Help with 555 as a delayed shutdown switch.

[ocean750]

Hello.

Could you please help me out ?

I am trying to build a circuit that would use two push button in order to turn ON and OFF a system.

When pressing the ON button, it will be latched using a relay.

I would like that when the OFF button is pressed, a timer from a 555 will count back some time, and when it is done, the system will shut down.

I would like to add a relay where the arrow is showing, and for that relay to be activated from the 555.

I tried a few times but could not get it to work.

How should I build the circuit ?



Thank you

 

[wwfeldman]

you might start by showing what you actually built, how you tested it, and how you know it doesn't work

the sketch shows two relays, one switch and no 555 and is inconsistent with what you wrote.

 

[ocean750]

Thank you for looking in to my question.

I did manage to activate a 555 circuit .

And now I understand that I did not explain my question well.

The ON button activates the entire system, including the 555 circuit.
And some other things after the Vo (I don't think it is relevant so I did not draw the rest).

I would like that when the OFF button is pressed the shutdown to be delayed.

I could not figure out how to power the relay after the ON was pressed and to be able to shut it down when delayed.

I did not build the circuit with the cutoff relay because I could not get it to "work" on paper.

 

[d123]

Hi,

You appear to like relays...

You could maybe re-think the design a little. My initial suggestion - without having considered detailed behaviour - would be:

'On' button flips a flipflop which drives an NPN or NMOS to control the relay coil.
'Off' button triggers a pair of 555 monostables - 555 #1 is the timing period which at time-out triggers 555 #2, 555 #2 is the short signal to reset the flipflop.

Something like that. Needs work, but is workable with a little thought.

 

[schmitt trigger]

The simplest solution is to use Linear Tech's Timer Blox family.
The LTC6994 appears that might do the job.

Of course, if you want to use a 555 as a monostable, it can also be done with some glue logic.

 

[ocean750]

Hi, thank you for the replays.
I looked for a 555 and relays solution from lack of other knowledge .
will be happy to use a better way,

 

[danadakk]

For future consideration, here is a graphical programming tool to program
either an Arduino board or Atmel chip. You drag and drop functional blocks
onto canvas and config, like assign what pin is for Power On (5 in this case)
and Power off (6 in this case). The tool then generates the Arduino C code
and programs the board / chip. You can see the code generated expanding
the right hand window.

I used, in this case, Arduino Nano board, ~ $ 2. You can also use basic 8 pin Atmel
chip, but have to use a Arduino board to program it. Slightly more involved
programming setup.

Program is mBlock, free to download.

Using board time delays fairly accurate, and mechanical push buttons are debounced
so no false triggers of relay. Also power up no relay glitching. Board as shown powered
from USB, can power board directly from another source easy to wire up as board has
regulator on it..

Delay to off is set at 10 secs, could be minutes, hours, whatever.

Lots of fun, to learn basic programming. Videos on web to learn.


Regards, Dana.

--- Updated Jul 16, 2020 ---

This is roughly the lashup -

Regards, Dana.

 

[d123]

Hi,

Previous two suggestions better than mine.

Even so, here's a 555 and flip-flop circuit that in simulations theoretically turns on immefiately and delays turn-off x seconds of choice with on and off pushbuttons.

It's just a simulation, not a breadboarded circuit, so caveat emptor...

You would need to play around with the RC values to see if it's a workable solution for the amount of delay you need.

Trigger signals replaced with pushbuttons/switches for the sake of clarity:

 

[Pjdd]

I have an idea but I need to know some details to finalise the circuit.
-What is the power supply?
-What is the load?
-How much delay do you need?
-What relay do you have (voltage and coil resistance)?

 

[danadakk]

For future consideration, here is a graphical programming tool to program
either an Arduino board or Atmel chip. You drag and drop functional blocks
onto canvas and config, like assign what pin is for Power On (5 in this case)
and Power off (6 in this case). The tool then generates the Arduino C code
and programs the board / chip. You can see the code generated expanding
the right hand window.

