Broadband AM radio with audio amplifer, for sniffing out sources of interesting AM from things like soda machines, swipe card readers, etc.
PDF of schematic:
The magic is in U2, TA7642 AM detector with built-in AGC.
Power is from 3 AA cells – should last a long time.
Coil is roughly 200 turns of wire-wrap wire on a Fair-Rite 4078377511 ferrite rod. I wound the core on a bit of (unshrunk) heat-shrink tubing which allows the coil to be positioned near the end of the middle of the rod, affording a bit of tuning.