cat5/6 cable from sensor to arduino

[ravi.2k17]

hi all,

can cat5/6 cable be used for communication between Arduino and pir sensor hc-sr501 safely? i want to use cable's 3 cores for vcc, gnd and data.
i have tested it myself and it is working fine but even though i want to confirm if this right choice? i find cat cables are cheaper than any single core jumper wire for long length.

thanks

 

[KlausST]

Generally yes,

But you need to wire them correctly.
In detail it depends on how noise is introduced, on length, load current, ....

Klaus

 

[ZASto]

PIR sensors do not communicate. They just change output state.
Beware that the output level is 3.3V so it is very susceptible to noise.

 

[betwixt]

It helps if you twist the PIR output wire with ground and also tie all the unused wires to ground at both ends. It keeps the ground impedance as low as possible and that helps to reduce interference pick-up.

Brian.

 

[ravi.2k17]

It helps if you twist the PIR output wire with ground and also tie all the unused wires to ground at both ends. It keeps the ground impedance as low as possible and that helps to reduce interference pick-up.

Brian.

what if i simply use single jumper wire around 22 AWG for data and keep gnd,vcc local wrt sensor? will it better than the cat 6 ? the cable length is around 25 meters

 

[betwixt]

I certainly wouldn't recommend that. There are two factors to consider:

1. the voltage you are sensing is produce relative to ground at the sensor and measured relative to ground at the MCU end. If the ground at the sensor already has a voltage on it from elsewhere, or if it is floating, the potential difference between the MCU input and MCU ground could stop it working or give intermittent results.

2. For the same reasons, a difference in ground voltage at each end could produce a voltage at the MCU sufficient to permanently damage it.

Always make sure the grounds are connected directly together and with an impedance as low as possible to minimize ground voltage differences.

Brian.

 

[ravi.2k17]

I certainly wouldn't recommend that. There are two factors to consider:

1. the voltage you are sensing is produce relative to ground at the sensor and measured relative to ground at the MCU end. If the ground at the sensor already has a voltage on it from elsewhere, or if it is floating, the potential difference between the MCU input and MCU ground could stop it working or give intermittent results.

2. For the same reasons, a difference in ground voltage at each end could produce a voltage at the MCU sufficient to permanently damage it.

Always make sure the grounds are connected directly together and with an impedance as low as possible to minimize ground voltage differences.

Brian.

might be this would be a basic silly question.
how come there would be a difference in GND voltage. Isn't it always 0V and connected to same mother earth?

i see CCTVs are some time powered locally and only send signal to centralized NVR (as opposed to connected and powered by directly using NVR via POE). what arrangements are done to reduce the voltage difference in this case?

 

[betwixt]

how come there would be a difference in GND voltage. Isn't it always 0V and connected to same mother earth?

That isn't a silly question at all and it comes up quite regularly on here.

The issue is that the Earth (as in the planet) isn't a perfect conductor, in fact on the surface much of it is a good insulator. So what do you use as a reference? In CCTV the shield on the video cable is used to join the camera and NVR grounds together but in other applications you can't assume 'ground' at one location is the same potential as 'ground' somewhere else. The direct connection between them does, as far as possible, make sure they have no voltage between them but it isn't safe to rely on independent grounds being equal. Consider that there is resistance in all wiring so wherever a current flows there will be a voltage drop. If the voltage drop is between a signal source and it's destination, the ground voltage will be appear to be added to the ground voltage at the other end and could either cause an error or in worst case make the voltage out of safe limits and cause damage.

Brian.

 

[KlausST]

Hi,

Inside your cellular phone ther is also "GND". It is the node where usually the battery minus is connected to and all IC GND are connected to.
--> it surely is not connected to earth GND.

*****
Let´s say you have a long pair of wires. Each wire resistance is 1 Ohm. On one side there is a 12V battery on the other side there is a lamp drawing 100mA.
on ech wire there is a voltage drop of 1 Ohm x 100 mA = 100 mV = 0.1V
seen from the batterie´s GND side one "far" end cable has 0.1V, the other has 12 V - 0.1 V = 11.9V

The lamp sees the difference between both = 11.9V - 0.1V = 11.8V

Klaus

 

[ravi.2k17]

Hi,

Inside your cellular phone ther is also "GND". It is the node where usually the battery minus is connected to and all IC GND are connected to.
--> it surely is not connected to earth GND.

*****
Let´s say you have a long pair of wires. Each wire resistance is 1 Ohm. On one side there is a 12V battery on the other side there is a lamp drawing 100mA.
on ech wire there is a voltage drop of 1 Ohm x 100 mA = 100 mV = 0.1V
seen from the batterie´s GND side one "far" end cable has 0.1V, the other has 12 V - 0.1 V = 11.9V

The lamp sees the difference between both = 11.9V - 0.1V = 11.8V

Klaus

>>> it surely is not connected to earth GND
so what is this GND, if this is not earth? so far i was under impression that GND is always earth. what is the GND in mobile phone case. what is this made of...

 

[KlausST]

Hi,

GND is what the designer decides to be "GND". It is the node where the voltage is referenced to - as long as there is no other reference mentioned.

Let's say you have a device with two 3V batteries in series:
-----[+ 3V battery -]-----[+ 3V battery -]-----
You are free to decide which of the thrre node is called GND:
(GND)----[+ 3V battery -]--- (-3V) --[+ 3V battery -]----- (-6V)
(+3V)-----[+ 3V battery -]---(GND)---[+ 3V battery -]----- (-3V)
(+6V)-----[+ 3V battery -]---(+3V)---[+ 3V battery -]----- (GND)
You may try it on your own. GND is the node where your "minus" or black wire of a voltmeter is connected to.

Klaus