Advice on power / signal re length of cable

Hi All

So, I am steadily building up my Arduino kit in prep for my night-time lighting set up using PIR's, SSR's, LED's and an Arduino (probably finally a Pro Mini).

I am presently unsure where I will mount the Arduino Pro Mini / power source etc once the whole rig is working, so have run all the necessary cables back to my desk in the study/bedroom where I have all my electronics kit etc.

The question I have is this: for the +5V items (PIR, SSR) should I use a separate +5V power supply or use the +5V from the Arduino - given that the cable length from my workspace to the PIR's is many meters.

Is there a limit to the length of cable run that the analogue/digital ins & outs, and +V supply and ground pins on the Arduino require?

Thanks all...

theMusicMan:
Hi All

So, I am steadily building up my Arduino kit in prep for my night-time lighting set up using PIR's, SSR's, LED's and an Arduino (probably finally a Pro Mini).

I am presently unsure where I will mount the Arduino Pro Mini / power source etc once the whole rig is working, so have run all the necessary cables back to my desk in the study/bedroom where I have all my electronics kit etc.

The question I have is this: for the +5V items (PIR, SSR) should I use a separate +5V power supply or use the +5V from the Arduino - given that the cable length from my workspace to the PIR's is many meters.

Is there a limit to the length of cable run that the analogue/digital ins & outs, and +V supply and ground pins on the Arduino require?

Thanks all...

For the past year, I have had an Arduino Nano anemometer and power supply in the kitchen and the spinning head with a reed switch about 50 feet away. You can convert that to meters. The wire is shielded and it directly connected to a digital pin on the Arduino. I have never had a problem with it.

So, my recommendation is to always use shielded wire, shield grounded on one end, only. In my case, the far end is not grounded, but is the return from the reed switch.

Paul

All wire has a resistance. Thick wire has less resistance. This is usually quoted in the wire datasheet (yes, wire has datasheets too) as Ohms per foot or Ohms per meter. For just 1ft or 1m, the resistance is usually very very small. But if you have many meters - remember you also have to consider the length of the ground wire - then the resistance may be significant.

If you have significant resistance and significant current, then you will have a significant voltage drop.

Let's pick a wire: 22g is a pretty common 'small' size. It has a resistance of 0.03 ohms/ft.
Distance? Pick 20ft but remember you have to add the length of the 5V and 0V wires, so 40ft total.
Current? PIR and SSR probably don't use a lot of current on average but there may be peaks when the current gets up to 500mA.

That gives us 600mV voltage drop. Do your devices still work on 4.4V supply? Can they still send valid digital data at that voltage?

Now switch to 14g wire, which is several steps bigger. Maybe you might see this size in the house wiring behind your light switches. Its resistance is 0.003 ohms/ft so the voltage drop is now one tenth or 60mV. That drop can be safely ignored for most purposes.

Or if your peak current is 50mA, keep the 22g wire as the drop is also 60mV.

For transmitting actual data along the wire (rather than just a switch status) then you may have to consider the capacitance and inductance of the wire. I2C data is especially sensitive to capacitance.

Fabulous response sir, thank you