One-wire over phone wire
May 25th, 2006
Here’s an initial take on an idea for running Maxim’s One-wire protocol over standard Cat3 phone cable for cheap, modular networking of a bunch of small temperature sensors…
The One-Wire protocol (which really requires TWO wires - one for ground, and one for data+power), is reasonable for a cheap local sensor network. But the real reason it’s great is that Maxim/Dallas makes a ton of awesome/cheap sensors that speak One-wire. I can imagine all kinds of uses - here are some of their ideas.
A standard phone line has four wires - here’s how Scott and I hope to use them for a short-haul one-wire network. The little black dot is the temperature sensor (in this case). It uses three of the four wires, but it’s also possible to run some of the 1-wire temp sensors in “paracitic” power mode, where it would only require the OW and ground pins. I don’t have a specific use in mind for the 12VDC pin.
(click the photo for a readable version)
If you’re a real over-achiever, here’s the sketchup file.
Phone wires are so nice, ’cause they’re flipped. You never* have to worry about polarity or “crossed cables”, or junk like that. This idea follows - by embedding small temperature sensors in a standard phone cable extender, a bunch of sensors can be daisy-chained together. Specifically, we stuck a DalSemi DS18B20 high-res temperature sensor (in a TO-92 package) inside a RJ-11 butt-end splice.
* never is a long time. - jengel
I was able to connect six devices directly to one pin of a PIC processor (via six two-foot phone cables), without problems. I suck a thirty-foot cable in there and things didn’t work so well. I wasn’t using a “real” one-wire driver (I was using software “bit-banging” on the PIC, rathre than Dallas’ PC driver). I’m sure you could get WAY better distance over Cat5, or with a “real” driver…
Here are a couple shots of the first prototype:
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