555 Timer Off Delay Configurtion - Page 1
Forum section: electronics / beginners

author: Instro_Spark
author_rank: Contributor
timestamp: August 10, 2023, 12:59:59 pm
content: Hello,I'm wondering if it is possible to configure the 555 timer as a delay off timer?When the switch is low the output is on and on the positive going edge of the switch then the timer activates.As per the timing diagram attached, except with the input polarity reversed.https://www.geya.net/off-delay-timer-relay-meaning-working-and-application/

author: BillyO
author_rank: Super Contributor
timestamp: August 10, 2023, 04:25:01 pm
content: I can't see how you could do it with a 555. Maybe with two of them, or a 556. But I think your best bet would be with a 74HC123 as it has both positive and negative edge trigger as well as both inverted and non-inverted outputs and houses two units. A lot of possibilities there. Better still a small microcontroller like an ATTiny85. It would be trivial and the chip is compatible with the Ardunio IDE.

author: ledtester
author_rank: Super Contributor
timestamp: August 10, 2023, 08:08:33 pm
content: Here is a product that might have the timing mode you are interested in:https://www.amazon.com/ACEIRMC-Controller-Delay-Off-0-01s-9999mins-Switching/dp/B09J2L4H2F/From the manual:https://m.media-amazon.com/images/I/B1GVAXm4XqS.pdfmode P7:Quote

author: sparkydog
author_rank: Regular Contributor
timestamp: August 10, 2023, 09:05:49 pm
content: An MCU is possibly overkill.This might be a job for an RC delay. Something likethis. Tune R and C as needed. I'm not sure you need the first resistor (though it will limit inrush), but the simulation is very unhappy if it's omitted. The FET is just for demonstration and is actually a poor choice since it's going to show significant analog switching in this application. In a real application, I would recommend a Schmitt trigger, which you could build with a comparator... or a 555. 🙂Actually, considering that most 555 applications have additional resistors and capacitors, this might qualify as "configur the 555 timer as a delay off timer".

author: Zero999
author_rank: Super Contributor
timestamp: August 10, 2023, 10:06:28 pm
content: I wouldn't say a microcontroller is overkill. 8 or 6 pin devices are commonly used for such simple tasks and often work out cheaper too.The classic delayed off 555 timer, switched by a transistor to invert the input would do the job.

author: BillyO
author_rank: Super Contributor
timestamp: August 10, 2023, 10:32:51 pm
content: Quote from: sparkydog on August 10, 2023, 09:05:49 pmAn ATTiny85 might be way more than you need, but there are options as low as $0.50 that could easily handle this task and other than a bypass capacitor you need no other components. Think of all the real estate you would save too.Actually, I don't think the solutions shown so far are what the OP is asking for.It seems he wants the output to go high when the trigger goes low, then once the trigger goes high the timer starts and the output goes low the timed interval later.

author: ledtester
author_rank: Super Contributor
timestamp: August 10, 2023, 10:45:57 pm
content: Quote from: Zero999 on August 10, 2023, 10:06:28 pmIt looks the output isn't enabled until the input goes low. However, this is the timing diagram from the OP's linked page:which shows that the output is enabled when the input is high as well as for the delay time after the input goes low.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 10, 2023, 11:39:58 pm
content: You could do something like this with a 555. Depends on what the "switch" is in reality. If it's an actual switch, you could put it directly across C and eliminate the leftmost diode & resistor.

author: BillyO
author_rank: Super Contributor
timestamp: August 10, 2023, 11:49:11 pm
content: Quote from: Kim Christensen on August 10, 2023, 11:39:58 pmYou could use a PNP transistor emitter-follower as the switch. This is basically using the 555 as a Schmitt trigger inverter.Maybe we should be asking the OP what the project is. Perhaps the inclusion of a 74HC14 might be handy for other things. Three of the inverters could be used to implement this functionality.

author: ledtester
author_rank: Super Contributor
timestamp: August 11, 2023, 12:46:00 pm
content: I breadboarded this circuit and observed that the output remained high for one delay period at power up.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 11, 2023, 03:04:40 pm
content: Quote from: BillyO on August 10, 2023, 11:49:11 pmYup... Or he might have a spare OpAmp or comparator which could also be used. Or may not even need this delay at all...Quote from: ledtester on August 11, 2023, 12:46:00 pmThanks, I forgot about that.To avoid the power up "feature" you can swap the R and C time components around. A resistor in series with the diode would also be need to limit the surge current through C when the switch closes. (Prevent dips on the supply rail)

author: ledtester
author_rank: Super Contributor
timestamp: August 11, 2023, 03:49:46 pm
content: Quote from: Kim Christensen on August 11, 2023, 03:04:40 pmCould you post an updated schematic? I tried a bunch of things but couldn't get it to avoid the start-up "feature". Thanks!

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 11, 2023, 04:18:24 pm
content: Quote from: ledtester on August 11, 2023, 03:49:46 pmMy verbal description was a bit off. This should work. R1 should be much larger (100x or more) than R2 to minimize the voltage drop across R2 when in the steady state with the switch on. A good starting point is to make R2 1K and then figure out the timing components (R1 and C1) from there. There will be a small delay before the output goes high due to the R2/C1 time constant. This could be a boon for immunity to false triggers or a bane if the user wants to detect very short closings of SW1.

author: Zero999
author_rank: Super Contributor
timestamp: August 11, 2023, 04:48:36 pm
content: Quote from: BillyO on August 10, 2023, 10:32:51 pmThe original poster wasn't clear. The circuit I posted does the same as that chart, but with the input inverted.Quote from: Instro_Spark on August 10, 2023, 12:59:59 pmTiming diagram from the site linked by the OP.QuoteI agree about using a microcontroller. The PIC12F509 or PIC10F202 are other possibilities.

author: BillyO
author_rank: Super Contributor
timestamp: August 11, 2023, 05:36:51 pm
content: @Zero999According to the chart you provided it seems that your circuit turns off after the set time even if the trigger is still low. It also turns off if the trigger is brought high. This in not the behavior the OP is looking for. If indeed he wants something that works like what he linked except with the trigger inverted then he wants the output to go high when the trigger is brought low, then go high a set time after the trigger goes high. Not when the trigger goes high and not before the trigger goes high, but only the set time after the trigger goes high.In any case, so far he's absent so maybe this was not that important to him.

author: Zero999
author_rank: Super Contributor
timestamp: August 11, 2023, 06:21:42 pm
content: You're right. I was having a brain fart. What's required is a pulse extender, but inverted.

author: ledtester
author_rank: Super Contributor
timestamp: August 11, 2023, 08:37:11 pm
content: An old thread:https://www.eevblog.com/forum/beginners/pulse-extender-circuit/