Archive for Photography

High Speed Air-gap Flash

A bullet hitting a solid brass rod.

If you just want to buy an assembled and tested airgap flash, I have made an updated versions that can be purchased at Information Unlimited.

In my quest to capture amazing high speed photographs I notice that when photographing shooting bullets the bullets were blurred. I found that standard xenon tube, which standard flashes use, is very bright for the energy put into it because of glowing xenon gas. The book Electronic Flash Strobe by Harold Edgerton explains all the calculations, but in practice this means all the flashes from Nikon, Canon and others that use xenon flash tubes have a minimum duration of 1/40,000th of a second. That’s fast enough for most things, but not for a shooting bullet travels around 1000 feet/second. In 1/40,000th of a second that bullet can travel about 1/3rd of an inch leading to blurry photographs of bullets.

To solve this I had to make a faster flash. I’m certainly not the first to do this. I think that was Harold Edgerton. He actually created a company called EG&G to sell a product called the 549 Microflash, but that company has been dissolved and the product discontinued. Sometimes you can still find these flash units on ebay, but the ones I saw were selling for $8K+. There is also a company called Prism Science Works making a modern version of these for researchers, but you’ll need really deep pockets to afford one of those. I saw directions on how to build one in the August 1974 issue of Scientific America and emailed Alan who had already built a few. After this research I realized I could build a sub-microsecond flash for just a few hundred dollars. A sub-microsecond flash means the flash duration is less than 1/1,000,000th of a second or about 25 times faster than a xenon flash.

Here is an image showing how an air-gap flash compares to a standard flash when photographing a 1000 feet/sec pellet.

I am a strong believer in sharing knowledge so I’ll explain how I made my air-gap flash, but I am knowledgeable with high voltage safety procedures. You should not build this because this flash will kill you. It really will. This flash requires charging a 35,000 volt capacitor that will easily kill a person for a single mistake. I have a safety checklist that I use ever time I plug in this flash and it still scares me. If it didn’t scare me then I shouldn’t be using it because this thing is dangerous. The below information is for educational purposes only. Do not build one! If you go against my advice and do build one, I am not responsible for any injury, death, or any other problems it causes.

Here is a list of the main parts I used to build my air-gap flash:

Here is an overall diagram of how all the parts fit together.

This is the schematic for the custom circuit board I made. Basically it doubles the 120V AC and uses an external signal to trigger the 45 kV transformer.

And here is a picture of my custom circuit board.

Here is a picture of the air-gap flash.

This shows what is plugged into the air-gap flash. 120V to run the 45K trigger transformer, 14V to charge the big capacitor, and a 3.5mm cable to use for triggering.

Here is a side view. Notice that the reflector has a thin sheet of Plexiglas over it to make cleanup easier. The metal reflector is also grounded in case the plexiglass tubes shatter, and they do shatter sometimes. I have found they last longer if I use latex gloves so no oils from my hands get on the glass tubes.

This is a close up of the glass tubes where the spark happens. The outer glass tube is just for protection and to reduce noise. The inner tube has the wire 45 kV trigger transformer inside. When that transformer is triggered the air ionizes and that cases the big capacitor to discharge in a 1.5 inch spark that creates the flash of light. To seal the inner tube I melt one end with a torch until it is sealed.

A top down view that shows how everything is connected.

This rod is very important. It is how I discharge the big capacitor after every use so I don’t die when I’m working on the flash.

Those extra wires and circles are there so I had an easy way to discharge the capacitor.

Note about using it with the Camera Axe. The Camera Axe only allows a trigger voltage of up to 20V. This has a trigger voltage of 240V so I needed to to put a high voltage protection circuit between this and the Camera Axe. There are many examples of these circuits on the web.

Here are a few results:

Glass rods being hit by a bullet.

A balloon being hit by a bullet.

Thanks Alan for your help and letting me use some of your amazing photos.

Comments (57)

Camera Axe 4

After six months of work, the fourth version of the Camera Axe is finally ready. For those new to the Camera Axe it’s a programmable camera and flash trigger with two sensor plugs that can be used for a wide range of sensors. It can do anything from timelapse to photographing a shooting bullet. For a little background information read about versions 1 and 3 of the Camera Axe. This version is open sourced under the Creative Commons Attribution NonCommercial 3.0 License. I do sell kits, assembled versions, and various sensors at my store.

Camera Axe Video:

Here is a list of improvements from version 3 to version 4 of the Camera Axe.

  • The screen is much larger and this makes the menuing system easier to use.
  • The case is is much nicer because I had custom cases made for this project.
  • Added two LEDs to indicate when the camera or flash is being triggered.
  • Much nicer 3.5mm jacks for the sensors and camera/flash plugs.
  • Added a dedicated external power plug for those wanting to use external power.
  • The buttons feel much nicer.
  • Batteries last four times longer.
  • A new microcontroller that has two times the space for more features in the future.
  • The software is a complete rewrite and has too many improvements to list, but I’ll mention the greatly improved intervalometer mode and the new fast trigger mode.
  • The kits are easier to assemble (no wires or drilling needed).

