Morse Radio
The radio is protected by epoxy. I used crystal clear epoxy in order to make the schematics as visible as possible. I did this so that people can study these schematics, learn about electronics again and start to rebuild the world we live in today. Since there most likely aren't any batteries left in a few thousand years there is a text readable on the radio explaining how to build a battery out of fruit. There is also a morse table readable on the radio in order to convert text to morse and back.
The radio is used by tapping a light dependant resistor on the sender, when this LDR is tapped a LED on the receiver side lights up. I used light instead of sound because light doesn't need any moving parts, making the radio stronger.