A 1-Wire network seemed really promising, so we decided tentatively to use it for temperature instrumentation. We set out to list what parts are needed: at least a computer<->1-Wire adapter, some sensors and cabling to put it all together. For the 1-Wire adapter, serial port connectivity was out of the question as it can not provide operating power and is being obsoleted, so USB was the next logical choice. As luck has it, Maxim manufactures part number DS9490R which is a USB<->1-Wire adapter that can provide up to 42mA operating current to the bus.
As for the sensors, we knew from previous experience that DS18B20 sensors would be suitable for our purposes. The DS18B20 is a 1-Wire temperature sensor with selectable resolution of 0.5, 0.25, 0.125 or 0.0625 °C (9..12 bits), 0.5°C accuracy from –10°C to +85°C and measurement drift of 0.2°C.
Maxim is a nice company and provides free samples of many of its products. However, their lead times, even when trying to buy from their web shop, were more in the order of months rather than weeks, so we set out to find suppliers from Finland. Having prior experience with sourcing parts, we quickly found out that Bebek in Hakaniemi and Partco supply DS18B20 temperature sensors and other parts off the shelf. The USB adapters were harder to come by on a short notice, but Tapio Haapala from F-Solutions lent us a hand.
Some effort was put into thinking about how to actually connect the sensors onto the wire. After considerable deliberation, we decided to use RJ11 female-female adapters (with holes drilled to sides for air flow) to house the sensors (more about this on part 3). We can then use the adapters to both extend the bus as needed and multidrop the sensors wherever needed.
Cabling (and to some extent, connectors) is the backbone of the 1-Wire network, so we picked 100m of 2-pair Cat5 cable, about 10 pieces of 6P6C modular connectors, enough 6P4C modular connectors and about 25 pieces of RJ11 female-female adapters . The 6P6C jacks connect to the USB-adapter, the 6P4C jacks are used to connect the RJ11 adapters together and the RJ11 adapters are use to house the DS18B20 sensors. The term RJ11 is used here liberally to mean 6 position wide modular jack with variable connected pin count (Wikipedia has an article about Registered Jack naming).
Our final shopping list looked roughly like:
- 2 DS9490R USB <-> 1-Wire adapters
- 20 DS18B20 TO-92 packaged temperature sensors
- 25 modular RJ11 female-female adapters
- 5 6P6C modular connectors
- 50 6P4C modular connectors
- 100 m reel of Cat5 2-pair data cable
Total price for these items is somewhere under 250 euros. The cable reel is about 65-70 eur, the USB adapters about 35-40 euros per unit and the temperature sensors can be had for about 3..4 euros per unit. The RJ11 adapters’ and connectors’ price is next to nothing.