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We used simple relays, with a small displacement. So we had to use a lever system to be sure that the targets could appear and disappear completely behind the plate. The levers are shown in the pictures.
The relays requires a voltage of around 30 Volt. So in rest the target and the LDR fixed on the target are behind the plate. On the lid we fixed a nail to extend the lever arm. The relay works as follow: when there is no voltage the magnet of the relay won't attract the lid. When there's a voltage of 30Volt, there will be a magnetic force of the magnet, this will attract the lid down. The nail is connected by a contact point fixed on the target arm. The target arm turns around a little nail (point 1) and therefore the target will move up and become visible. It is very important that there is a diode parallel placed with the relay and also inverse polarized with respect to the relay. We encountered serious problems when we tested our programs in the pic when there was no diode! This had something to do with the fact that if you drop the voltage from 30 Volt to 0 volt there is a emf generated. So if you put there a diode like we described we short - circuit this emf.