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Proper sensor calibration is critical for accurate parking space detection. This guide covers VL53L0X configuration, distance thresholds, and installation best practices.

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VL53L0X Sensor Overview

The VL53L0X is a time-of-flight (ToF) laser ranging sensor that measures distances by calculating the time light takes to bounce back from objects.

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Key Characteristics

  • Measurement Range: 30mm to 2000mm (approximately 3cm to 2 meters)
  • Accuracy: ±3% at typical distances
  • Field of View: 25° cone
  • Measurement Time: ~30ms per reading
  • Interface: I2C (default address 0x29)

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Distance Thresholds

The firmware uses a fixed threshold to determine occupancy from S-Parking.ino:78-82: 
if (measure.RangeStatus != 4 && measure.RangeMilliMeter < 400) {
  nuevoEstadoSensor = 0; // Ocupado fisicamente (Occupied)
} else {
  nuevoEstadoSensor = 1; // Disponible fisicamente (Available)
}
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THRESHOLD
millimeters
default:"400"
Distance threshold for occupancy detection
  • Less than 400mm → Space is occupied (status = 0)
  • Greater than 400mm → Space is available (status = 1)
  • RangeStatus = 4 → Out of range or no valid measurement

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Why 400mm?

The 400mm (40cm) threshold is chosen because:
  1. Car undercarriage height: Most vehicles have ground clearance of 150-250mm
  2. Sensor mounting height: Typically 2-3 meters above ground
  3. Safety margin: 400mm provides buffer against false positives from debris or puddles
  4. Reliable detection: Large difference between “car present” and “empty space”
The threshold of 400mm assumes the sensor is mounted at ceiling height (2-3 meters). If you mount the sensor differently, you’ll need to adjust this value.

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Calibration Process

1

Measure Installation Height

Determine the actual mounting height of your sensor:
  • Ceiling mount: 2.5 - 3.0 meters typical
  • Pole mount: 2.0 - 2.5 meters typical
  • Ground mount: Not recommended (sensor will see car undercarriage)
Record this value for threshold calculation.
2

Test Empty Space Reading

With the parking space empty, check the sensor reading:
  1. Open Serial Monitor at 115200 baud
  2. Add temporary debug code to print raw values:
void readLocalSensor() {
  VL53L0X_RangingMeasurementData_t measure;
  sensor.rangingTest(&measure, false);
  
  // DEBUG: Print raw sensor values
  Serial.print("Range: ");
  Serial.print(measure.RangeMilliMeter);
  Serial.print(" mm, Status: ");
  Serial.println(measure.RangeStatus);
  
  // ... rest of function
}
  1. Note the typical empty space reading (should be close to installation height)
3

Test Occupied Space Reading

Park a vehicle in the space and observe the readings:
  • Most vehicles will show 1500-2000mm (sensor sees car roof, not ground)
  • Some low-profile vehicles may show 1200-1500mm
  • SUVs/trucks may show 1800-2200mm
If readings are inconsistent, check:
  • Sensor is pointed straight down (not angled)
  • Field of view isn’t obstructed
  • Vehicle is properly centered in the space
4

Calculate Threshold

Choose a threshold value between empty and occupied readings:
Threshold = (Empty Reading + Occupied Reading) / 2
Example calculation:
  • Empty space: 2500mm
  • With car: 1800mm
  • Threshold: (2500 + 1800) / 2 = 2150mm
Round down for safety margin: 2000mm or 2100mm
5

Update Firmware

Modify the threshold in S-Parking.ino:78:
if (measure.RangeStatus != 4 && measure.RangeMilliMeter < 2000) {
  nuevoEstadoSensor = 0; // Occupied
}
Re-upload the firmware and test thoroughly.

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Physical Installation Guidelines

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Optimal Mounting Height

The sensor should be mounted directly above the center of the parking space: 
        [Ceiling/Mounting Surface]
                  |
                  | 2.5m - 3.0m
                  |
              [VL53L0X]    ← Pointed straight down
                  |
                  | 25° field of view
                  ∨
        [====================]
        [   Parking Space    ]
        [====================]

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Mounting Recommendations

Best for: Open parking lots, temporary installations
  • Mount sensor 2.0-2.5 meters high on pole
  • Position at edge of parking space, angled toward center
  • Account for angle in threshold calculations
  • Use sturdy pole to prevent swaying
Considerations:
  • Requires trigonometry for threshold adjustment
  • More susceptible to weather
  • May need recalibration after wind/vibration
Best for: Perpendicular parking against walls
  • Mount on wall at front of space
  • Sensor points horizontally toward parking space
  • Threshold measures distance to car front
  • Height: 0.8-1.2 meters (bumper to mid-door level)
Considerations:
  • Different threshold range (detect car presence, not distance to ground)
  • Use shorter threshold (e.g., 1500mm for detection)
  • May miss motorcycles or compact vehicles

