Six months after the Pearl Harbor bombing, we provided an easy guide to making your own affordable airplane detector. Although this instrument wasn't as sophisticated as the ones used by army searchlight crews, it was sensitive enough to detect a conversation occurring three blocks away. Civilians using this device could reasonably locate planes in a five to ten mile radius. The microscope installation pictured at left used a rock-wool plug, a shielded microphone cable, a water-tight metal shield, and a static microphone cartridge, among other materials. The "ear" could be taken from the horn of a phonograph, but you could also make it from a sheet of metal. The stand was made from scrap lumber, and the entire device was battery-powered. Simply plug earphones into the jack to begin listening.
Read the full story in "Homemade Plane Detector"