Published June 13, 2019 © GPL3+

Hot Wheels Car Photogate

Measure how fast your Hot Wheels car moves at different points on your track or use it to tell how fast your race was.

IntermediateFull instructions provided8,268

Things used in this project

Hardware components

Arduino UNO

Through Hole Resistor, 200 kohm

Through Hole Resistor, 2k ohm

High Brightness LED, White

Adafruit Photo transistor light sensor

Hand tools and fabrication machines

Soldering iron (generic)

Solder Wire, Lead Free

3D Printer (generic)

Story

Students needed to measure how fast their Hot Wheels cars went along the track, so I built several inexpensive photogates to help them.

It uses a photo transistor to detect when the car passes and can calculate the speed of the car at whatever position you put the sensor holder. A white LED was used instead of an IR LED so it was more obvious the device was on and connected properly.

3D-printed photogate

The 3D printing of the photogate is optional. During the proof-of-concept phase, the prototype of the sensor and LED holder was made from hot gluing cardboard pieces together, and it worked just fine, though not very durable.

One sensor can measure relative speed of one car (assuming a typical length car).

A single photogate only measures relative speed for an individual car

Two photogates can be used to be more precise, but the distance must be measured by the time seen in the Arduino serial monitor to make the calculations.

Two gates can determine the exact speed for multiple cars

Below is an example of the single photogate and the serial output as the car passes.

Schematics

Code

/* PhototransistorVoltage Hot Wheels Timer
 * Power phototransistor with 5v and GND
 * Connect the phototransistors yellow wire into A1 
 * Optional: Use a piezo buzzer connected to pin 4 and GND.
 */

const int buzzerPin = 4;  // other end of buzzer connected to GND
float threshold = 0.25;    // voltage where phototransistor shows car is in the way
const int interval = 5;  // accuracy of timer in milliseconds

void setup()                                 // Built-in initialization block
{
  Serial.begin(9600);                        // Set data rate to 9600 bps
}

float v1, timerCount;
boolean waitFor2ndTrigger;

void loop() {
  tone(buzzerPin, 440*6, 50);                   // Sound "ready"
  timerCount = 0;
  waitFor2ndTrigger = true;
  v1 = volts(A1);                                // measure voltage from phototransister in A1
  tone(buzzerPin, 880, 150);                     // Sound for start of waiting for trigger
  Serial.println("Waiting for trigger...");      // wait for  phototransistor to be dimmed
  while (v1 > threshold) { 
   delay(interval);                   // otherwise it's too fast to notice button press ((when one is used)
   v1 = volts(A1);                    // measure voltage from phototransister in A1
    }
  Serial.print("Started...");                    // Display "Start"
  
  while (waitFor2ndTrigger) {              // count time until phototransistor receives light again
    delay(interval);                       // Delay for defined time
    timerCount = timerCount+interval;      // Add time to counter
    v1 = volts(A1);                        // check A1 phototransistor
    waitFor2ndTrigger = (v1 < threshold);  // is "false" when phototransistor receives light again
    }

  // end timer count and display results
  Serial.println("Stopped");             // Display " sec." & newline
  tone(buzzerPin, 880, 50);              // Sound at finish
  delay(80);
  tone(buzzerPin, 880, 80);
  Serial.print("Final Time = ");          // Display "Final Time = "
  Serial.print(timerCount/1000);          // Display timerCount in #.### format instead of milliseconds
  Serial.println(" seconds");             // Display " sec." & newline
  Serial.print("Speed = ");               // Display "Speed = "
  Serial.print(72/(timerCount/1000));     // Display speed based on 72 mm car
  Serial.println(" mm/sec");              // Display "mm/sec" & newline
  Serial.println("");                     // print blank line
  delay(3000);                            // wait 3 seconds and reset
}
                                           
float volts(int adPin)                       // Measures volts at adPin
{                                            // Returns floating point voltage
 return float(analogRead(adPin)) * 5.0 / 1024.0;
}

/* PhototransistorVoltage Hot Wheels Timer
 * Power each phototransistor with 5v and GND
 * Connect the 2 phototransistors yellow wires 
 *       into A1 & A2 to detect start and stop of car.
 * Use a piezo buzzer connected to pin 4 and GND.
 */

const int buzzerPin = 4;  // other end of buzzer connected to GND
float threshold = 0.25;    // voltage where phototransistor shows car is in the way
const int interval = 10;  // accuracy of timer in milliseconds

void setup()                                 // Built-in initialization block
{
  Serial.begin(9600);                        // Set data rate to 9600 bps
}

float v1, v2, timerCount;
boolean v1trigger, waitFor2ndTrigger;
int count=0;

void loop() {
  tone(buzzerPin, 440*6, 50);                   // Sound "ready"
  timerCount = 0;
  count = 0;
  v1 = volts(A1);                      // measure voltage from phototransister in A1
  v2 = volts(A2);                      // measure voltage from phototransister in A2
  Serial.println("Waiting...");       // wait for either phototransistor to be dimmed
  while (v1 > threshold && v2 > threshold) { 
   delay(interval);                   // otherwise it's too fast to notice button press
    v1 = volts(A1);                    // measure voltage from phototransister in A1
    v2 = volts(A2);                    // measure voltage from phototransister in A2
    }
  v1trigger = (v1 < threshold);  // find which was triggered 
  Serial.println("Start");                    // Display "Start"
  tone(buzzerPin, 880, 150);                          // Sound for start of clock
  v1 = volts(A1);
  v2 = volts(A2);
  waitFor2ndTrigger = true;                // check only A1 phototransistor
  
  while (waitFor2ndTrigger) {              // count time until 2nd probe is triggered
    delay(interval);                       // Delay for defined time
    timerCount = timerCount+interval; 
    v1 = volts(A1);
    v2 = volts(A2);
    if (v1trigger) {
      waitFor2ndTrigger = (v2 > threshold); // check A2 phototransistor
    } 
    else {
      waitFor2ndTrigger = (v1 > threshold); // check A1 phototransistor
    }
  }

  // end timer count and display results
  tone(buzzerPin, 880, 50);                   // Sound at finish
  delay(80);
  tone(buzzerPin, 880, 80);
  Serial.print("Final Time = ");       // Display "v  Final Time = "
  Serial.print(timerCount/1000);       // Display timerCount in #.### format
  Serial.println(" sec.");             // Display " sec." & newline
  Serial.println("");                  // print blank line
  delay(3000);                         // wait 3 seconds and reset
}
                                           
float volts(int adPin)                       // Measures volts at adPin
{                                            // Returns floating point voltage
 return float(analogRead(adPin)) * 5.0 / 1024.0;
}

Credits

nfarrier

1 project • 8 followers

I'm a retired science and technology teacher, and have an amateur radio license. I enjoy programming and making things.

Hot Wheels Car Photogate

Things used in this project

Hardware components

Hand tools and fabrication machines

Story

Custom parts and enclosures

Photogate Holder

Schematics

1 photogate

2 photogates

Code

Single LED Photogate

Dual photogate code

Credits

nfarrier

Comments

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Hot Wheels Car Photogate

Hot Wheels Car Photogate

Things used in this project

Hardware components

Hand tools and fabrication machines

Story

Custom parts and enclosures

Photogate Holder

Schematics

1 photogate

2 photogates

Code

Single LED Photogate

Dual photogate code

Credits

nfarrier

Comments

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