Thank you for all of your help so far, however you don't seem to think that a MUST is a MUST. I have thought of most of solutions provided for me already, and I am thankful for them. Its just that these solutions do not meet the needs of this project. It's already almost on post 40 and people are still recommending solutions that do not fit my needs. So I will try and clarify what must be done, and hopefully you can help me with what I can not figure out.

Constraints: One end of sensor MUST be connected to 24VAC the other end to ground.

Thank you for the suggestion of a series of resistors to drop the voltage going into the 5V pin of the micro. However this would not work directly because its connected to ground. A work around would be to branch off of the switched output end and use resistors to drop it down to 5V.
This still leaves a problem of a "short circuit" from 24VAC to ground minus the power lost going into the micro-controller.

The resistance of the sensor is about 100ohm.

 

It has 3 wires coming out
1) Common
2) Normally Closed
3) Normally Open

Both the N.O. and N.C. can pass a current up to 5A.

Constraints: One end of sensor MUST be connected to 24VAC the other end to ground.

You are contradicting yourself.

 

The resistance of the sensor is about 100ohm.

Wait a minute...

Your sensor operates a relay? Why didn't you say so in the first place?

Okay, your 100Ω relay coil will draw 0.24A at 24V. This will energize the contacts, bringing the "N.O" and "common" together, and separting "N.C." and "common." When your rain bucket gets full, the relay will temporarily deenergize, and the reverse will happen momentarily. You can connect 5V to "common" and take your signal off of N.O. or N.C. as you wish.

 

Thank you for your help, that would work perfectly, except for my mistake.

I feel stupid for posting the resistance of 100 ohm for the sensor. It was a mistake on my part I meant to say 1 ohm, which would still mean that it could pass 24A of current if it did not burn out.

Edit: I measured the one connected to my house, which for some reason has 100 ohm, but the one out of the box has 1 ohm. So assume that the resistance is 1 ohm, sorry for the mistake (there must be a poor connection on the one that's in use)

 

I shall refrain from saying what's on my mind, for I would be booted off the forum if I did! Grrrrrrrrrrrrrrrrrrrr!

 

For those who might be still interested in a practical AC current limiter,

Elektor Electronics has published such a project in the June (English) issue.
It is aimed at helping users not exceed their current limit at campsites.

 

http://www.rainbird.com/pdf/turf/man_RSD.pdf

If this is the sensor in question, then I think you are wrong and that it doesn't need 24 volts connected.
This is a passive device, it's not "powered" by anything. However, one STRONG possiblity is that it is a switch in series with a capacitor.

In which case it would not allow 5 VDC to pass. Or 24 VDC. It will only allow AC current to pass. Now if you happen to have the 24 volt AC adapter that comes with the rainbird controller. You can get around this. (Note any AC adapter should work)

Hook one side of the adapter to the Normal Closed on the sensor, hook the common on the sensor to resistor 1. Hook the other side of resistor 1 to resistor 2. We will call the point where these connect junction A. Hook the other side of resistor 2 to the other side of the adapter. We will call this connection junction B.

You will have to figure out what values to use for resistors 1 and 2. I think a 1 to 7 ratio will work well. But thats a rough guess I haven't done the math on it. Make sure that you use high enough values to limit the current.

Now hook a bridge rectifier between junction A and junction B. Hook the negative side of the rectier to ground. Between the negative and possitive side hook a capacitor to even out the signal. Again haven't done the math on it. I'ld just choose something really big rated for perhaps 10 Volts. After hooking up the capacitor measure DC voltage off the possitive side. If it's higher or lower than 5 volts. Tweek with resistor 1 and 2 until it's right. Now measure the AC voltage. If it's higher than say .05 put a bigger capacitor in.

Now disconnect the Normal Closed line of the sensor replace it with the normal open side of the sensor and then hook the possitive line to the microcontroller.

That should do it.

Note this is only necessary if you use the 24 VAC power supply that comes with Rainbird controllers.
Zenock

I am curious about one thing. If it "REQUIRES 24VOLTS" or even an AC current. How were you able to measure the resistance at 1 ohm? I doubt your ohm meter was providing 24 volts to it.