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Build a One-wire weather – Oww

Build a One-wire weather – Oww

Oww is a software project, to communicate with the Dallas 1-wire weather station and other 1-wire sensors, presenting live weather readings, logging data to file, uploading data to weather Web sites, and sending data to other programs through a command line interface.
If, like me, you think it’s fun to keep tabs on the weather, you might be pleased to discover the budget weather station kit sold by a firm in the USA. Dallas Semiconductor [1], of Texas, are well known as manufacturers of integrated circuits. One of their devices may be found in some Acorn models. Lift the lid on a Risc-PC for example, and towards the front from the processor slots you can locate a socketed 3-pin device that looks like a transistor. This is a DS2401 silicon serial number, that holds a unique 64-bit code, laser-engraved at the factory. This was chosen by Acorn to give a unique machine ID.

What’s clever about this line of chips is the 1-wire bus that connects them to the rest of the system. This is a bi-directional serial data line, that allows communication between the host and many Dallas chips without the need for a clock line. ‘1-wire’ is a bit of a fib of course, as for any current to flow there has to be an ‘earth’ connexion as well. But even a power line is optional, as an internal capacitance gets charged up off the serial line, providing the required drive voltage.

Dallas now have a broad line of 1-wire devices [2], including switches, counters and analogue-to-digital converters. They even do a Java-based computer in a tiny stainless-steel canister (called an ‘iButton’), like a calculator battery, that holds secret keys and has its own encryption engine for tasks such as e-mail encoding or as an alternative to stamps in the US postal system.

The weather station kit is a bit of a departure from their normal range, mainly intended as a promotional item I think, rather than as a serious contender to the professional systems that cost a great deal more. You can buy it on-line at the iButton web site [3] for $79. They will add a charge for delivery (mine came by Federal Express) and HM Customs & Excise will want to charge you too (usually via the couriers).

As kits go this one doesn’t take much putting together. The main body of the weather station is a plastic moulding, split down the middle. The anemometer, with its three cups, projects from the top, and the wind vane from beneath – see Fig. 1. In the middle is a circular printed circuit board (PCB), on which are all the 1-wire chips – see Fig. 2. The 1-wire bus is connected through US-style (RJ11) telephone sockets, which are a smaller version of 10baseT ethernet sockets (RJ45). This is a little unfortunate for us (outside America) as the tools to make up cables are more expensive than those for our domestic telephone connectors; around £15 from Farnell [4] is the cheapest I’ve found.

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