If you need a simple hardware solution, you can probably adapt
something like this.
Since the output goes low when triggered, it would be relatively easy to drive a mosfet or optocoupler directly with it. The mosfet could in turn switch a relay.
Of course I may be over simplifying the problem slightly as these devices are relatively crude, and their output may need latching and/or de-bouncing. For that, add an Arduino pro mini clone (or if you are old school, a 555 timer).... but... as I said before... if CHDK could do the trick with no additional hardware, that would be ideal.