555 monostable circuit for PC hibernation

Thread Starter

DJS

Joined Sep 13, 2019
30
Hi,
I want to build a circuit using 555 that would monitor 12V battery and turns on relay momentarily (relay ON time should be around 0.5 seconds) when the battery voltage reaches 11.8V. This circuit i would like to use for mini PC that is powered by 12 V battery directly and hibernate it by wiring the relay contact directly to power button. I know windows allows hibernation if power button is momentarily pressed when windows is running.

I have tried 555 circuit having resistor divider at Pin 2 (to create trigger at 11.8V) and RC in Pin 6 (threshold to keep output HIGH momentarily) to achieve above but it is not resulting as per my requirement. Below is what i have noticed.

1. Out put at pin 3 changes to HIGH when battery voltage drops to around 11.8V which is OK.
2. The problem is that the output remains HIGH until Pin 2 voltage becomes more than 11.8V.

For PC to go into hibernation i just wanted the output to become HIGH just for moment, say 0.5 seconds. Any suggestion from expert will be appreciated.

PS: Circuit diagram attached.

Best,
Deepak
 

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ericgibbs

Joined Jan 29, 2010
13,652
hi DJS,
Welcome to AAC.
One option would be a second 555 set up as a Monostable to give a single pulse, when triggered by the Vbat sense 555.
E
 

danadak

Joined Mar 10, 2018
4,057
You need a little more intelligence in design, there needs to be a test
that if PC did not shutdown a pulse is re-issued. And of course if
PC does shutdown to stop issuing pulses otherwise PC will restart.

PC is shutdown if (if I recall correctly) 5V is not present from PSU.
There is a power_ok signal I think from PSU. Or just take 5V signal
out of MOBO power connector.

I had the opposite problem, to start a PC in cold weather at a remote site.
And I wanted it only to start up when the sun rises. So used a UP to
measure 5V rail and implement the test for daylight (used a solar cell)
to generate the pulse.

As part of the logic there was hysteresis built into code to make sure
system noise, when measuring PSU 5V power up/down, is "stable".
Also there were delays in code after issuing pulse to allow PSU to
ramp up, In your case to shutdown. I issued .5 sec pulse, then waited 5
sec delay before doing measurements/tests.

I used a PSOC for that design, but any UP with A/D can do it.

The signal from UP came from an output pin configured as open
collector for the start button.


Regards, Dana.
 
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danadak

Joined Mar 10, 2018
4,057
PC start button is pullup so open collector driving button to
ground is active state. The processor was configured with
the pulse out to PC as open drain as its cmos.

Regards, Dana.
 

Thread Starter

DJS

Joined Sep 13, 2019
30
hi DJS,
Welcome to AAC.
One option would be a second 555 set up as a Monostable to give a single pulse, when triggered by the Vbat sense 555.
E
My circuit was setup as Monostable which should have made the output momentarily high when the battery voltage reaches around 11.8V. I am wondering why the circuit is not working as single shot. Anyways, i can add one more 555 stage as you have suggested if that solves the problem. Will you help me with the diagram how it should be connected with my original circuit.

Best,
DJS
 

ericgibbs

Joined Jan 29, 2010
13,652
hi DJS,
I will look at your circuit in simulation, let you know.
E

Update:
Could you check this diagram, is it the same as your circuit.?
It is not showing any switch action.

031 Sep. 14 13.35.gif
 
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ericgibbs

Joined Jan 29, 2010
13,652
hi.
This is what we get with the 3k4.
As the Trig pin 2 is held below the trigger voltage it will keep the 555 On, ie; Vout will be held high.
The Circuit could be modified in order to give a single high pulse.
E
 

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Thread Starter

DJS

Joined Sep 13, 2019
30
hi.
This is what we get with the 3k4.
As the Trig pin 2 is held below the trigger voltage it will keep the 555 On, ie; Vout will be held high.
The Circuit could be modified in order to give a single high pulse.
E
Dear Eric,
Yes this is exactly what is happening with this circuit. Isn't this circuit supposed to give one shot momentary HIGH output when battery reaches 11.8V and then remain LOW until next cycle of battery low condition occurs?

And may i request for suggestion what modification is required to achieve my requirement?

BTW which simulation tools are you using?

Best,
DJS
 

Thread Starter

DJS

Joined Sep 13, 2019
30
Dear Eric,
Thank you for being so quick in replying and spending time for my problem. Really appreciated. I will try with your suggestion and update you the result. Once again many many thanks.

BTW which simulator are you using?

Regards,
DJS
 

ElectricSpidey

Joined Dec 2, 2017
1,814
Dear Eric,
Yes this is exactly what is happening with this circuit. Isn't this circuit supposed to give one shot momentary HIGH output when battery reaches 11.8V and then remain LOW until next cycle of battery low condition occurs?
A 555 needs a pulse on the trigger that is shorter than the timing cycle, otherwise it stays high until the trigger goes high.
 

Thread Starter

DJS

Joined Sep 13, 2019
30
Hi Eric,
I tried the circuit. I can see a momentary pulse output at C3 but it is not driving the FET and so is relay not turning ON. I am using IRFZ44N FET as i dont have 2N7000. I think the problem is that the gate drive current after C3 is too small and weak. I am stuck. Do you have any suggestion?

Best,
DJS
 

ericgibbs

Joined Jan 29, 2010
13,652
hi DJS,
Thats a high power MOSFET with a highish Vgs threshold, most likely not being fully turned On.

E
034 Sep. 15 07.56.gif

EDIT:
Checking the sim with a IRFZ44, shows it working.
Try a 1uF cap in place of the 470n.
What is the operating voltage of relay coil.?
 

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Last edited:

Thread Starter

DJS

Joined Sep 13, 2019
30
Hi Eric,

Tried with 1uF and IRFZ44, but it is not working. Then i somehow managed to get 2SK941 which seems equivalent to 2N7000. It worked wonderfully. Now the circuit is working as expected.

My personal thanks to Eric for assisting me in resolving the problem. It has been great pleasure discussing the issues with experts like him. Many thanks Eric once again.

Eric, i am working on one more circuit about 48V Solar Circuit and have some issues there too. If you don't mind, will it be ok if i continue that problem in this this thread or would you suggest me to create new thread as the topic is different current one?

Warm regards,
DJS
 
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