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Installation Checklist

  • Sensor is securely mounted (no movement or vibration)
  • Field of view covers entire parking space
  • No obstructions between sensor and ground (pipes, lights, etc.)
  • Sensor lens is clean and free of debris
  • Weatherproofing applied if outdoors
  • Cable routing is secure and protected
  • Power supply is stable (no voltage drops)
  • Tested with multiple vehicle types

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Handling Edge Cases

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RangeStatus Error Codes

The VL53L0X returns a RangeStatus value indicating measurement quality:
StatusMeaningFirmware Handling
0Valid measurementUse reading
1Sigma fail (low signal)May be valid
2Signal fail (no return)Treat as out of range
4Out of rangeTreat as available
From S-Parking.ino:78, status 4 is explicitly checked: 
if (measure.RangeStatus != 4 && measure.RangeMilliMeter < 400) {
  // Status is good (0, 1, or 2) AND distance is close
  nuevoEstadoSensor = 0; // Occupied
}

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Common Calibration Issues

Symptoms: LED always red, even when space is emptyPossible causes:
  • Threshold too high (detects ground as occupied)
  • Sensor pointed at angle (sees nearby wall/object)
  • Obstruction in field of view
Solutions:
  • Lower threshold value
  • Adjust sensor angle to point straight down
  • Remove obstructions
  • Check for debris on sensor lens
Symptoms: LED stays green even with car presentPossible causes:
  • Threshold too low (doesn’t detect cars)
  • Mounting too high (out of sensor range)
  • Sensor not pointed at parking space
Solutions:
  • Increase threshold value
  • Lower mounting height
  • Adjust sensor aim
  • Test with serial monitor debug output
Symptoms: LED flickers between red and green randomlyPossible causes:
  • Threshold too close to typical readings (no margin)
  • Vibration causing measurement noise
  • Objects passing through field of view (people, carts)
Solutions:
  • Add hysteresis (require 2-3 consistent readings before state change)
  • Stabilize mounting
  • Adjust threshold with larger safety margin
  • Implement software filtering (moving average)

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Advanced: Implementing Hysteresis

To reduce false triggering, you can add hysteresis with two thresholds: 
#define THRESHOLD_OCCUPIED 350  // Below this = definitely occupied
#define THRESHOLD_AVAILABLE 450 // Above this = definitely available
// Between 350-450 = maintain previous state

void readLocalSensor() {
  VL53L0X_RangingMeasurementData_t measure;
  sensor.rangingTest(&measure, false);

  int nuevoEstadoSensor;

  if (measure.RangeStatus != 4) {
    if (measure.RangeMilliMeter < THRESHOLD_OCCUPIED) {
      nuevoEstadoSensor = 0; // Definitely occupied
    } else if (measure.RangeMilliMeter > THRESHOLD_AVAILABLE) {
      nuevoEstadoSensor = 1; // Definitely available
    } else {
      nuevoEstadoSensor = estadoLocalSensor; // Maintain previous state
    }
  } else {
    nuevoEstadoSensor = 1; // Out of range = available
  }

  // Only send update if state actually changed
  if (nuevoEstadoSensor != estadoLocalSensor) {
    estadoLocalSensor = nuevoEstadoSensor;
    sendStateToCloud(estadoLocalSensor);
  }
}
This prevents rapid state changes when readings hover near the threshold.

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Testing & Validation

After calibration, perform comprehensive testing:
1

Basic Functionality

  • Empty space → Green LED
  • Car parks → Red LED within 2 seconds
  • Car leaves → Green LED within 2 seconds
2

Vehicle Variety

Test with different vehicle types:
  • Sedans
  • SUVs/Trucks
  • Motorcycles (may need different threshold)
  • Compact cars
3

Environmental Conditions

  • Test in rain/snow (if outdoor)
  • Test with shadows and sunlight changes
  • Test with people walking near sensor
4

Extended Operation

  • Monitor for 24 hours
  • Check for drift or false triggers
  • Verify cloud synchronization works correctly

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Next Steps

LED Indicators

Configure RGB LED wiring and behavior

Hardware Overview

Review complete hardware architecture

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