hi . i will apreciate the help to build a circuit with RC timer to enable a mosfet N for 2 seconds to turn on a phone when 5v are present ....(ignition car key)
just that... No 555 only wish RC and n ch mosfet from old motherb have around... just a simple circuit ... when 5v dissapears just turn off the phone no problem... it is to turn it on my need.

 

You have a proposed solution, but we don’t know what problem you are trying to solve.

Also the details of how this works are lacking. Is the phone powered by the same 5V used to trigger the turn on? This statement seems to imply that:

when 5v dissapears just turn off the phone no problem...

But a typical phones will not operate without a battery and removing the charger power does not turn them off.

Also, how is it turned on? Typically a long press of a momentary button. Are you planning on using the MOSFET to short the button for two seconds? if so do you know the polarity and voltage seen by the switch?

 

So, what values of R & C do you think you might need to achieve that result, Can, you obtain those components in a useable package at a reasonable price?

 

I have the same question.
How exactly do you intend to turn the phone on and off?

 

thanks.... the circuit is just an automated switch... all i need to achieve is a two seconds (mod by cap value maybe) closed circuit, a switch... the phone android samsung s4 will turn on if i connect (the already soldered wires to the sotck micro switch )... no polarity issues here, is a switch.... the phone will have no battery, i have a 5v power supply with proper resistor and diode to the terminals and works fine.... yes the same 5v is going to be used to power the phone and the mosfet to turn it on.....im using a rom that cannot auto power when ac adapter is plugged in, in case you wonder why i need an electric solution for this....

RESUMIENDO


I turn on ignition in my bike, the 12v to 5v converter gices 5 to this CIRCUIT!! that allow me to close the switch for just 2 seconds , using the mosfet, then my phone turns on.... i said i dont care turning off part

 

Rload represents the load of your Phone. It might be much less current.
The actual time delay will depend on the mosfet specs. Do you have a mosfet part number?

 

no polarity issues here, is a switch

But it does matter to the MOSFET, it will only work one way. If you use relay instead the polarity will not matter.

 

thanks eetech00
i Will try with some mosfets around i have Irfz44n but it is too big, much A useless, i think, so would use one of many motherboard channel N from macbook scrap boards.... note that ignition switch will not turn off in many minutes or hours, when the car/bike reaches destination, so i wanted to tell you that the 2 seconds should be mosfet on and then off (but car 12v and 5v) will still be present... is this what the circuit does right? thanks eetch00 for understanding my idea and give a clear solution

 

is this what the circuit does right?

It does!

 

thanks eetech00
so i wanted to tell you that the 2 seconds should be mosfet on and then off (but car 12v and 5v) will still be present... is this what the circuit does right? thanks eetch00 for understanding my idea and give a clear solution

Yes. That is what it does.
You'll need to find out how much current your phone uses to start, then choose a mosfet with the appropriate current capacity. The mosfet VGS(th) value will affect the length of the time delay, but you can adjust the value of R4 until the desired time delay is achieved.

 

the phone android samsung s4 will turn on if I connect (the already soldered wires to the stock micro switch )... no polarity issues here, is a switch...

From that statement I thinking maybe what's needed is high side switching, still feasible using a N-mosfet with this circuit: