Endevor:
Ok, so I'm a little new to this concept and I'm still fighting myself in not claiming connecting grounds from different components is evil. I'm running into an issue now in trying to connect the grounds from two different voltages.
I'm currently trying to design a PCB that takes in an initial 12v 8a power source for some LED drivers and motors. I need a step down to 5v to run some sensors and relays. When I did this on a breadboard, I was using this:
http://www.amazon.com/gp/product/B009HPB1OI/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1.
I thought you could link the 5v ground and the 12v ground (Essentially a wire shooting from the 5v+ to the relay, then returning to 12v-) based on the fact the Arduino running all these components is running off the 5v from the step down, yet still has a ground line to the 12v- for the LEDs. When I connect it in this fashion however, my output to my devices is 12v.
Am I missing something? Do my 5v devices need their own ground and the only connection to the 12v- is back through the 5v-?
Is there an easier way to do this on a PCB than just mounting this mini board to it?
Thanks
First of all, let me provide a minor correction for you. What you are talking about is "circuit common" which is not the same thing as "ground" (although most people erroneously interchange the terms).
Now, normally circuit commons can all be tied together. The only reason to separate them is to isolate the noise of one from the other.
Say you designed an audio amplifier with a switching power supply. The switcher produces ton of electrical noise, and the audio amp has a high gain. So obviously, you would make each circuit on a separate PC board (or on the same board with separate circuit commons) and possibly even metallic shielding between the two to further prevent "transmitting" noise to the sensitive circuit.
For what you are doing, you can probably have all the circuit commons (i.e. the negative sides) all tied together with no problem.
If you use relays, be sure to place snubber diodes across the coils. The inductive kickback of an unsnubbed coil can destroy the transistor switching it, as well as induce noise into other parts of the circuit and cause problems (like an Arduino reset, for example).
Hope this helps.