There’s more information information about sensors, reprogramming, flash/camera cables, and much more at www.CameraAxe.com.

Here are a few images taken with the Camera Axe.

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Projectile Sensor #2

Due to a few request I decided to make some improvements over my original version of this sensor.

The major improvements are a new 3 PCB design, which makes this much more durable than the previous version. The other huge improvement is using the Honeywell SD5600 Optoschmitt Detector which has a fall time of 15 ns. Previously I was using a standard photo transistor which had a response time of 15 us. This new design is 1000x faster! This actually matters on faster bullets since sometimes the old sensor wouldn’t notice a fast small bullet. The SD5600 never misses.

The only downside to this new design is that it’s a little more expensive. Here’s the BOM.

  • Female Header
  • 2x 90 Degree Male Header
  • 2x 3.5mm Jacks
  • 2x IR Emitters
  • 2x Honeywell SD5600
  • 2x 1K Resistor
  • 2x Male/Male 3.5 mm Cable
    • This sensor has two IR sensors spaced exactly two inches apart. The user inputs the distance from the sensor to the desired position of the projectile when the picture is taken. Based on the time it takes the projectile to travel those two inches between the sensors, a velocity for the projectile can be determined. Since bullets and other projectiles basically travel at a constant velocity, it is easy for the microcontroller to calculate the delay in microseconds until the picture is taken.

      PCBs



      Here’s a link to the Eagle files I used to create the PCBs.

      Using the Projectile Sensor with the Camera Axe

      There are 3 PCBs and everything is labeled so it’s supper easy to plug them together. You will need the 3.0.04 or newer version of the Camera Axe software. Turn on the Camera Axe and hit menu until you get to the projectile menu. Set the distance you want the bullet to be from the second gate when the picture is taken and then push the right button until you get to the “Trigger on” menu for the projectile sensor and set this to low. Now hit the set button. If the sensor continuously displays the speed of the projectile that means the sensor boards aren’t lined up correctly. Look at it from the side and bend the boards so the emitter is pointed directly at the detectors. Once it’s sitting there waiting for a projectile put your finger through the two sensors and it will report back to you the speed of your finger. Once you get this it’s working.

      Mounting the Projectile Sensor

      I mounted a projectile sensor to my pellet gun and it is working great. The pellet’s velocity ranges from 985 feet/second to 1060 feet/second. Below are a bunch of pictures of how I did this mounting.



      Results

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    Photographing Splashing Droplets

    Up until now I’ve been using a photogate sensor and the Camera Axe to take pictures of water and milk droplets. After some research I found manypeopleonline were using solenoid valves to create droplets and take pictures of them. The big advantage to this method is it’s easy to collide drops which is was very difficult and random using my old method. I decided I’d make a new valve sensor (pre-built version available here) for the Camera Axe and document how to make your own since I didn’t find any detailed instructions or part lists on the web.

    Building It

    I knew I wanted to have a way to trigger my camera on a fairly long exposure in a dark room (I use a 1 second exposure). Then make a water droplet. Wait a little while. Make a second water droplet that would collide with the first droplet’s splash. And then wait a little more until the collision before triggering the flash. With this in mind I started making the different pieces I needed and connecting them together.

    The only new circuit I needed was a simple motor driver circuit to drive the solenoid. Below is the one I designed and here are the PCB files I designed in Eagle.

    Valve Sensor Circuit

    Next I had to find and order the parts.

    After assembly here is what it looks like:

    I also made a new version of the Camera Axe software with the valve sensor. You can download this new version (3.0.03) from CameraAxe.com.

    Using it with the Camera Axe

    Plug your camera into Camera/Flash1. Plug your flash (or flashes using a splitter cable) into Camera/Flash2. Plug this new valve sensor into Sensor1. Below is a picture of my setup. It has two flashes, a camera, the Camera Axe, and the valve sensor.

    Go to the valve sensor menu. Set drop1 size to a good starting size like 80. Set drop2 delay and drop2 size to 0 (we will start with only a single drop). Set Flash delay to around 200 ms. Then turn off the lights and press the “Set” button. This will trigger the camera and the flash. Now adjust the flash delay by 10 ms increments until you have a good droplet picture. Below is a video sequence of 20 images stepping through a milk drop splash. The images go from 220 to 420 ms.

    If you want to do colliding drops timing is more complicated. As a starting point I’d suggest a drop1 size of 80, drop2 delay of 40, drop2 size 50, and a flash delay of 200 ms. Then adjust the flash delay until you find the time of collision. Then you can start adjusting other timing parameters to get all sorts of different types of pictures.

    You can find lots of photos (including droplet pictures) on the Camera Axe flickr group. Below are a few of my favorites from yesterday.

