Home > Prototyping, Tech geeks, Temperature monitoring > Sensor Bus PCB Implementation

Sensor Bus PCB Implementation

DS18B20 Interface Board SchematicI began the process to implement my temperature monitoring circuit by breaking out the sensor plug board into its own PCB. This seemed to make alot of sense, as the board would be butted up against one of the enclosure’s faceplates.

It also allowed me to test out a larger implementation of my 3.5mm stereo plug idea for the DS18B20 temperature sensors. The schematic, shown here, is extremely simple and is essentially the 3 active pins for 4 3.5mm stereo plug jacks ganged together with a corresponding header pin. Once the schematic was worked out, creating the PCB was extremely easy.
DS18B20 Interface Board
I placed 4 of the jacks on each board, which is about the upper limit for the number of sensors I wanted on a single OneWire bus. Alot of reading (and some experimentation) proved to me that there is a point where too many sensors on the OneWire bus causes the whole thing to stop working – it seems related to the overall length of the sensor wire and the resistance or impedance associated with that. I am not an Electrical Engineer, so the exact cause eludes me – but I seemed to be OK so long as I had 4 or less sensors on a bus.
InterfaceTop InterfaceBottom
Once the board file was layed out in Eagle, I uploaded the design to OshPark.com and they sent it off to their fabricator to be produced. I think the 3 copies of the interface board cost me about $15, $5 of which were shipping. After a very short wait (less than 10 days), the manufactured circuit boards arrived in the mail, exactly as the OshPark renderers had depicted them.
A few minutes with the soldering iron and minor assembly with some stackable standoffs, and I had assembled interface boards to test with the prototype circuit (in place of the DS18B20s that were just plugged into the breadboard).
They worked perfectly. I plugged sensors into each of the 8 jacks, and they all registered / displayed temperatures as expected. The next step was to mount this bus (as well as some power circuitry) into the faceplate for the enclosure. I wish I could say that all of my drill holes were perfectly placed and spaced, but I’d be lying. I got the job done with a titanium step bit, but I would not characterize some of the spacings to be “snug”. It’s really hard to keep the drill from wandering when you don’t have a drill press. At any rate, the pictures below show the enclosure with the 8 sensor plugs (5 of them have sensors plugged in), the DC power jack, and the power switch.
The main board has been ordered from OshPark, and hopefully I’ll get it in a couple of weeks. I had to go through a couple iterations to have the LCD, main board, and auxiliary boards (like the MicroSD board) all oriented properly within the enclosure. I’ll post more on the main board once it comes in. For now:

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