Comment by gary_0 2 days ago
I designed the electronics for a heavy-duty industrial 3D printer and used a 555 in the failsafe circuit (alongside the manual e-stop). If it didn't get reset by a heartbeat from the embedded computer/software, it would unpower the heaters and actuators.
Yes, a simple purpose-made chip designed to be used in safety-critical situations, with high tolerances for voltage etc, would probably be better. Although one thing the 555 design has going for it is that a seasoned EE could take one look at the physical circuit and know exactly what it does.
But I would never trust anything that ran software for something like this.
The chips themselves add a bunch of new failure states to consider beyond software bugs, too. Maybe a bad wire or component puts too much load on the microcontroller's wee internal pin drivers and they melt into a permanent "on" state. Or a voltage fluctuation browns out the chip on boot, partially randomizing its RAM or registers. Or the chip manufacturer fixes some errata or discontinues a particular part number and now a pin you've left floating has become a hardware heisenbug. Or the wrong bit flips in your EEPROM after being in a hot machine for a few years. Suddenly a boring 555 looks pretty good. (Keep in mind, we're talking about "turn off heater after pulses stop", not "abort launch sequence if tank 3 pressure low". The latter is way above my pay grade.)
That's the only use for one that isn't (always) a design smell - it makes a really nice missing pulse detector, better than you can easily do with comparators. But if you have the budget, a purpose-made watchdog chip or a tiny microcontroller really can make a better watchdog.