Jeff,
I assume from your reference to Velleman that you are using their light detector circuit kit. Almost all of their relay outputs have SPDT contacts. If I'm correct so far, you need an "edge detector" that reacts to change of the relay state. In this circuit, the relay on the left is in the light detector. With that, a capacitor, and a small reed relay you will have a set of contacts that momentarily close when your detector relay switches. The ON-time of the reed relay depend on its coil resistance and the value of the capacitor.
Ken