Probe and LCD both need 5V, How do I accommodate this? (First Project)

Hi, this is my first Arduino project and I'm building a temperature correcting pH probe with LCD/keypad. Although I've had some programming experience with C++ and Java I have no knowledge about circuits.

My issue is that when I run my probe with Atlas Scientific pH stamp and AT Mega2560, I get a fluctuating pH +/- .2 on 3.3V VCC. When I stick it into the 5V it runs fine. Is there something wrong? If I can't fix it how can I run both this probe and my LCD simultaneously if they both use the 5V pin.

Sorry if I jumbled up vocabulary or anything...hopefully I can get this wiring out of the way soon so I can start writing ha.

It sounds like you are exceeding the current draw of the on board 5V regulator.

How are you powering the Arduino?

I was initially running it through my USB. I've had some power supply issues with my USB before so I tried it with a 9V adapter as well. Both had the same problem.

Just to clarify, the probe runs fine on 5V but fluctuates on 3.3V even though the stamp is rated for anywhere between 3.3-5.5V.

Thanks for the help.

It is possible the mega is not registering a High logic level at 3.3V , ideally you want 5V input High level. Otherwise it can be caused by long wires or improper grounds. I am not sure why you can’t use the same pin for both, I’m sure the current won’t be exceeded. You could even power the ph module from an output pin set to high, I am sure that the 14mA current is within limits, but you should look it up.

I'm pretty sure I could use the same pin for both, there isn't much draw from a probe but how would I go about wiring something like that?

Do I just form a parallel circuit between the 5v and ground on a breadboard?

edit: Tried shortening the wires. Still the same fluctuation on 3.3V. Is there a way to test if the 3.3V pin is faulty?