How would I wire one of the DC plugs with the switch built-in. It has three prongs comming off it. Its for when you plug in the wall power it switches from the battery power. Thanks in advance.

The middle one goes to the +9 volts (or however many volts you are using) on the system (probably on the 7805). One side goes to the + end on the battery, and the other goes on the + end of the adapter

I thought one of the prongs go to the ground? What do you mean by the "positive end of the adapter"?

I think spongebuell is talking about a spdt switch for the problem. I've heard that there are dc plugs that automaticaly switch, but I haven't seen one in any electronics store near me so I can't really help.

Yeah, I did mean a SPDT switch. You shouldn't need anything to go into the ground. As for the adapter, there is always a + and a - end, use the +. Usually the inside of the plug is one, and the outside is the other.

Plug the wall adaptor into the jack. Test the pins with the multimeter until you find a voltage across two pins. Think of these as V+ and V- and call the third pin NC. Now unplug the power. Check continuity between NC and the other pins.

If NC connects to V- when nothing's plugged in: Connect the positive terminal of the battery to V+ on the jack and Vcc on the system board. Connect the negative terminal of the battery to NC, and connect V- from the jack to ground on the system board.

If NC connects to V+, connect: battery(positive) -- NC ; battery(negative) -- V- -- system ground ; V+ -- system Vcc.

This is the dc jack w/ switch I have right here.

http://www.radioshack.com/product.asp?c ... d=274-1583

Can you give me info on this one? Thanks.

I'm not sure to how to wire it, but with switching jacks like that it normally has nc and no labelled lightly on it near those wiring points.

Wow, you're in luck. That looks just like the power jacks I've been dealing with at work for the last couple of days.

In that picture, the pin on the right, the longest pin, might have a lightning bolt on it, is connected to the center pin in that picture when no jack is plugged in. Connect that pin to the negative terminal of the battery.

The center pin in the picture is connected to the outside of whatever power plug you use. I'll assume you've got a plug with the center positive and outside negative. Connect the center pin to V- or ground on your system.

The left pin is connected to the pin inside the jack. Connect that pin to V+ from the battery and V+ on the system board.

If you have a metal case, be careful. The case is grounded when you plug in external power. If you want it grounded all the time, you need another connection to the case.

You said the left pin is connected to the battery AND the system V+. I don't get it? Doesnt that mean constant power will flow from the battery to the system. And since that very same pin is connected to the positive pin on the wall adapter won't that mean if the external power was plugged in, constant power from the external source AND the battery will flow in at the same time? Or am I confused somewhere? Thanks for your patience.

no, what the switch does is switch between the - sources of the batteries and dc adapter. Without the - contacting the board the circuit is open and voltage will not flow through it.

Yep, electricity needs a loop to flow. You can break that loop at any point to switch power off.