STAR Grounding for Sensors

Hi there! I read the grounding advice, for motors and what not. I see that it can flow and turn devices on and is good for different voltages. My question, is this acceptable for sensors? In my example, the arduino is reading from a sensor on a car and is being supplied voltage from the 12v system (after being reduce to a safe level). Is it ok to only run a 5v signal to the sensor from the Arduino? And use a common ground on the chassis? Or is this poor practice?

Star grounding is important in the case of RF or low level signals*. For automotive projects it is less important and a metal monocoque is usually a good, low impedance conductor anyway for higher current interconnects.

Sensors, audio and RF all count as low level signals.

  • Edited to be clearer.

agentgengar:
Hi there! I read the grounding advice, for motors and what not. I see that it can flow and turn devices on and is good for different voltages. My question, is this acceptable for sensors? In my example, the arduino is reading from a sensor on a car and is being supplied voltage from the 12v system (after being reduce to a safe level). Is it ok to only run a 5v signal to the sensor from the Arduino? And use a common ground on the chassis? Or is this poor practice?

If you can look at a circuit diagram for modern cars with many many sensors, they run the sensors gnd wires back to the controller that is using the sensor output.
If you use the chassis return, then currents from other devices will cause noise problems with the sensor signal.

As an example, the 100 or so Amps that flows when you start the engine or the 50 or more Amps that the alternator supplies to charge and keep the cars electronics/electrics running.
Tom.... :slight_smile:

TomGeorge:
If you can look at a circuit diagram for modern cars with many many sensors, they run the sensors gnd wires back to the controller that is using the sensor output.
If you use the chassis return, then currents from other devices will cause noise problems with the sensor signal.

Yes, my answer was perhaps not as comprehensive as it should have been... Low level signals should be star wired.

So if Im understanding things correctly, In the case of signals like say, a square wave where Im looking for a pin to go HI, It would be ok? (Pulses from transmission counting rotations) But in cases where I want to try to get an accurate resistance value from say a water temp sensor or something I should run the wire back?

The cause for confusion is due to the fact Ive seen it both ways flipflopped even for the same car a lot of the time. Some sensors have 2 wires some dont. o2 sensors will often only have 1 wire. while others have 2 (Or more but those are operating differently and I'm aware of that). Water temp sensors depending on who makes them have a common or devoted ground.

I figured what it mainly came down to was how the ecu was setup to look for resistance. Thus my question about the Arduino if it was able to get its values without using a ground pin, but instead just sharing a common ground.

Common narrow band O2 sensors are almost digital in that they only really say "rich" or "weak", rather than produce an analogue output like a temperature sensor. If everything is using a common ground, the impedances are low enough for it not to be an issue. Ibus signals are referenced to local earth, whereas CAN bus is balanced. So I would say look at the sensor. If it is a two wire device, it is probably designed for a signal earth rather than chassis.

Ahh. So lets see here. So for a digital thermometer than can be ran in "leech" mode using only the 5v sensor (Assuming this is basically just a low data transfer canbus) it can use common ground with the chassis.

In the case of watching for my square wave pulse common would be ok as well? Its a 3 pin thing, Ground, voltage, and output. Sensor doesnt care how much voltage it receives, but just outputs when ever its made 1 rotation. Since its not using a shared thing like canbus this is also acceptable?

Basically as long as its not trying to do a serial communication its good to go?

AJLElectronics:
Star grounding is important in the case of RF or low level signals*.

At RF stray inductive and capacitive coupling dominate over any IR effects and star-grounding is not
relevant - groundplanes and shielding boxes are used to stop signal crossing over to other parts of the circuit.

Star grounding matters for low-level low frequency circuits when in the presence of high-level low frequency
signals, as that's when IR voltages are problematic.