Just a rough sketch but that's what I use. But if i use the 5v pin in the Arduino everything looks fine. Btw, i use a breadboard for the connection of 5v, 3A power supply and the TDS sensor.
I will also use a pH and sensor. That is really my only problem for my project. It occurs also if I use the LCD. When I use external power supply, it does not print the texts
Your diagram should contain all components and connections. You mention e.g. a LCD but it's nowhere to be found. It should also show where the 12V goes (Vin, barrel, ...)
Assuming the 12V goes to the barrel or Vin, the voltage is reduced to 5V by a voltage regulator on the Uno. Due to lack of cooling, that voltage regulator can not provide much current; if you draw too much, it will overheat and shut down.
There is only one 5v pin in the arduino. I am obliged to use an outside power source to supply my sensor.
The problems is that when i use the outside power source, the sensor connected to that do not function well. The value readings are very high and unconsistent, but when i switch and use the 5v pin from the arduino, it works fine.
How can I use an outside power source?
This also happen When I use LCD. If I use an outside power source, it doesnt lit the text.
You can use the 5V pin to power more than one sensor, as long as the maximum allowed current draw is not exceeded (TDS and pH sensors will not).
pH sensors are extremely sensitive to outside electrical disturbances, and in most cases, no other powered sensor can be present in the sample solution.
When I use LCD. If I use an outside power source, it doesnt lit the text.
That suggests a serious wiring error. For help with that, post a hand drawn schematic of the wiring, with all pins, parts and connections clearly labeled. See the "How to get the best out of this forum" post for instructions on posting.