Creating a water drop photograph timer - DIY Project.

Hey all.

I love water drop photographs but it's been a few years now (probably more like 15) since I attempted anything like that. The results back then were average so I think it's about time to build something new and give it another go. I'll be doing this over a few posts, starting with some of the concepts in this post and then moving on to prototyping, building and testing and then, of course, getting some results (hopefully)

Concepts

The basics concept around taking photographs of fast moving objects such as a water drop in motion is to use a flash to expose the scene as opposed to the camera shutter. Once you get your head around this concept then things start to make sense....

Imagine yourself in a pitch black room with a camera on a tripod. You open the shutter on the camera for 30 seconds and the image you get out of that exposure is totally black - of course; there are no lights. Now imagine you do the same but this time, at some point during the 30 seconds you fire off a flashgun to illuminate the scene. It is the flashgun doing the exposure, not the camera. The amount of time the flash is on for, or the power of the flash is what determines how the scene is exposed

This is the fundamental concept behind pretty much all flash photography and the reason why cameras have a flash sync speed. This is the quickest that your camera can synchronise to a flashgun and is normally around 1/250th of a second. The process of opening the shutter, firing the flash and then closing the shutter is done during this period. If you close the shutter before this, you might find that part of your image is dark as the shutter was half closed while the flash was going off.

The other concept that it is useful to understand for high-speed photography is how the duration of the flash can be adjusted by increasing or reducing the power of the flash. Flash power isn't always how bright the flash bulb is, it is also determined by how long the flashlight is turned on for. For example, a Vivitar 283 flashgun on it's lowest power setting would flash for 1/30000th of a second. That is way faster than any shutter, and can freeze the fastest of motion without problems.It can also fire of multiple exposures without fully draining the capacitors inside the flash. Of course, with this comes the problem of lack of light which is then going to manifest in either higher ISO ratings/more grain and/or less depth of field and potentially less sharp images. A good balance between motion freezing and power is required.

So hopefully now you can grasp the concept here.

• Drop fluid using some kind of device
• Open the camera shutter
• Fire flash with lowish power at precisely the right time to freeze the motion
• Close the shutter

Getting the timing right on this is imperative. We are talking in the realm of milliseconds each way is the difference between getting the image you want and missing it completely.

As you can imagine, not only do you need to have a consistent timing between shutter and flash, you need to produce drops consistently. There's no point setting up a system where the drops fall at different rates each time.

So, I am going to use a trusty Arduino Uno to do all the timings and hardware control

An Arduino Uno

I am going to need to control the following.

• Something to drop the water in a controlled manner - I have seen some people using a peristaltic pump (the kind they use to deliver fluids in hospitals), but I prefer the cheaper alternative of a small fluid/gas solenoid valve. This is basically a valve that you can open with a 12-volt current to allow liquid to come through. That removes the need for a pump of any kind and allows the system to be gravity fed.
  One thing to note about a solenoid is that it is driven by 12 volts. Arduino only outputs 5v, so on its own it will not be able to drive the solenoid. That means that the solenoid will have to be on its own 12v circuit and then the Arduino will have to switch it with something like a TIP110 transistor (which basically is a switch that uses a low power on-off function to switch on a high power appliance)
• Something to fire the camera shutter.
• Something to fire the flash.
  Both firing the shutter and firing the flash require the "shorting" of a cable. In the case of the shutter, I will modify a cheap Canon cable release. For the flashgun, a modified sync cable should do the job. In both cases, however, there is a problem. Neither of these mechanisms will accept a voltage applied across the terminals. They are both passive and just need a "switch" to close the circuit to fire them, just as a shutter button is a button that completes a circuit.

An Arduino can only output a electrical current from its pins and is not a switch. In order to fire the shutter/flash, that current needs to close a switch. The best way to do this is with an optocoupler (or opto isolator). This is effectively a tiny led that, when powered, closes a circuit. That way, no current makes it to the shutter release or flash cable. The image on the right here is an optocoupler. A current across pins 1 and 2 will close the circuit across pins 3 and 4.

I have the solenoid and some of the cables on order so can't get things fully built yet, but I do have most of the other parts available. I should be able to create a prototype using LEDs to signify the drip happening, the shutter firing and the flash firing while I wait for parts to arrive. That will allow me to get to work on the code.

Initially i'll be putting the timing values directly into the code, but eventually I will have a 16x2 LCD screen and some menu buttons which will allow me to configure the settings without having to recompile and upload the code every time. Most of that code and the parts will be repurposed from my time-lapse rail project.

For this post, that is all I am going to go into, the next will hopefully contain a basic prototype using LEDs to show the code works and that the concept is sound. There are a number of Arduino libraries that you can use to improve the responsiveness of the Arduino when it comes to time-sensitive applications so I'm looking forward to giving them a try.

Thanks for reading and stay tuned for the next instalment.

Mark