    Comments (10)

    Photographing a Speeding Bullet

    I’d like to start with a big thanks going to Chris Callander.  When he saw the Camera Axe project he shared his idea for a new sensor with me.  I took his ideas and developed this new projectile sensor and updated the Camera Axe software to support it.  I doubt I would have ever came up with this new sensor idea without Chris.  The ideas that get shared in the open source community are great!  The hardware and software for this project are shared under the Creative Commons Attribution 3.0 License.

    You can purchase the Camera Axe and this projectile sensor at this store or built it yourself (parts list below).

    This new projectile sensor is designed to help photograph speeding bullets on the Camera Axe platform, but since I used the Arduino development environment, anyone using an Arduino could easily adapt the this hardware and software to their purposes.

    This new sensor has two IR sensors spaced exactly two inches apart.  The user inputs the distance from the sensor to the desired position of the projectile when the picture is taken.  Based on the time it takes the projectile to travel those two inches between the sensors, a velocity for the projectile can be determined.  Since bullets and other projectiles basically travel at a constant velocity, it is easy for the microcontroller to calculate the delay in microseconds until the picture is taken.   I ran into a few gotchas in the software while I was writing it.

    • Needed to use digitalRead() with the sensors and not analogRead() (analog read is too slow at 100 us)
    • Used integer math because floating point math is very slow in Arduino
    • Had to be careful with my order of operations or I would overflow 32 bit unsigned integers in certain cases

    I’m very pleased with the results.  I’ve tested it with an airgun that travels at a relatively slow 500 ft/sec (half the speed of sound), but my calculations show that the Camera Axe’s 16 MHz ATmega168 chip can easily predict the position of a high speed riffle (several times the speed of sound) to within a fraction of an inch.  I had known that this should work, but it was still amazing to see it working perfectly in the real world, considering all the micro fluctuations that I didn’t account for in the calculations.  In the end, things like gravity, air resistance, and quantum forces just didn’t matter 🙂

    [Update: Alan Sailer correctly pointed out the IR transistor I’m using only works at 15us, this would limit photos to around 2x the speed of sound, see the comments for options on faster transistor options.]

    Below is a schematic of this new projectile sensor sensor and here are the Eagle files.

    If you’re planning to build your own, then this is my parts list

    Here is a list of updated files:

    • Updated Camera Axe label.  Nothing special here.
    • Updated Camera Axe source code to support this new sensor.  One nice feature of the projectile code is that I print out the feet/sec or the cm/sec that the bullet was traveling.  While this wasn’t needed for photography; it is still fun to see the numbers.
    • Updated user manual to include the software changes.

    Using the Projectile Sensor with the Camera Axe

    First, visually line up the IR LEDs to the IR transistors so that they are pointing at each other. Then just plug the 3.5mm cord from Sensor1 on the Camera Axe to Sensor1 on the projection board. Next do the same for sensor2. Plug a flash into Camera/Flash1 on the Camera Axe. Turn on the Camera Axe and go to the sensor menu. (Optional if you want to change from inches to centimeters press [Menu]+[Set]+[Left] while turning on the Camera Axe to enter the special menu to change from English units to Metric units.) In this menu you can set the distance from the sensor to where the projectile should be on the picture (0->999 cm/inch).

    You can test your the projectile sensor’s set up by putting your finger in front of the first sensor. After one second a message will display saying that the “second trigger failed”. After this message goes away, put your hand in front of the second sensor and then your other hand in front of the first sensor. This will basically simulate an infinitely fast projectile and the Camera Axe will display the speed of a fast projectile (something more than 45 times the speed of sound).

    This sensor should generally be placed at the end of the gun barrel so you don’t need to worry about the bullet hitting the circuit board or sensors.  The bullet/projectile must pass through this sensor before it hits the target.  In my setup I have everything firmly bolted to a table and trigger the gun with a string from a safe distance.

    Pretty Pictures

    Last but not least, here are a few of the pictures I took with this new sensor.  Taking these pictures was possible with the Camera Axe before I had the projectile sensor using a microphone or a laser sensor, but it involved a lot of trial and error.  With the projectile sensor, every picture is what I was shooting for (pun intended).

    First, visually line up the IR LEDs to the IR transistors so that they are pointing at each other. Then just plug the 3.5mm cord from Sensor1 on the Camera Axe to Sensor1 on the projection board. Next do the same for sensor2. Plug a flash into Camera/Flash1 on the Camera Axe. Turn on the Camera Axe and go to the sensor menu. (Optional if you want to change from inches to centimeters press [Menu]+[Set]+[Left] while turning on the Camera Axe to enter the special menu to change from English units to Metric units.) In this menu you can set the distance from the sensor to where the projectile should be on the picture (0->999 cm/inch).

    You can test your the projectile sensor’s set up by putting your finger in front of the first sensor. After one second a message will display saying that the “second trigger failed”. After this message goes away, put your hand in front of the second sensor and then your other hand in front of the first sensor. This will basically simulate an infinitely fast projectile and the Camera Axe will display the speed of a fast projectile (something more than 45 times the speed of sound).

    Comments (41)

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