My second idea is more trivial and probably doesn't have useful applications--or at least, not any that could not be done more safely and reliably by electronic or electromechanical systems. Briefly, it is a light-sensitive chemical fuse for explosives, and is inspired by the chemical-time-delay fuses used by Allied amphibious saboteurs during the Second World War to set off "Limpet" magnetic mines below the waterlines of enemy ships in port. This fuse, the operation and history of which are explained in great detail on pp. 98-99 of Dorling-Kindersley's "The Ultimate Spy Book," used a mixture of water and acetone to dissolve a celluloid disk restraining a spring-loaded firing pin; when the disk dissolved, the pin was released to mechanically initiate the primer charge. The concentration of acetone in the water-acetone mixture determined the rate of dissolution of the celluloid and, hence, the delay before firing. Fuses were equipped with interchangeable glass ampoules containing various concentrations of acetone and water which had been calibrated to produce various delays ranging from several days to a few hours and identified by color-coding the ampoule glass. An orange ampoule, for instance, produced a delay of 6 to 9 hours, depending on the ambient temperature. The fuses were armed by turning a screw which crushed the ampoule, spilling its contents onto the celluloid disk and beginning the process of dissolution.
My own idea works along the same general principle, but uses a different chemical system to release the firing pin in response to daylight. The charge is placed during the nighttime hours and is armed by removing a cap covering the transparent reaction chamber. When the sun rises, the light enters the exposed reaction chamber and initiates a radical chain-reaction between liquid bromine and a suitable alkane, producing the corresponding haloalkane and, most importantly, hydrobromic acid. The acid dissolves a thin metal disk restraining the firing pin and thus initiates mechanical detonation as in the Limpet mine. Because the reaction is a chain process, the presence of even a small amount of radical initiator, such as that produced by heat-induced homolytic decomposition of molecular bromine, could ultimately cause premature release of the pin. Such an eventuality would render the fuse useless and would be dangerous to the operator. Thus the system must be stabilized by the addition of a few percent of a radical inhibitor such as TEMPO (2,2,6,6-tetramethylpiperidinoxyl). This would prevent "substoichiometric" exposure to light and/or heat from initiating the reaction.
"Tuning" the system to produce the proper combination of substrate alkane, reactant concentrations, and disk metal and thickness would be the object of some applied research. The design criteria are that the system be shelf-stable, heat-resistant, shock-resistant, economical, and fast-acting under the appropriate conditions.
Certainly there are electronic systems already in existence that could serve analogously as a photofuse. Thus some advantage must accrue to the use of an all-chemical system to justify the development expense. The chemical system proposed is fairly straightforward and is derived from a basic reaction found in any respectable sophomore organic chemistry text. For this reason, such a device is relatively obvious and may already have been developed, patented, manufactured, and/or used. Likewise, there may be a more esoteric photosensitive reaction that could be better made to serve the same purpose. I'd have to peruse the patent and academic literature to determine these questions, and given that no compelling demand for the chemicomechanical switch seems to exist, such researches are probably not worth the effort. Lastly, I would point out that, although the device I propose has been described hereinbefore as a fuse for detonating explosives, it could in fact be applied to any single-use photoswitching application; one simply would substitute the firing pin with a spring-loaded electrical switch or mechanical linkage that would activate whatever mechanism.