Hi, new electronics student here with a project in mind to light up a small sculpture with some LED's. I think I can manage the LED portion, but it's powering it I'm having trouble with. I have an ambition to make it possible to power it with both a wall outlet or by battery, but have a system in place that ensures the circuit will only draw from one or the other to avoid double feeding. I've gotten my hands on a circuit simulator called MultiSim through school I've been playing with to try and work this out, but I'm still very new to it.
My idea is this; for portability, I would have some 12V battery source that could be drawn from, but, to also have a port where I could plug in a 120 to 12V converter to draw from a wall outlet, both options being toggled by some form of switch. So to do that, I figured I'd use some kind of relay that when unpowered would complete the circuit for the battery source and light the sculpture. Then when plugged in, the relay would be powered and toggle, closing the circuit from my wall outlet and opening the battery one. Would this all work like I think it would? It makes sense in my head, but I can't figure out how to wire it up in multisim to confirm my idea as I'm not all that well-versed in the hardware I would need.
I'll also be designing the base for the sculpture where all this will be hidden, so the size of the hardware and circuit is flexible, but I'd like to keep it small as possible. Any and all help or feedback would be appreciated.

 

You can simply add a diode at the 12v battery pack and on at the adapter (use a 13.5v Dc wall adapter to insure the adapter provides power preferentially). When the higher voltage is active, no current will flow drop the battery. When the Adapter is disconnected, power will flow from the battery.

 

If you buy a DC power socket, such as this one, which fits most plug-top power supplies, you’ll find that is has a built-in switch which will disconnect the battery when a plug is inserted.

 

Welcome to AAC!

So to do that, I figured I'd use some kind of relay that when unpowered would complete the circuit for the battery source and light the sculpture.

You can do it with a power jack:

You connect the positive terminal on the battery to terminal 1 and negative to 3. When nothing is plugged into the power jack, the battery powers the circuit. When a power plug is inserted, it breaks the ground connection for the battery and connects the power plug to terminals 1 and 2. Terminal 2 is connected to circuit ground.

 

Using a switched jack is a very good idea. Alternatively, depending on your power requirements, a board like this one might be a good option. It would also allow you to use a lithium cell and charge it. I don't know how much power you need, though,

 

Sorry for the late reply. I posted this before class and didn't expect such prompt responses.

You can simply add a diode at the 12v battery pack and on at the adapter...

So to make sure I'm understanding, this system would replace the need for a relay?

If you buy a DC power socket, such as this one...

You can do it with a power jack:...

This sounds like it would drastically simplify my project. While I was kind of looking to challenge myself with building some fancy circuitry (By my standards), I am also tempted by this.

Using a switched jack is a very good idea. Alternatively...

The lighting circuit I have in mind works between 9 and 12 volts. That does bring to mind a thought I had, which was to build a system that would be able to charge the battery as well. I was just concerned it may be beyond my scope of understanding.

Thank you all for the suggestions. I'll take it all into consideration and hopefully have some prototypes to show off soon... Or burnt components and more questions. We'll see.

 

Why not let two diodes select the adapter or the battery automatically? It is called an OR gate, adapter OR battery.