Circuit Help - Hooking relay to ignition with timer to power PC on/off

[CaptainKahuna]

First, a little background; I installed a PC in may car to play mp3s, all the details and pics are here: http://www.mp3car.com/usersites/bombboyer

The electronics question:

What I have now is a switch for my PC mounted in the dash, and an inverter in the trunk. The inverter's low voltage alarm goes off when I crank the car, so I installed flip switch in the glove box so I can give the inverter power after I crank the engine, then I hit the PC power button

I want to replace this system with one that is more efficient. When I crank the car, I want a 5 sec delay before the power to the inverter is connected. Then, when I turn off the car, I want power to the inverter to be cut.

There also needs to be an "override" flip switch to enable or disable this whole system, so I can run the PC with the car off if I want to.

I need your electronics expertise to design and build this circuit. If I'm not being clear, LMK.

 

[blahblah99]

Well assuming that you don't need to shut down the PC before cutting off power, you can do achieve what you want by using a relay hooked up to your ignition wire with a bypass switch connected to your battery or permanent power. The ignition wire is 12V when your key is on ACC or ON position.

Or you can have an RC network with a time constant of 5 seconds connected to a comparator with some hysteresis, whose output is fed to drive a transistor to drive a relay.
The RC network gets it power from a wire that gets 12V when your key is on the ON position. That way the relay won't power the inverter until the capacitor in the RC network gets charged up to 12V, which will trigger the comparator.

Or you can use a microcontroller.

 

[Geniere]

Preview Message [ CLOSE WINDOW ]

You needn't build your own, as time-dela relays are available to do exactly what you want - link below:

http://relays.tycoelectronics.com/datasheets/Time_Delay_Relays/CH_DS.pdf

You might check Radio Shack, an auto parts store, or a decent electronic parts store.

You'll want one with a 12vdc coil and delay on operate feature. I like the potentiometer type, as 5 seconds may be too little or too much.

If you want to build your own, a Google search for "time-delay relay" will provide circuit diagrams. Unless your geared up for it, I don't think it's worth the hassle to save a few bucks.

Regards

 

[Geniere]

Not sure how preview massage, close window got into my post

 

[CaptainKahuna]

How would I wire an override switch in conjunction with that?

 

[Geniere]

Here?s how I would wire it:

From the fuse protected voltage source that?s hot only when ignition is on, to the + coil pin of time delay relay. The (-) pin of the coil goes directly to ground.

From a fuse protected voltage source that?s hot regardless of ignition key status, to one of the normally open contacts of relay. The mate of that contact goes to the inverter. These contacts close after 5 seconds.

Bridge those contacts with the ?over-ride switch? (parallel the switch with contacts).

If the switch is on, the inverter will always be powered. When switch is off, the inverter will be on only when ignition is on for more than 5 Seconds and will go off only when ignition is off.

If you want to turn inverter off while ignition is on, you?ll need another switch to break circuit between relay with over-ride switch and the inverter.

Regards

 

[CaptainKahuna]

Nice, thanks for the ideas.