I recently bought a new camera; it’s a Canon Powershot A800. Although it’s only a basic compact camera, I specifically chose one that can run CHDK; this is a replacement firmware that allows access to just about every feature imaginable. One thing I wanted to try was to take regular shots over the course of a night so that I could combine them to create a photo of star trails. I quickly realised that powering the camera with batteries limited shooting to a couple of hours and the cost of batteries was going to start mounting up. With canon wanting about £50 for an official mains adaptor it was time to make my own. This is what I came up with:
The first challenge was to interface my power supply with the camera. There is no convenient DC port on the side, there is just a little slot for the lead of the official adapter to enter the battery compartment. Instead of a normal plug the lead is terminated by a lump of plastic shaped like a couple of AA batteries. Without access to any way of nicely moulding plastic I went for a slightly less high-tech approach: cardboard rolled around a pencil. I formed 2 tubes the right size and used electrical tape to hold them in place while I super-glued them together.
Next I needed to attach electrodes to the end of the form; continuing the theme of ‘things lying around on my desk’ I used a drawing pin and a screw. I threaded the wires down the tubes, soldered the terminals on and glued them to the end of the tubes.
I cut some slots in the other end of the tubes to contain the leads and sealed everything in place with my glue gun; I had to trim the glue down to fit in the battery compartment, although it’s still slightly tight. A surface with less friction than hot glue would definitely be an improvement.
Now that I had the ability to get electricity into the camera, I needed to supply it with the correct electricity in order to work. I was replacing 2 AA batteries, this meant I required a nominal 3V. I tried a variety of regulation circuits based around a bare transformer without much success; the official power supply can supply up to 2A, while I was struggling to get more than 1A from my circuits built with the components I had to hand. I decided to change tack and use a factory built DC adapter. The one which was lying around was a 5V 1.5A one from an old broken iPod dock; while this wasn’t quite the 3V 2A of the official supply, it was close enough to use with a little tinkering. I started off by aiming for 2.4-3.2V (the voltages of a pair of NiMH and Alkaline cells respectively); this meant loosing 2-2.8V off the measured open circuit voltage of the adapter of 5.2V. I started off by putting 4 diodes in series, each one has 0.7V across its p-n junction while conducting for a total drop accross the 4 diodes of 2.8V. The camera would not power on so I tried 3 diodes for a nominal voltage of 3.1V, the camera would now power on but gave a low battery warning and turned itself off when I tried to take a photo. 2 diodes gave a nominal voltage of 3.8V which was a little high, but this dropped as soon as the camera started drawing current; the camera now powered up and took photos perfectly. Most cameras should be fairly resilient to differences in power supplies, but there is obviously a small risk that applying too high a voltage could cause damage; always start at the low end of the estimated voltage range and work up.
The camera will happily sit powered by this and take photos until its card is full. My first attempt of taping it to a box on a window sill at night still had plenty of room for improvement on the photography end, but definitely proved that the power supply is up to the job. This is a composite of several hundred photos of Magdalen College library taken out of my bathroom window over the course of about 4.5 hours.