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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 b
by gary_0 8mo 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.
- buescher 8mo agoThat'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.
- gary_0 8mo agoI would absolutely not want to use a microcontroller or a complicated chip for something like that. Simplicity is the point.
- buescher 8mo agoSupervisor chips are not complicated. In some ways simpler than a homebrew analog watchdog, and the good ones will handle failure modes a simple watchdog won't, like those that result in an oscillating output.
- gary_0 8mo agoYes, 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.
- buescher 8mo agoIt depends on the system's potential failure modes and what's required by your safety standard, not on one engineer's opinion of what's "best".
- 15155 8mo agoModern 32-bit microcontrollers are cheaper than 555s.
- buescher 8mo agoNot if you go to the cheap "Asian brands" like you're thinking with micros, plus with your cheap micro you'll need a reset controller. And budget isn't all BOM cost.
- 15155 8mo agoWho provisions dedicated reset monitors on $0.06 MCUs?
- deleted 8mo ago[deleted]
- fmlpp 8mo agoYes, but uses software, so you have another level of added complexity that may be or may be not desirable.
- 15155 8mo agoFor every task you could also use a 555 timer for (with dedicated analog support complexity,) you are talking about tens of lines of user code at most. Even if you had to do everything directly with registers, the amount of C or Rust here is minuscule.
- gary_0 8mo agoThe 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.)