Technology has moved at such a furious pace that what would have been most secret military technology a few decades ago is now surplus on eBay. Case in point: [msylvain59] picked up a Soviet-era K-13 IR seeker used to guide air-to-air missiles to their targets. Inside is a mechanical gyroscope turning at over 4,000 RPM, a filter made of germanium to block visible light, and a photoresistor. It’s sobering to think you can get all of this in a few small packages these days, if not integrated into one IC.
Fitting on top of a missile, the device isn’t that large anyway, but it is nothing like what a modern device would look like. A complex set of electronics processes the signal and moves steering actuators that control fins and other controls to guide the missile’s flight. You can see a video of the device giving up its secrets, below.