So, I’m doing a project where I have two 5v circuits that share a positive and I want to sense when they have power running through them. If I ground both negatives then the circuits will connect and when one circuit is powered the other will be powered too. Is there any way I could connect the positive to 5v on the arduino and both grounds to input pins or something? I really have no idea, thanks!
and both grounds to input pins or something?
No that makes no sense.
Your description of what you want to do is very vague. Can you add some detail?
What are these two circuits?
How do you want them to behave?
Why do you want to shut down one and have the other still work.
Have you got a schematic of each along with how you propose to power them?
Attache a wiring diagram. The Shakespeare type diagram is not good enough to find errors. It only consumes time and efforts to get understood.
So I’m taking the blinker signals from a golf cart. The wiring harness from the turn signal lever has 3 pins on the top row. The first pin is the negative for the left blinker, the middle is the positive for both the left and the right, and then the right pin is the negative for the right blinker.
When the cart is turned on there is a 12v signal going through both circuits and when one blinker turns on the circuit loses all power but the opposite circuit stays powered.
So, I want to be able to read when one of the blinkers is turned on and when there isn’t power running through it. But since the negative is the only thing differentiating the two circuit I can not combine both of them and by grounded them. If I did that when one blinker is on the arduino would read that both were on.
So I need a way to keep both grounds separated so I couldn’t just ground the negative and then connect the positive to an input pin since the positive is the same for both blinkers.
Also I’m using lm7805’s to take the 12v down to 5v so the input pins can handle it. Please let me know if you don’t understand something with my explanation I couldn’t find a wiring diagram for the turn signal lever.
Please, no more Shakespeare. Attache a wiring diagram. Pen, paper and a good photo often works well.
Look again, 12 volt systems do not turn lights on and off by switching the common/ground, the +12 supply is switched in the directional signal.
You read 12 volt signals in an Arduino with simple voltage dividers, 7805 regulators are for power supply use, not signal level translation.
Optocouplers work extremely well in such cases, to protect Arduinos from the hazards of amateur miswiring.
No electrical connection between the golf cart and the Arduino is required.
so I couldn't just ground the negative and then connect the positive to an input pin since the positive is the same for both blinkers.
No.
Use an opto isolator, that is what they are for.
s1ant:
... words ...
Railroader:
The Shakespeare type diagram is not good enough to find errors.
s1ant:
... blah blah ...
Railroader:
Please, no more Shakespeare
Thanks for the chuckle in these tough times.
(Actually I think the OP makes more sense than old Bill did, but that might just be me)
Hi,
When the cart is turned on there is a 12v signal going through both circuits and when one blinker turns on the circuit loses all power but the opposite circuit stays powered.
So when the indicator lever is in the middle position, both lights are ON or OFF?
Have you used a DMM to prove your wiring tome?
Tom.. 
When you connect the positives together, that is what in most circuits will be called "ground" or "0V".
The original "ground" then becomes -5V or -12V.
After all, "ground" or "0V" is an arbitrarily chosen reference point, which commonly is the lowest voltage present in a circuit. Some circuits have the 0V at a mid point, so both positive and negative voltages. It's uncommon to use the 0V as highest voltage and have everything else negative, but also nothing preventing you from doing just that.
If you do it like that, you probably need 7905 regulators instead (or something at least halfway modern, of course).