I used, in this case, Arduino Nano board, ~ $ 2. You can also use basic 8 pin Atmel
chip, but have to use a Arduino board to program it. Slightly more involved
programming setup.

Program is mBlock, free to download.

Using board time delays fairly accurate, and mechanical push buttons are debounced
so no false triggers of relay. Also power up no relay glitching. Board as shown powered
from USB, can power board directly from another source easy to wire up as board has
regulator on it..

Delay to off is set at 10 secs, could be minutes, hours, whatever.


View attachment 162909

Lots of fun, to learn basic programming. Videos on web to learn.


Regards, Dana.

--- Updated Jul 16, 2020 ---

This is roughly the lashup -


View attachment 162912

Regards, Dana.

This would be solution using Atmel Chip, using Arduino Nano to program it. Again using
mBlock to create code.

Regards, Dana.

 

[d123]

Hi,

Whoops-a-daisy... In the second schematic with the switches/pushbuttons, U1 555 needs a pull-up resistor between the parallel RC (R11 and C1) and pin 2 (trigger) of e.g. 100k to Vaux. I forgot to add it to the schematic but it must be there for the timer to work properly.

If you are comfortable with code, Dana's solutions are good.

 

[ocean750]

Thank you all for your time suggestions and help.
actually I do like to code, but with this task using a controller is not an option.

regarding the questions:
the power supply is a 48 V battery, and I intend to drop it to 12 v or 5 for this circuit.
the load is actually a relay that would activate another circuit, so the final current will be very low.
the delay will probably be about 3 minutes.
I have a general relay that I bought at digikey, will buy a specific relay if there is a need.

 

[Pjdd]

Thank you all for your time suggestions and help.
actually I do like to code, but with this task using a controller is not an option.

regarding the questions:
the power supply is a 48 V battery, and I intend to drop it to 12 v or 5 for this circuit.
the load is actually a relay that would activate another circuit, so the final current will be very low.
the delay will probably be about 3 minutes.
I have a general relay that I bought at digikey, will buy a specific relay if there is a need.

Shown below is what I had in mind. But while a 3-minute delay with a 555 is possible, it requires large values for the timing elements R1 and C1. With such values, you have to be careful to select a capacitor with low leakage and keep the PCB surface clean.

This design uses the same 12V supply for the circuit and the load. If a different supply is needed for the load, the relay should have two separate contacts.

A design using fixed logic ICs like those from the CD4000 series could achieve the same goal without the need for large timing elements or for programming and a microcontroller. But the circuit will be slightly more complex.

 

[danadakk]

Of course design may or may not be concerned about -

1) Power up and brownout glitching of timing elements in-appropriately turning on relay.

2) Button press bounce creating false triggers.

3) Simultaneous button press, how do you want to handle that logically.

4) Desired timing accuracy of delay over T and V.

5) Effects of rapid discharge of supply, eg. discharge of large timing caps thru
pins on 555 thru internal protection diodes.....that otherwise do not occur.

Case for UP, as modern UPs eliminate power up glitching on outputs, have accurate T and V
timing (relative to a 555), are immune to bulk timing cap T and V effects and accuracy. Of
course handle 2 and 3 above easily as well.

Just a thought....


Regards, Dana.

 

[d123]

Hi,

Agree about 555 messy imprecision for long-duration timing due to capacitor sizes needed. Pjdd's circuit is few parts, nice.

I checked my simulated version for 3 minutes delay, and it would need a reset in case there was a need to re-energize the relay coil quickly after the off-delay has timed out. Here is a version that auto-resets (drains the second important capacitor quickly)... that requires three more components... Not great in comparison to other suggested approaches, very pedestrian and too many parts.
CD40xx family can work from (3V) 5V to 15V, as I'm sure you know .

 

[ocean750]

Thank you for all of your help and effort