Resistance

Resistance is nice and all, it’s easy to know what you’re dealing with when you have control of everything. Things like initial voltage, and expected voltage and an idea of the resistance you’re looking for. But when you’re given a device, and you’re told “I want this value, find out how to get it” you have to do a little searching with a lot of unknowns.

Of course I am talking about electronics, and more specifically, micro-controllers. A project I am working on needs to take a COTS (Commercial Off The Shelf) item and do special things with it. What am I working with? It’s a temperature controller. Sure, you can see the temperature on the unit itself, but I want to see that in a database. That database is going to reside on a web server somewhere. And the only access I will have to that web server and database is on the computer on the other side of the room.

The answer? You got me. Just kidding, it’s micro-controllers. I am going to use an arduino (or a clone) and then use and xbee radio (on the zigbee protocol) and communicate the data to a similar setup that connects directly to the computer. Ahh, now you get it, resistance comes in the form of adversity and complications in moving the data. Nope, the resistance comes in the form of a set current being passed to the already installed temperature probe. That probe is placed wherever we need temperature, in this case a vat of beer, and the voltage returned from the probe is a value derived through the resistance. So I pass 5 volts in and depending on the temperature, I will get somewhere around 1.335 volts back with something like .0049 volt increments per degree.

That’s just an estimate. How did I find all this? Well, first you take the COTS temperature controller and google it. That gives you its good max and min ranges. But then you have to google the temperature probe as well because it can (and in this case does) have different ranges. Next you hook up a DMM (Digital Multi-Meter) and see what those voltages are because the white papers do not specifically list them. In our case its 0 – 5 volts which is excellent because that’s the exact range our micro-controller runs on.

In the googled white papers I got a list of the resistance, in ohms, returned by the probe for each degree from -40 to 100C but I don’t need the ohm, I need the voltage because I can’t measure resistance on the micro-controller. So now I have to either formulate the voltage based on known max and min values, or just set up a hash table. The first way sounds better and also more precise. The second way, well to me just seems like a cop out.

Currently I am working on making that first way happen. I have most of it ready to go but need to test with multiple temperature units, since we will be watching more than one vat of beer. So hopefully soon we can check out the current temps for our favorite beer at the local brewery and we can do it through a web interface from anywhere in the world!