camp/day1/test_all_sensors.py

The script initializes the PiCar-X hardware and drops you into an interactive menu.

Use the menu

You see a menu like this:

============================================================
  PiCar-X  —  Sensor & Actuator Test Menu
============================================================
    1.  [ — ]  Ultrasonic Sensor
    2.  [ — ]  Grayscale Sensor
    3.  [ — ]  Servos
    4.  [ — ]  DC Motors
    5.  [ — ]  Camera
    6.  [ — ]  Battery / Speaker

    a.  Run ALL tests
    r.  Show results summary
    q.  Quit
============================================================

Type 1–6 to run a single test.
Type a to run every test in order.
Type r to print a results summary at any time.
Type q to quit (the summary prints automatically on exit).

You can re-run any test as many times as you want. The menu updates with [PASS], [FAIL], or [SKIP] after each test.

Confirm each result

At the end of every test, the script asks:

Did the <test name> test work?  [y]es / [n]o / [s]kip:

Press y if the hardware behaved as expected.
Press n if something was wrong.
Press s if you want to skip it for now.

You can always come back and re-run a test to change your answer.

Press Ctrl-C during any test to skip the remaining readings and jump straight to the pass/fail prompt.

Each test in detail

Test 1 — Ultrasonic distance sensor

What does this test do?

The script reads the HC-SR04 ultrasonic sensor 10 times, one reading per second. Each reading shows the distance to the nearest object in centimeters, along with a small bar chart.

Reading  1/10  |  Distance:   23.45 cm  |  #######################
Reading  2/10  |  Distance:   12.10 cm  |  ############
Reading  3/10  |  Distance:     — cm    |  (no echo — nothing in range)

What you should see: distance values that change as you move your hand closer to and farther from the two round “eyes” on the front of the robot. Expected range: 2–400 cm. A value of –1.00 (displayed as —) means “no echo received” — the sensor didn’t detect anything within range. This is normal if nothing is in front of the robot. PASS criteria: values change when you move your hand near the sensor.

If every single reading is — (no echo), the ultrasonic module may not be connected. Power off the robot, check the 4-pin cable between the sensor and the Robot HAT, then re-run the test.

Test 2 — Grayscale sensor

What does this test do?

The script reads all 3 channels (Left, Center, Right) of the grayscale sensor module 10 times, one reading per second. It also reports the cliff detection status and line-following status for each reading.

   #      Left    Center     Right    Cliff?    Line status
   —      ————    ——————     —————    ——————    ———————————
   1     1024.0   1100.5     980.3        no         0
   2      320.0   1095.2     970.1        no         1

What you should see:

High values (800–1400) on light surfaces (white paper, light-colored desk).
Low values (100–400) on dark surfaces (black tape, dark paper).

PASS criteria: values are responsive to surface changes. Place something dark under one sensor at a time and watch that channel drop. If you calibrated in the previous step, the Cliff column should show no on a normal surface and YES when the sensor is lifted off the table.

If all three channels read very similar values regardless of surface color, the sensor may be too far from the surface. Make sure the robot is sitting flat on the table with the grayscale module close to the surface.

Test 3 — Servos

What does this test do?

The script sweeps all 3 servos through their full range of motion (–35° to +35°), one at a time:

Steering servo — turns the front wheels left and right.
Camera pan servo — rotates the camera horizontally.
Camera tilt servo — tilts the camera up and down.

Each servo sweeps out, sweeps back, and returns to center (0°).

What you should see: smooth, continuous movement on each axis. All three servos return to center when the sweep finishes. PASS criteria: all 3 servos move without grinding, stuttering, or stalling.

Servo movement is jerky or stuttering

This almost always means low battery. The servos draw significant current during movement. Charge your batteries and try again.

One or more servos don't move at all

Power off the robot and check the servo cable connections on the Robot HAT. Each servo plugs into a specific 3-pin header — make sure none are loose or plugged into the wrong port.

