Multiple Power Source Switching Circuit

Is this possible?

  • This is possible

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  • This is impossible, you're an idiot

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Thread Starter

MechTurtle

Joined Mar 30, 2017
9
Hello, I'm relatively new here and I'd like some help with a compact powered breadboard project I've been working on.

I don't have much in the way of design yet, but my goal is to have a breadboard connected to two power wires (one V+ and one ground), and the two wires to be attached to a circuit capable of switching between a 3V double AAA battery pack, a 9V battery, and a 5V DC power jack. The only condition is that my limited arsenal of parts only gives me access to one ON/ON toggle switch and a handful of ON/OFF toggle switches. I understand that this issue could be resolved with a simple rotary switch, but I would prefer it didn't have to come to that.

I've bounced around the idea of using a bistable multivibrator, but I don't think it would be as seamless with three different power sources.

My current plan of action is to have a single toggle switch to delegate between 3V and 9V, but both are usurped if the DC jack is plugged in. This is the most elegant solution I could come up with and it sounds reasonably doable, but I can't seem to figure it out.

To recap, the circuit needs:
- a toggle switch to control whether the circuit is on or off
- another toggle switch to switch between 3V and 9V
- a DC jack that takes precedence when plugged in

I would appreciate any two cents on how I might go about this or if a setup like this is even possible. Thanks in advance!!!
 

crutschow

Joined Mar 14, 2008
23,816
You just need a DC jack with three connections, such as this.
It has a contact which connects to the external power when plugged in (2 to plug), and the internal circuit when not (2 to 3), as shown in the schematic below:
Pin 1 (center) is common, pin 2 is power to the circuit, and pin 3 is the internal power source (3V or 9V)
upload_2017-9-5_15-29-46.png
 
Last edited:

crutschow

Joined Mar 14, 2008
23,816
So do you mean something like this? (with the "battery jack" in the image representing the 9V/3V connection, and the Vcc/V+ wire representing the positive wire attached to the breadboard)
I think you have two pins reversed.
I believe C corresponds to pin 2 in my schematic which goes to the circuit Vcc/V+, and S corresponds to pin 3 in my schematic which goes to battery pos+.

Do you have a diagram of the internal connections for your jack?
 

Thread Starter

MechTurtle

Joined Mar 30, 2017
9
Oh, I assumed the orientation of the two positive terminals didn't matter and C and S were interchangeable. So pin 3 has to be connected to the battery and pin 2 goes to the circuit? This is what my dc jack looks like so I guess if that's the case then yes, I do have it backward.
 

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crutschow

Joined Mar 14, 2008
23,816
I assumed the orientation of the two positive terminals didn't matter and C and S were interchangeable
No.
Think about it.
If you connect it as you show then the battery and external supply would be shorted together when plugged in, and the circuit V+ would be open.
 

philba

Joined Aug 17, 2017
960
You'll need a wall wart with the center pin as ground for that to work. For some reason, center pin positive seems to be much more common..

You can also use schottky diodes on each of the two inputs or PMOSFETs for less of a voltage drop.
 

crutschow

Joined Mar 14, 2008
23,816
You'll need a wall wart with the center pin as ground for that to work. For some reason, center pin positive seems to be much more common..
If that's what he has then he could wire the circuit to switch the common (minus) side of the batteries and external supply instead of the positive side.
 

Thread Starter

MechTurtle

Joined Mar 30, 2017
9
this is starting to delve slightly outside my area of expertise so i'll try out your previous suggestion and see if that works. Thanks for the help!
 

crutschow

Joined Mar 14, 2008
23,816
this is starting to delve slightly outside my area of expertise so i'll try out your previous suggestion and see if that works. Thanks for the help!
If you have a 5V wallwort with a plug and you can determine the polarity, I can post a wiring diagram of how to connect the socket to your circuit.
 

Thread Starter

MechTurtle

Joined Mar 30, 2017
9
So I've mapped out the rest of the circuit and put the circuit on a breadboard where the Battery ground leads and the "-Board" lead to 1 on the DC jack. The way I have it on the breadboard is that point 4 of the ON/ON switch is connected to 3 on the DC jack and point 5 of the ON/OFF switch is connected to 2 on the DC jack. The circuit works fine until I plug in the power cord, then the voltage is displayed as somewhere between -114 volts and -194 volts.

Could you please explain one more time which connections I should make to fix the circuit?
 

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Thread Starter

MechTurtle

Joined Mar 30, 2017
9
Yup that works! I had to switch the wires so that 2 on the jack was to the batteries and 3 was connected to the circuit, but it works like a charm!
 
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