Highspeed photography 101 – Lesson 7

Highspeed Photography 101

Table of contents

Introduction
Lesson 1: Freeze the motion with your shutter
Lesson 2: Freeze the motion with your shutter II
Lesson 3: Freeze the motion with your flash
Lesson 4: A small but powerful helper: Arduino
Lesson 5: Trigger your camera with an Arduino
Lesson 6: Trigger your flash with an Arduino
Lektion 7: Working with sensors: Light barrier
Lektion 8: Working with sensors: Sound trigger

Lektion 7: Working with sensors: Light barrier

Introduction
In the last lesson we saw how one can trigger a flash with an Arduino. In this lesson we’ll add a light barrier to trigger the flash in the right moment.

Rainbow splash

Idea
One of the biggest difficulties in highspeed photography is to trigger the flash at the right moment. If you try this without additional helpers (try & error), it is possible that you take many photos and still haven’t got the right moment.
To trigger the flash in the right moment, we use sensors. Light barriers and sound trigger are often used in highspeed photography.

Equipment
– Camera
– Flash
– Remote trigger for flash (optional)
– Arduino
– Schmitt photogate trigger (hiviz.com)

Camera- / flash settings
How to setup the camera for this kind of photos is described in detail in lesson 3. How to connect and trigger a flash with an Arduino is described in lesson 6. For this kind of photo we don’t use any special flash mode as HSS or ‘repeating flash’. We setup the flash to trigger ‘normally’, just once, with very low power settings. (Remember: The lower the settings the shorter the flash duration)

Arduino
The basic Arduino setup is the same as in lesson 6. We’ll add a light barrier as sensor. I choose the hiviz.com sensor as this is a very cheap assembly kit and it has a very good step-by-step tutorial to build it. We don’t need the delay unit, if we’re using the Arduino.

It is very easy to connect the light barrier with the Arduino; we connect the output of the light barrier with an analog input of the Arduino. If you’re using the hiviz.com kit the output of the light barrier is the anode of the SCR from the ‘Schmitt photogate trigger’. (Until version 10: Row 13 A-E, version 10 and above: Row 10 A-E)

Arduino with connected light barrier

We’ll setup the Arduino to trigger the flash as soon as there is a signal from the light barrier.

//hiviz connected here
int signalPin = 4;
//flash connected here
int flashPin = 11; 

//delay between signal from sensor and firing flash
int preflashdelay = 200; 

//delay after flash until the unit shall be ready again
//usually I put the exposure time, to ensure that
//I don't have unwanted multi exposures
int postflashdelay = 5000;

// The setup() method runs once, when the sketch starts
void setup()
{
  // initialize the flash pin as an output:
  pinMode(flashPin, OUTPUT);
  // initialize the signal pin as an output:
  pinMode(signalPin, INPUT);
  //setup serial console
  Serial.begin(9600);
}

// the loop() method runs over and over again,
// as long as the Arduino has power

void loop()
{
  //Read value from signalPin
  int reading = analogRead(signalPin);

  //print reading
  Serial.println(reading);    

  //if we're about a certain value
  if(reading > (100)){

    //debug output
    Serial.print("Flash triggered @ ");
    Serial.println(reading);

    //sleep for preflash delay
    delay(preflashdelay);

    //fire flash
    digitalWrite(flashPin, HIGH);
    delay(50);
    digitalWrite(flashPin, LOW);

    //sleep for postflash delay
    delay(postflashdelay);
  }
}

Mount the light barrier somewhere, where it will get interrupted. If you create “drop images” the drop will interrupt the light barrier. I mounted the light barrier on my ‘sliding table‘ so that it gets interrupted as soon as the slider arrives in its end position.

Light barrier

Taking the picture
Since I built the ‘sliding talble’ I wanted to take a photo with different colored glasses. Therefore I modified the slider of my ‘sliding table‘ to hold more glasses. Due to the modifications the slider got a bit massier therefore I accelerated it by hand.
I like this example as it shows that there is no big difference between triggering the flash with a sound trigger (last time) and light barrier (this time).

Setup rainbow splash

Creating the image is fairly easy: Turn of the light, start the camera, accelerate the slider, flash gets triggered, clean up.

Conclusion
In this lesson I showed, how we can extend an Arduino with a light barrier. If you react on the signal we get from the light barrier, you’ll trigger the flash in the right moment.