Test 4 — DC motors

Prop the car up on a book or box, or clear a large space on the table before running this test. The wheels will spin and the car will drive forward and backward.

What does this test do?

The script runs both DC drive motors at 30% speed:

Forward for 2 seconds.
Pause for half a second.
Backward for 2 seconds.
Stop.

What you should see: both wheels spin forward together, then both spin backward together. PASS criteria: both wheels spin and the car moves in the correct direction (forward first, then backward).

Car goes backward when it should go forward

The motor direction needs to be swapped. Run the calibration script again and use option 2 (motor direction):

sudo python3 ~/camp/day1/calibrate_picar.py

Select option 2, then follow the prompts to reverse the motor direction.

Only one wheel spins

Check the motor cable connections on the Robot HAT. One of the two motor connectors may be loose.

Neither wheel spins

Make sure the Robot HAT power switch is on and batteries are charged. Motors require more current than sensors — a low battery can power the Pi but not the motors.

Test 5 — Camera

What does this test do?

The script captures a single still image and saves it to ~/test_capture.jpg. It uses picamera2 if available, falling back to libcamera-still. After capturing, it prints the file size so you can verify the image is real.

Image saved to /home/car/test_capture.jpg
File size: 1,245,678 bytes
[INFO] Looks good — file is large enough to be a real image.

What you should see: an “Image saved” message with a file size greater than 1,000 bytes. PASS criteria: the file is created and is large enough to be a real image (not a 0-byte placeholder).

You can copy the test image to your laptop to view it:

scp car@picar.local:~/test_capture.jpg .

Open the file on your computer to confirm the camera captured a real photo.

"Camera not found" or capture failed

The CSI ribbon cable is likely loose. Power off the Pi, gently reseat the flat ribbon cable where it plugs into the camera port on the Pi, and make sure the locking clip is pushed down. Then power back on and re-run the test.

File size is 0 or very small

The camera module may be defective or the ribbon cable is partially disconnected. Try reseating the cable at both ends (the camera board and the Pi board).

Test 6 — Battery and speaker

This test has two parts.

Battery voltage

What does this test do?

The script reads the battery voltage from the Robot HAT’s ADC (analog-to-digital converter) on channel A4. It converts the raw ADC reading to a voltage estimate.

ADC raw value : 3150
Est. voltage  : 7.62 V
[INFO] Battery looks healthy (> 6V)

The PiCar-X uses 2S 18650 Li-ion batteries (two cells in series). Here is how to interpret the voltage reading:

Voltage	Status	Action
~8.4 V	Full charge	You’re good to go
7.0–8.4 V	OK	Normal operating range
6.0–7.0 V	Low	Charge soon — some features (speaker, motors) may underperform
Below 6.0 V	Critical	Stop and charge immediately — the cells can be damaged by deep discharge

You can monitor battery voltage at any time without running the full test suite:

python3 ~/camp/day1/battery_check.py

For live monitoring that updates every 2 seconds, add the --watch flag:

python3 ~/camp/day1/battery_check.py --watch

Press Ctrl-C to stop the live monitor.

Speaker

What does this test do?

The script plays a 4-second sine wave test tone through the Robot HAT’s onboard speaker, then follows up with a text-to-speech phrase (“Hello. Picar X speaker test.”) using espeak.

What you should hear: a steady tone for about 4 seconds, then a robotic voice saying “Hello. Picar X speaker test.” PASS criteria: you hear the tone and/or the voice clearly.

The speaker is powered through the battery, not the USB port. You need charged batteries (above 7 V) for the onboard amplifier to produce audible sound. If you don’t hear anything, check your battery voltage first.

Results summary

After you finish all six tests (or whenever you want a status check), press r at the menu to see the summary:

============================================================
  TEST RESULTS SUMMARY
============================================================
  [PASS]  Ultrasonic Sensor
  [PASS]  Grayscale Sensor
  [PASS]  Servos
  [PASS]  DC Motors
  [PASS]  Camera
  [SKIP]  Battery / Speaker

  5/6 passed, 0 failed, 1 skipped, 0 not run
============================================================

Your goal: all 6 tests show [PASS].

