length of sensor wires

Hi guys, This is my very first post. and it is concerning what will hopefully be my very first arduino project. I am a noob and have very little experience in electronics but do have some basic knowledge.

In order to get my wife to sign on to my new hobby I have cleverly disguised inside of a household chore that she has been wanting me to tackle for a few years. Lawn sprinklers.

I was envisioning a backyard system controlled by soil moisture sensors and possibly i want to look int a way to delay watering if rain is in the forecast. But I will need to do some research on that part.

My question is: If I place moisture sensors around my yard, say 6 of them, is there a limit to the length of wires I could run back to my arduino. the sensors would be,say, 10, 20, 30, 50, 40, 30 feet away. I am figuring that the signal would be lost over such long runs. also, although it would be more expensive, would using xbees be an option?

Thanks so much for any help Mike

Do you have the specs on the moisture sensors? What load resistance/impedance is specified? How much voltage do you get out of the thing?

If there's a problem it will be noise pick-up which will result in unstable (possibly unusable) readings.

You get more noise pick-up with high impedance (and long wire-runs). [u]Shielded cable[/u] can help a lot to block noise-pick-up.

No, I dont have sensors yet. I cant even seem to find any moisture sensors yet. This idea is in it's infancy. I am just trying to see from you guys if its even possible.

Hi, Look for:

opensprinkler http://arduino.cc/forum/index.php?topic=75415.0

GardenBot: http://gardenbot.org/howTo/

-- Se the H-bridge-based Moisture sensor: http://gardenbot.org/howTo/soilMoisture/#The_local_circuit_-_simple_voltage

I don't THINK you need so many sensors, unless almost all the water the lawn gets is from sprinklers in many zones.. Big greenhouses use a "Pilot Plant" and water according to it.

Let us know how it goes...

Thanks so much Terry! This info looks great. If I am brave enough to tackle this project, I will report back to you guys.

A project for a rainy day? (sorry!).

Perhaps an atmospheric temperature & humidity sensors would help in estimating the true demand for water (and control the amount of watering). Or perhaps the soil moisture responds to this enough already.

Ahhhh! good one.

yes, I was thinking of temp sensors.

I also want to take info from NOAA and delay watering when possible rain is in the forcast. But that seems pretty advanced. I dont know if I can educate myself enough by springtime to make that happen.

Some sensors, at least one temperature sensor I have seen, give serial data rather than analog. With those the length of your wires would be limited by serial hardware which you can work with (read: add parts) to extend.

You could put a stand-alone ATtiny with an analog sensor to make a serial device. ATtiny45 is less than $2 and way overkill but I dunno what less you could use. I bet someone here knows the answer to that!

Added note: Whatever you hook up, put opto-isolators between your sensor wires and your more expensive controller unit. One even semi-close lightning strike is going to induce a LOT of current down those wires!

Great advice.
Thanks so much!

I made a moisture sensor with plaster of paris and it works great. I tried it with longer sensor wires and I am getting more fluxuations in my readings. Where they were steady before I am getting readings between 35 and 14 a pretty large swing relative to before. could this be interference?

between 35 and 14 a pretty large swing relative to before

Read some of the discussion on GArdenBot and google this subject.

I read that the PlasterOfParis and similar probe age pretty fast and vary unless used with AC signals like the H-Bridge examples..

You can also use an L293N H-bridge- Pin compatible with the Ti part shown.

Here: http://arduino-direct.com/sunshop/index.php?l=product_detail&p=296 (See data sheet)

DISCLAIMER: Mentioned stuff from my own shop...

thanks again for your help . you have been very helpful

I was wondering. Is there a line of code that would take the average of the last say 10 sensor readings and return that value instead so to smooth the readings?

say to use the average to turn my sprinkler valve on at a reading of 10.

I have allot of learning to do still but I was just wondering

Long bare wires act as antenna besides having inherent resistance. Antenna are affected by outside radio waves. It's not much but it's enough to blur signals, like a radio tuned off station enough to start picking up the next. If you use shielded communications cable, you don't get that problem. It's still best to digitize close to the analog capture, get it fresh before it spoils. An ATTiny has A/D, is serial-capable and data-logger cheap. Very few pins, trivial memory but fast. It could get power from the CAT-5 cable in a PVC pipe under the lawn. A good size pipe makes it easy to add or remove cables later. The CAT-5 is electrically shielded as long as you ground the shield.

I have seen Automotive Grade AVR's listed in catalogs. I think maybe they're already 'hardened' to some extent.

I read that the PlasterOfParis and similar probe age pretty fast and vary unless used with AC signals like the H-Bridge examples..

You can also use an L293N H-bridge- Pin compatible with the Ti part shown.

Here: http://arduino-direct.com/sunshop/index.php?l=product_detail&p=296 (See data sheet)

I really appreciate your help allot but I am totally new to e.e. and I really wouldnt know how to wire the h bridge. thats my problem.

Do you have and can you use a breadboard?

Yes I do and can. I understand how resistors, caps, transistors, Volt regs. And some others work. And I have an Arduino. And I can do basic stuff with that. Basically, I can follow schematics but I don't Know how to design stuff myself.

Here's good explanations on motors and what: http://www.thebox.myzen.co.uk/Workshop/Introduction.html