For something feeling like a fairly specific IC, I remember seeing many projects that use it throughout the years in wacky ways - and seeing it makes me happy to know that the sentiment for this little piece is shared.
There's a lesson in there somewhere.
https://hackaday.com/2011/08/05/building-a-computer-out-of-5...
Turns one a single frequency that’s remotely close to one of the two tones of a dial tone will convince it. Wasn’t sine wave either but not a problem! 555 powered by a 9V battery.
555s are such delightful little guys. I used a pair of them, plus an ebay telephone line driver, to make an old telephone ring: https://hardfault.life/p/telco-2
One timer runs at the ~20Hz ring frequency, and the other runs at ~0.2Hz on a 20% duty cycle. The slow one's output feeds the enable line of the fast one, so you get 1-second burts of ringing, then a few seconds of silence, then ringing... just like a normal phone.
I moved about 5 months ago and haven't had time to get back to this project. The goal is to build a little phone company in a box, so I can have all my old PCs talk to each other with their modems.
building an amplifier from transistors is sometimes simpler/cheaper than using an op amp. And some designs don’t need the benefit of op amps.
On the other hand, building a timing circuit from discrete components is less obvious. And the 555 does so much of what often needs doing.
Also, the design was not patented so they were commodity chips right away.
i have the page archived, but it's called A New Solar _ Wind Charge Controller Based on the 555 Chip (2_7_2026 12
I can upload the webrip if anyone wants it