It’s OK if the speaker shows [SKIP] because your batteries are low. You can always test the speaker later after charging. Everything else should be [PASS] before you move on to Day 2.

Troubleshooting

Symptom	Likely cause	Fix
All tests fail or script errors on launch	Robot HAT not powered on	Turn on the power switch on the Robot HAT board
Ultrasonic always reads `—` (no echo)	Sensor not connected	Check the 4-pin cable between the HC-SR04 module and the Robot HAT
Servos don’t move	Low battery	Charge batteries above 7 V and re-run
Camera error or “not found”	CSI ribbon cable loose	Power off, reseat the ribbon cable, power on
No speaker sound	Battery below 7 V	Charge batteries — the amplifier needs sufficient voltage
Motors go the wrong direction	Not calibrated	Run `sudo python3 ~/camp/day1/calibrate_picar.py` and use option 2
Only one motor spins	Loose motor cable	Check both motor connectors on the Robot HAT
Grayscale values don’t change	Sensor too far from surface	Make sure the robot sits flat with the sensor module close to the table

Quick fixes with the troubleshoot script:

Speaker not working? Run bash ~/camp/troubleshoot.sh and select option 1.
Camera not working? Run bash ~/camp/troubleshoot.sh and select option 2.

Day 1 wrap-up

Checkpoint: every student ends Day 1 with all sensors showing [PASS] and a confirmed SSH connection to their robot.

What you accomplished today

Imaged an SD card with Raspberry Pi OS and configured Wi-Fi, SSH, and Pi Connect.
Connected to your PiCar-X over SSH for the first time.
Installed all SunFounder libraries (robot-hat, vilib, picar-x).
Calibrated the steering servo, motor direction, and grayscale cliff threshold.
Validated every sensor and actuator on the robot with the test suite.

What’s coming on Day 2

Tomorrow you start writing real programs for your robot:

Movement programs — drive in patterns, steer with code, and control speed.
Text-to-speech — make the robot talk using espeak.
Line following — use the grayscale sensor to follow a track.
Computer vision — process camera frames to detect objects and colors.

Homework: leave your robot powered on and connected to Wi-Fi overnight. Tomorrow morning you should be able to SSH right in and start coding without any setup delay.

Welcome

Class Recordings

Day 1: Setup and Calibration

Day 2: Code & Computer Vision

Day 3: GenAI and Cloud LLMs

Day 4: Vision AI

Day 5: AI Ethics & Final Project

Time estimate

Running the test suite

Each test in detail

Test 1 — Ultrasonic distance sensor

Test 2 — Grayscale sensor

Test 3 — Servos

Test 4 — DC motors

Test 5 — Camera

Test 6 — Battery and speaker

Battery voltage

Speaker

Results summary

Troubleshooting

Day 1 wrap-up

What you accomplished today

What’s coming on Day 2

​Time estimate

​Running the test suite

​Each test in detail

​Test 1 — Ultrasonic distance sensor

​Test 2 — Grayscale sensor

​Test 3 — Servos

​Test 4 — DC motors

​Test 5 — Camera

​Test 6 — Battery and speaker

​Battery voltage

​Speaker

​Results summary

​Troubleshooting

​Day 1 wrap-up

​What you accomplished today

​What’s coming on Day 2

Time estimate

Running the test suite

Each test in detail

Test 1 — Ultrasonic distance sensor

Test 2 — Grayscale sensor

Test 3 — Servos

Test 4 — DC motors

Test 5 — Camera

Test 6 — Battery and speaker

Battery voltage

Speaker

Results summary

Troubleshooting

Day 1 wrap-up

What you accomplished today

What’s coming on Day 2