"power good" and "power bad" with the 555? - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: June 17, 2025, 09:19:12 pm
content: I recently did a design that was essentially a timer. It had two functions:Power good: supply voltage continuously above 20V for 1 second (other stuff gets triggered by power good)Power bad: supply voltage continuously below 18V for 5 miliseconds or more (other stuff gets triggered by power bad)In retrospect, I could have used one or more 555 timers.I have not dealt with the 555 in many years. Only having ever blindly constructed a few 555/556 circuits when I was a hobbyist.Much to my embarrassment, even after a fair bit of professional work I still view the 555 as sort of complex and physically big compared to all of the various circuits that can be whipped up with transistors, comparators, etc.I'd like to become more confident with the 555 since it is such a commodity. It is very versatile - although that flexibility creates some potential for footgunning oneself.I was wondering if anyone could point me to an example of designs that use the 555 to implement a "minimum on" and a "minimum off" period for setting a bit (eg. driving a RS latch or the equivalent). In this case it would be, if the POWER_GOOD signal is 'good' for 1 second continuously without ever going 'bad' set the output bit. And if the signal is 'bad' for 5 milliseconds continuously without going 'good' clear the output bit. (or similar, whatever logic polarity works)I have a suspicion that maybe a single 555 could be fenangled into performing both (delay) functions if one was clever. And two or three 555s certainly could do it and provide the 1 bit of output memory in a more conventional design.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 17, 2025, 10:32:19 pm
content: What does"continuously ... for 5 milli seconds"mean?For the rest, the 555 does not have any sort of voltage reference. the whole thing is radiometric. As a result, timing parameters vary very little regardless of the power supply voltage (I think 5V and 15V are the limits of the NE555).You can probably make it work with a 555 and a TL431 (which is about the cheapest voltage reference you can get).But there is a whole class of "reset controller" and "brownout" control / protection IC's.

author: incf
author_rank: Frequent Contributor
timestamp: June 17, 2025, 11:12:03 pm
content: It means something along the lines of:there is a digital signal POWER_GOOD output by something like a TL431 that has some filtering and hysteresis (eg. high = power is good, low = power is bad)and that signal must maintain the same value (low) for 5 milliseconds continuously without going high for a single nanosecond. (otherwise the 555 ignores it and the memory does not get cleared, memory element reset = 0)at the end of the 5 milliseconds a RS latch (or similar memory element) gets cleared (memory element reset = 1)clearing takes priority over setting in the memory elementthe 555 stops trying to clear the memory element at some point (either when POWER_GOOD changes, or after some period of time)

author: WattsThat
author_rank: Frequent Contributor
timestamp: June 18, 2025, 04:27:39 am
content: As far as I’ve seen, which is a shallow dive, processor supply supervisors don’t have the timing and dual level features required.IMO, this requirement looks like a resistive divider, TL431 (or other suitable reference), and an atTiny5. Done.I seriously doubt you could get your bom cost under the above parts with a cobbled together pile of discretes and one or two 555’s or a 556.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 18, 2025, 04:46:23 am
content: I agree that a small microcontroller may be the best option. Even though the task at hand is "extremely simple" for a microcontoller, these things cost somewhere between a few cents and 50ct (for the small ones in low quantities). You would also need a voltage regulator because uC's only work on low voltage (max 3V3 or 5V) but as you need some voltage reference anyway, you can also use the voltage regulator as the voltage reference. Most uC's also have an ADC, timing is also trivial (especially if the RC oscillator (typical deviation a few percent) is good enough.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 18, 2025, 04:58:10 am
content: Quote from: incf on June 17, 2025, 09:19:12 pmThat can be done with standard parts : An OV UV detector, open drain, like the SOT26 CS305 etc and a SET/RESET latch.The RC's on the open drain do the timing, and the latch alternates GOOD / BAD.Quote from: incf on June 17, 2025, 09:19:12 pmAs others have mentioned multiple 555's is not a great fit, it moves from 'conventional' into 'ugh' as more bits are cobbled on.An OV/UV detector already has a reference included, and if you need a delay, simply add a RC to the Open drain parts.The 555 has a RESET / notSET action, but it more naturally does 'pulse for T' action, rather than 'wait for T' you need here.