Article History

Digital temperature sensors

Some ideas for temperature sensors, probably using Dallas/Maxim's 1-wire sensors.

Current sensors

The current sensors and cabling I use were ‘designed’ back in 2002 when I lived in Sydney.

The wiring consists of a twisted pair of wires, taken from a Cat5 cable. The Cat5 cable is pretty cheap, and there's four pairs to every cable, so it's super cheap. Power to the sensors is purely parasitic because of this arrangement, which can be a problem for long cable runs. As pointed out by Brian Lane, Dallas Tech brief 1 recommends connecting a Schottky diode across the wires at the end of long cables.

My sensors are simply soldered to the wires of a twisted pair, covered first in Selley's® All Clear (a clear water-proof sealant), and then white shrink-wrap tubing.

My cables are attached using mono 3.5 mm ‘phono’ inline plugs and sockets. These seem to have been the cause of some reliability issues.

Better cabling

Tech brief 1 recommends wires carrying power not be adjacent to the 1-wire data wire (page 22, figure 22 on page 23). This would allow flat 4-core cable to carry power and data great distances.

Better software

I am still using Brian Lane's ‘digitemp’ software to read temperature values from the sensors. This software is somewhat limited. I would do better to use OWFS for exploring the 1-wire bus and reading data from many different types of sensors.

For wireless sensors (see below), I would like to make the software sensor-driven instead of the current host-driven i.e the sensor nodes decide when to take a reading and transmit it to the host. I would also like to extend it to support more than just temperature readings. At one point I was working on ‘Weather Station’ software but nothing has come of it. Perhaps I will work the other way, extending my current ‘digitemp’ software as is needed.

Other types of connections

Ways of building 1-wire sensor networks without dedicated cabling.

Internet

Using owserver from the OWFS project, 1-wire networks can be accessed over any IP network, either locally or even globally.

Wireless

I would like to use Jean-Claude Wippler's JeeNodes to create a wireless sensor network. Each JeeNode is an Arduino-compatible board with a simple radio module. The I/O pins of the ATmega microcontroller are arranged into four identical “ports” and he makes a bunch of “plugs” which connect to these ports. Or things (like 1-wire sensors) could be simply connected straight into the board.

Since each sensor would be close to the node it's connected to, there's probably no need to worry about Schottky diodes or even parasitic power.