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Author Topic: Firecracker (CM17A) range  (Read 3555 times)

bkenobi

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Firecracker (CM17A) range
« on: January 10, 2013, 11:13:48 AM »

I know the reported range of the CM17A is up to 100ft.  However, that's X10 optimism I presume.  If it's anything like the motion sensors, that really translates to ~15' through one wall.  In my application, I'm interfacing the controller through an Arduino.  I don't know how much power the Arduino provides through the serial port, so I don't know if this setup can provide the optimal range in any case, but how far is the TRUE range of the CM17A?  In my quick testing with the Arduino/CM17A located in my office, I was unable to reliably send signals to the CM15A antenna in the attic.

For this specific project, the question is primarily academic as I will be putting this setup within 5' of the Radio Shack antenna on my CM15A so I would assume that there would be no issue with range.  However, I could see an Arduino/CM17A setup being a reasonably cheap way to interface potential future projects to X10 for cheap.  I am likely going to install an ethernet shield on the Arduino at some point to provide data from sensors to Pachube, so at some point I might be able to control things via a network command sent to the HA computer.  For now though, CM17A at ~$7/ea on ebay is by far the cheapest way to easily control X10 from an Arduino...assuming range issues are not a show stopper.

Brian H

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Re: Firecracker (CM17A) range
« Reply #1 on: January 10, 2013, 11:29:22 AM »

Glad you got it working in your project.

The CM17A is kind of an odd module.
It uses the DTR and RTS Serial Port Signals for power and turning On and Off the transmitter.
Some data here if you don't have it yet.

http://software.x10.com/pub/manuals/cm17a_protocol.txt
« Last Edit: January 10, 2013, 11:43:51 AM by Brian H »
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dhouston

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Re: Firecracker (CM17A) range
« Reply #2 on: January 10, 2013, 11:50:38 AM »

I doubt that it's being starved for power. Even a PC serial port is limited to about 80mA as a power source. The CM17A has a 3V3 (IIRC) LDO regulator so, if the Arduino serial port is supplying +5V, it should be adequate. I don't recall which PIC is used but it probably operates on 50mA or less.

The FCC output power limits are very low. 100' in free air is optimistic and my rule of thumb is 20-30% of free-air rating for indoors. However, both the CM17A and CM15A rely on LC tuning so it's likely that they are operating on slightly different frequencies, further limiting range.

There are far simpler ways to do what you wish. Having the Arduino directly control a 310Hz transmitter is one of them.
« Last Edit: January 10, 2013, 11:53:55 AM by dhouston »
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bkenobi

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Re: Firecracker (CM17A) range
« Reply #3 on: January 10, 2013, 01:12:41 PM »

Building an rf receiver into an arduino project could simplify things assuming a library already exists.  The real attraction to the cm17a was the cheap price, easy 3-wire connection, and readily available library for sending commands.  Thanks for all the great feedback as usual!

dhouston

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Re: Firecracker (CM17A) range
« Reply #4 on: January 10, 2013, 01:30:46 PM »

Get this transmitter...
and replace the SAW resonator with...

The first link has links to example code/libraries
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