I don't have a problem with the woodworking aspect but thought it would be a neat idea if I could incorporate some inexpensive electronics that would sense a height greater than 42" and light a small bulb at the top of the lighthouse.

I was thinking of a low voltage circuit that could be powered by a battery. My talent in electronics is rudimentary.

Could anyone suggest how this might be done?

-You could use the garage door sensor that keep the door from closing is something is blocking. The send a beam from transmitter to receiver and generate a signal when the beam is broken.

-Perhaps a copper plate on the floor and another plate 42" above the floor (on the wall). The easiest way to get a complete circuit would be with an old oil-ignition transformer, but you might want to use something low voltage that detects capacitance between the two plates.