[VK2ZAY] has a thing for 555 chips. Before the ready availability of microcontrollers, the 555 was the hardware hacker’s swiss army knife. After all, even though the chip is supposed to be a timer, it is really a bunch of simple pieces you can use to make a timer: a pair of comparators, a few transistors, and a flip-flop. You can use those parts in many different ways, and a timer is just one of them.
[VK2ZAY] used one as a key component in a simple spectrum analyzer. The 555 generates a ramp voltage which alters the frequency of an oscillator. The oscillator mixes with the input signal and a fixed-frequency superregenerative detector creates an output voltage proportional to the input signal strength. You can see a video of the whole setup, below.
Simple and elegant. Not the best performing spectrum analyzer you can find, but it does the job and it fits in an Altoid tin. Generating a ramp isn’t too far of a stretch for a 555, but we’ve seen [VK2ZAY] abuse 555s far worse.