Timed swich

[Nice Hat]

ok heres the deal....most of you know that in most newer cars you can take the key out and still use the window and radio for about 2 or 3 min.....what i wanted to know is there something i can whip up that would do something like that....i only know a little about this kinda stuff so im not rely sure where to start

 

[jbelectric777]

This may work

555IC w/ a IRF510 MOSFET , I guess you can use the timer as an off delay and feed the fuse box from the mosfet. Normal power would cut out from the ignition switch but be delayed via the 555 holding the 510 for a few minutes. An even better way would be to designate only those circuits you want delayed off. Good Luck

 

[Nice Hat]

ok so if i understand correctly...i run power from the battery to the MOSFET to the 555 with cap that determines the amount of time it stays on????then how do i trigger it to do this??

 

[Nice Hat]

look at the atached diagram whould this work??if so could it be triggered usng a normaly closed cirut while the car is on then once there is no voltage it will start the timer???any help whould be really apriciate

 

[jbelectric777]

add a CR and holding circuit

I was lookin at using just a FET and 555 but I dont think that will work because of the high state the 555 is in all the time. The way I would do it would be to use a 12V control relay. It sounds like you have already figured out a way to do it, if that does'nt work , use a CR. When the car ignition switch is closed it will bring in (and hold) a control relay with the holding contact in series with the FET, when the car is turned off you want a charged capacitor to discharge via a certain size resistor until the voltage at the trigger drops below 2/3 (8V) When your trigger changes states it will drop out the FET which will drop out the CR and cant be closed again until the ignition is turned on again... Good Luck

 

[jbelectric777]

actually the 555 should be able to hold a relays coil all by itself without using an FET, so a charged cap thru a resistor until dropping below the needed 2/3 should be easier dont ya think ? les complicated anyway