I work on a lot of night photography including High Dynamic Range (HDR) of urban scenes. Early in the process of taking the photographs for these art projects I found that I needed a way to automate the process of taking a series of photographs with different shutter speeds. I was manually timing the exposures starting with the shortest time and increasing by one exposure value (EV) each time and using the preview on the camera to determine if I had one that was long enough. This manual process had the potential for timing errors that would cause problems later in the process.
I started by researching the wired remote connection for my camera and how it operated. My camera has a connector with three pins, a ground pin, a focus pin, and a shutter pin. I determined that as long as the shutter pin was grounded that the shutter was held open. I also found that the focus pin needed to be grounded long enough for the auto-focus system to complete the process.
I decided that the camera should be electrically isolated from the timing circuitry to protect it from possible damage if the timer shorted out. It would be a simple matter to revise the circuit to a common ground if it was needed. To achieve this I decided to use a pair of reed relays to connect the wires running from the camera as needed.
The timing electronics are based on an Arduino Uno with a custom shield and a TFT screen shield in a protective frame. The firmware is at Github and is opensource.
I have previously made a couple of blog posts about this project: Increasing exposure length interval meter, and My Variable Interval Timer for long exposure photography