555 circuit to hibernate computer

[Deepak Shakya]

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 power 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 the output remains HIGH unless input at PIN 2 is removed.

For PC to go into hibernation i just wanted the output to become HIGH just for moment, say 0.5 seconds. Any suggestion what changes is required in this circuit to achieve this?

PS: I have tried to upload my circuit diagram but it says upload failed.

Best,
Deepak

 

[AnalogKid]

Please post your schematic.

ak

 

[Deepak Shakya]

https://drive.google.com/file/d/1c6OGfOyPqzGD8-i6kN7gWT19GMWFRq4X/view

Dear ak, here is the circuit diagram. Appreciate your suggestion.

 

[AnalogKid]

This is an electrical engineering forum. A schematic is a bit more than a suggestion.

Separate from that, your circuit will not work because a 555 is not a true monostable. As you have discovered, once the trigger input falls below its threshold voltage the output will change state and stay high as long as the trigger voltage is low. This is a problem anytime the trigger pulse is wider than the desired output pulse, and your circuit is a version of that. One solution is to add several parts to the Trigger input.

Another is to abandon the 555 and use a true monostable circuit, where the output pulse width is completely independent of the trigger input (once triggered). Because of the slowly changing input, I suggest a 74AC132 quad Schmitt trigger NAND gate for 5 V operation.



In your case, R1 and the switch are replaced by the battery voltage divider. The other two gates can be used to drive the LED.

ak

 

[Deepak Shakya]

Dear AK,
I tried the above circuit but it is not working as is required for hibernation. The output always stays ON irrespective of level of battery voltage. Since it was not working. I tried same circuit without feed back that is removed the jumper cable that connects the output of U2 and is fed to U1. Surprisingly then the circuit started working with correct output that becomes high for about one second. This is exactly what is needed for PC hibernation. But one problem surfaced which i don't know how to solve. This circuit has become so sensitive that its output changes when other loads (such as fluorescent tube lights, water pump etc) are switch ON or OFF. I have heard that such problem arises due to external noise. I even tried to isolate the noise by inserting paper capacitors in all points where changing voltage is sensed by circuit. But non of those helped in solving this problem. Any suggestion how to make the circuit immune to external noise?

Deepak

 

[AnalogKid]

What are the values of the two resistors you use to attenuate the battery voltage down to logic level? Based on those, you can calculate a noise filter capacitor that makes the circuit immune to short-term fluctuations.

ak