Bi-Directional Communication Systems using RF

dan2004

09-10-07

Hello all,

I want to build a system that will control a pool heater. So for example: if the temperature of the pool goes below a certain degree than I want the heater to go on. The temperature would be controlled from a panel inside the house. The heater is about 100 feet away from the house. I saw that Parallax makes components·for bi-directional communication systems. That would be the Parallax 433MHz RF Modules.

http://www.parallax.com/detail.asp?product_id=28180

Would anyone be able to recomend a good book on RF circuits for beginners that would use Parallax Basic Stamp products? I searched the site but was not able to find anything. Any help would be greatly appreciated.

Thanks!
Dan··

Post Edited (dan2004) : 9/10/2007 8:37:41 PM GMT

Chris Savage

Dan,

The RF Modules all communicate serially so it’ almost like having a wired connection, but with a few important things to keep in mind. For starters the target controller at the pool should have failsafe code to not allow the temperature to go above a certain temperature should it somehow get an invalid packet, such as set the temp to 800 degrees!

That brings us to preventing such things by using checksum and preamble packet headers which the receiver will respond to only if there’s a match. The packet should be rejected if the checksum fails. I hope this helps. Take care.

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Chris Savage
Parallax Tech Support

dan2004

Thanks Chris,

I appreciate the info.

Could you recomend a path for me to follow? I am totally new to Basic Stamp. I was an Electronics Technician for several years but never had any training in RF, so I don't know anything about it.· I've covered the entire website and read about all the different boards, Stamps, and so forth. There are several options but I want to make sure that whatever I purchase can expand into the final project. I think that my final objective of automating the pool would come much later. All I would want to do now is·simply turn on a light bulb 100 feet away from the house, wirelessly. If I could learn everything just to do that, I feel that I would be well under way into understanding what I need to know.

What would·your recomendation be·on how to get started?

Thanks Chris, or anyone else that could offer any advice.·

Dan

Chris Savage

Dan,

There’s no fixed solution for everyone. It really depends on a lot of factors in how you’re exchanging the data. If I were doing it I might try something like having the controller transmit the data every second or so. Using an LCD Display with a keypad interface, such as the Matrix Orbital unit we sell will make things easier because it can buffer keystrokes and generally make reading the keypad easier if you aren’t familiar with scanning a matrix keypad. Data might be sent as:

!#$TEMP:xcc

Where x is the byte temperature value and cc is the 16-bit checksum of all characters sent, except the !#$. The target/pool controller would receive these packets and WAIT for the !#$ before inputting characters. There are a lot of ways this could be done. The target only needs to check for serial messages in between checking the temperature and turning the heater element on/off. You should employ timeouts in the serial routines so they don’t hangup. Even then you will need to make sure that you check for noise in your area so the RF Modules aren’t saturated with noise which keeps the system from timing out. I hope this offers some help. Take care.

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Chris Savage
Parallax Tech Support