A crystal-set radio was a LOT of people's introduction to electronics, I remember building one with my dad at age 6 or so. We were a fair distance from the stat

A crystal-set radio was a LOT of people's introduction to electronics, I remember building one with my dad at age 6 or so. We were a fair distance from the station so the signal was faint, but the earphone reproduced enough to hear the news and most importantly, the station call letters. They really are extremely simple things, and you can use the base-emitter structure in any BJT if you don't have a discrete diode around. The trick is knowing how many turns of wire to put on the inductor, but if you're stuck in a foxhole for who-knows-how-long, you have time for a lot of trial and error.