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Sensors, Actuators, and Robotics » Remote Temperature Sensor

November 04, 2009
by luke
luke's Avatar

I'm trying to figure out the best way to read the outdoor temperature so I can graph it over time. I have one of those weather stations with a remote temp sensor, so I was thinking maybe I could read that somehow. I know the device transmits periodically at 433 MHz, but that's about all I know about it.

Any suggestions?

-Luke

November 05, 2009
by mikedoug
mikedoug's Avatar

Luke -- I've been scheming on exactly that same project. In fact, I just emailed the nerds yesterday with my thoughts and ideas on how to accomplish it. I wasn't planning on using an existing transmitter, but to create my own. I want to be able to have multiple sensor boxes around the house monitoring light and temperatures in freezers, fridges; other various temperatures (indoor, outdoor, upstairs, downstairs); light and sound sensors in the kid's room for night-time naughtiness detection; and perhaps a distance sensor attached to the garage door letting you know it's down.

My plan is to have sensor boxes where you can attach multiple sensors to each box. You configure the box with an identity which it broadcasts with the results of the ADC conversions. The plan is to have the sensor boxes run for a REALLY long time on a 9V battery -- so power conversation at the sensor box is very important.

Lastly would be the home-base box which has the receiver. The receiver would have an LCD display to report information as it comes in, some LEDs and a buzzer for indicating trouble conditions (deep freezer temp at 20F, etc.). It would also report, in the raw, all information received over an RS232 link to a computer where a program could do whatever it wanted with it.

On the computer side I plan on having my Linux box receive the data and dump it all into RRD files. That way, like you, I can look back at the historical information any time I wanted.

November 05, 2009
by mrobbins
(NerdKits Staff)

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Luke,

Do you have the receiver unit too? My guess is that if you open it up, you'll find somewhere on that circuit board there's a serial signal being passed around that you could tap. If you had an oscilloscope/logic analyzer you could probably find a few potential candidates (probably antenna --> some chip doing the RF decoding ---TAP HERE--> some microcontroller running the display).

mikedoug,

We got your e-mail and e-mailed you back. This would be a really useful project and I think you are on the right track!

Mike

November 05, 2009
by luke
luke's Avatar

Mike,

Yeah, I did try tapping into the receiver. There is a separate PCB with three wires: power, ground, and signal. The receiver seems to run at around 3V, but 5V from the nerdkit seemed to work... In any case, I couldn't make heads or tails of the signal wire. I don't have an oscilloscope/logic analyzer, which is part of the problem. I wrote a little Java program that graphs output from the nerdkit, but it didn't seem to help with analysis...

mikedoug,

Sounds like a cool project. Lots of stuff involved, so good luck! For my project, I just want to track indoor/outdoor temperature, and the on/off state of my furnace. Then I can graph it and see how much I'm heating the outside.

-Luke

November 07, 2009
by Hugh
Hugh's Avatar

"For my project, I just want to track indoor/outdoor temperature, and the on/off state of my furnace"

Luke, a possible power source for your furnace might be a peltier module (do a search on eBay for peltier or thermoelectric). It's solid state and is powered by the temperature difference between the two sides. The higher the difference the more voltage is produced. You will want some sort of heatsink on the cold side though.

Peltier modules used to be pretty darn expensive and were over $100-150 USD just a couple years ago. Now you can pick them up for around $8-20 each retail and $6 if buying in bulk of 10 or so.

You have two types - TECs and TEGs. Both are basically the same thing except TECs (thermoelectric coolers) are used at lower temperatures and have electricity applied TO them. when you put electricity into them they become a heat pump - one side gets really hot and the other side will ice up. TECs are put together with a lower temperature solder. TEGs (thermoelectric generators) are made with a higher temperature solder and thus can take higher temperatures which allow a bigger temp difference between the hot and cold sides.

However, TECs are fine for experimenting with and are generally cheaper than TEGs. Most of the TECs that I pick up on eBay can handle an input of 8.5amp, max 127.5 watts, 0-15 volts, with a working temperature of -40 to 85C. They are 40 millimeters on a side, used, and sealed against moisture.

You will see woodstove fans that are powered with these types of modules that run for $50 to $100+ even though the parts run about $10-15.

It's interesting stuff and they are fun to play with as a possible power source any place that you have waste heat (even solar waste heat). There are no moving parts and like solar cells have a working lifespan of around 20 years though there are some additional factors that come into play (heat-cooling cycles, moisture, etc.).

You will need a heatsink to make the most of one or both sides. Using one or a couple you might be able to trickle charge a power source every time your furnace kicked on.

Hugh

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