Hey all, I'm building this power supply where I use ~12V from the cigarette lighter jack in a car and have it run through a voltage regulator( LM 7805 ) to power : atmel microcontroller, UM12 modem, LCD and a GPS receiver.

I need to add a brown-out circuit ( most microcontrollers have one built in ) but i'm not sure how to connect this circuit to the rest of the modules.

Any bit of help would be greatly appreciated.

 

With respect to battery powered systems the concept of a brown-out is out of place. In a mains powered circuit it refers to a temporary dip in the voltage. If the power supply cannot handle such a dip in the AC voltage, it's regulated output will dip. In automotive systems recoverable brown-outs don't generally happen. A load dump transient is probably of greater concern.

To answer your question there are probably several different ways you could approach the problem. If you use a diode to charge a capacitor from the +12V and compare the value on the capacitor to the +12V you will get an indication that the +12V has dropped below the capacitor voltage before the capacitor has a chance to discharge.

 

What PapaBravo said, plus when you start the car, or hit major peaks with a 1kw+ audio amp, you can have major voltage dips as well.

What kinds of problems are you having?

The PIC's and AVR's mostly have brownout detection built in. Just enable it and the cpu should just reset itself if this happens.

If this is a commercial product, I would highly recommend moving to a switching power supply. That 7805 is really inefficient.