cable specs and help mounting sensors

hello,

i am fairly new to the world of arduino/electronic engineering,
so i got some questions before i start building something …

i would like to attach some sensors to a (most likely metallic) structure and then of course wire the cables to the arduino somewhere separate.

  1. is there a maximum length of the signal cables? which diameter and type of cable would be the best?

  2. i also wondered if there is some standard mounting system? - i.e. put a sensor on/into something which has holes for mounting … because sensors do not seem to have mounting holes in general since they are tiny

thanks in advance
didi

I think you left out the rather important piece of information telling us what sensors you want to use. It would be helpful if you could post a link the datasheet for the sensors you want. If you don’t know what sensors to use, you will need to say more about what you are trying to do.

upps, sorry for that.
i want to build a cage-like structure (out of metal or wood, but not sealed, approx. 2x2x2m). then i want to detect motion and distance of a human being.

i am not sure if IR distance sensors or sonar sensors are better - i will try. till now i found the standard sonar (LV MaxBotix Sonar EZ0, EZ1 or EZ2) or IR sensors (Sharp GP2Y0A21YK or GP2D12) which are both analog.

is there a maximum length of the signal cables?

yes but it depends on the signal. Even with this information it is imposable to say without consulting the data sheet of the actual sensor. As a very rough rule of thumb anything over 5 meters needs careful consideration.

i also wondered if there is some standard mounting system?

no you just make it up with what you have.

ok, can you elaborate on that (the signal). what should i look for in the sensor datasheets. i did not find anything useful till now.
do you have any good links about that topic (wikipedia, textbooks, …). i just need hints or keywords to look in the right direction …

i will keep the 5m consideration in mind. a rule of thumb like that what i was looking for. so i can start and not go in the totally opposite direction …

regarding the mounting system - i found something in different electronic supply stores, but it is not standard. it is mostly metal plates with lot of holes where you can mount your sensors on…

First of all you need to distinguish between an analogue interface and a digital one.
With an analogue output, some sensors output a voltage signal, it is the impedance of the signal that determines how long it will travel before it picks up too much interference to be useful. The lower the impedance the longer it will travel. Something like a microphone has a high impedance and so needs an amplifier at the microphone end to not only boost the voltage but also lower the impedance. After that it can travel a long way down a mike cable. Generally amplification at the sensor is a way to boost the distance.

With a digital interface it generally goes so far and then you run into trouble all of a sudden. Anything over 2 meters is a problem for most logic level signals. You can then do the same trick by boosting the voltage and lowering the impedance, some of these tricks have names like RS232 or RS485. This last one uses two lines that go in opposite directions, that is called a differential output. Also the speed it changes matters, the faster it goes the shorter the distance is.
If you want really fast and long you have to go for optical fiber.

As I said it is complex and you could probably spend a few years studying transmission methods.

regarding the mounting system - i found something in different electronic supply stores, but it is not standard. it is mostly metal plates with lot of holes where you can mount your sensors on…

don’t get me wrong but i think the investment in a hot glue gun isn’t a bad idea for you if you like it more hightec get a drill,a screwdriver, some screws and some wood/metalplates. ::slight_smile: seriously you want to spend money on a small plate with holes? just for holding a small sensor in place that weight about seriousthinkingface 5gramms or so you can hold this things in place with bubblegum or wires you don’t need!? :-?

hello Grumpy_Mike,

thanks again for all your valueable insights! i really appreciate your help.

ok, the thing about “amplification” and the tricks (RS232 and RS485) sounds worthwhile to investigate. thanks for the nice summary. i will have to dive into that materia.

i think optical fibre is not an option, because it is too expensive and also an overkill. but at least now i know that i have to think about putting arduinos closer to the sensors or use some of your suggestions!

what about the length of the USB cable to the arduino?! i hope up to 5m without a USB repeater works well enough …

@fujii:
the thing why i want to avoid such mountings is that the sensors need to be place and oriented quite exactly and the whole thing needs to be assembled and disassembled a lot of times. so i prefer spending money on a clean and wearless system.
i am not so fond of pulling the trigger of my gluegun every time infront of the performance while i probably already have a lot more to do …

wood or metalplates sounds good though, thanks for the input.

okay if you need it exact just holding it there with a wire isn’t that good… i don’t know if there is a problem with the signals but … what about magnets? i mean once you set the metalplate in the right position for the magnets removing/assembe the sensor will be really easy. sounds cheap and lowtec, i kinda like it ^^

edit: just made it a little bit more readable :wink:

that is actually a really nice idea!!! muchas gracias :sunglasses:

what about the length of the USB cable to the arduino?

Yes 5m is the normal maximum length.