Canon Camera Shutter Release Cable

Using a Canon handheld camera with the APM 2.5 looks to be an easy low cost method of collecting some aerial imagery.  We have a couple of suitable cameras to try out:

Canon Powershot A2300:  16MP, $100 at Walmart

Canon Powershot SD800IS:  7.1MB, $25 off Craigslist

Both cameras are supported by the Canon Hack Development Kit (CHDK) which adds two valuable features to these low cost devices:  scripting and a remote shutter release via the USB port.

The remote shutter release simply monitors the presence of a voltage on the USB power pin.  Following the instructions here, I made up a test cable using a battery and a momentary contact switch.  Testing with various voltages, it seems to need at least 4.1V to reliably trigger the shutter.

CHDK Remote Cable Test Rig W/4.8V Battery

To connect to the APM, we'll need to use one of the digital I/O pins (3.3V) as a trigger, with voltage supplied by the 5V supply pin.  Testing with the battery indicates that the current drain is negligible, so here is the circuit we came up with, based on a 4049 hex inverting buffer:

CHDK Camera Circuit

Here's the completed circuit, needs some strain reliefs, heatshrink, 

A small ceramic capacitor is bridged across 5V-GND as a buffer.  On initial testing, it works!  Next steps are to test with the APM and fine tune the scripting on the camera and autopilot.

Many thanks to my friend paulb for the expert advice, parts, soldering, and correction of errors.  I did, however, supply the beer.

Here is a discussion and some code for scripting the camera.