Temperature and Current sensor evaluation for the Home

Dec 10, 2008 in Home Automation

I’ve researched what type of cheap current and temperature sensors I would like to prototype for my project and decided upon the following:

1. An NTC Type Thermistor from Cantherm (CWF1B104J3950) that provides relatively high precision, reliability, and response time. This type of sensor also allows for a fairly simple conditioning circuit.

2. An Integrated-circuit precision temperature sensor (LM34) from National Semiconductor. This sensor provides a very linear output (+10mv/°F scale) at a very low cost. The problem with the IC type sensor is that it requires a more complicated power supply if you wish to measure temperatures around or below 0° Fahrenheit. I’m prototyping these since I already have a decent supply of them.

3. The CR Magnetics’ CR8410 current-sensing transformer. This a relatively cheap current transformer that is capable of measuring up to 20 amps. You need to take precautions when using this type of device since there is a potential to create high voltages when reading house current so if you aren’t sure, get someone who is experienced with these.

These parts were ordered from Digikey. I use both Jameco and Digikey when buying electronic components and I’ve always had good results with both companies.

Also, some thanks to the people over at All About Circuits, I wasn’t sure on the transformers and they were there to help me out.

When they arrive, I’ll set these components up to do some initial testing and evaluation. In the meantime, I’ll start some initial work on the Computer measurements I discussed earlier as well as a solution for converting the conditioned signals from the current and temperature sensors to a digital signal a computer can understand.

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3 comments

Home-Automation » joliet blog » Blog Archive » 3 Ways To Spend Less On X10 AM466 ... on December 10, 2008 at 7:15 am

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Toby on December 16, 2008 at 6:40 pm

I am a big fan of the LM34. The LM34CAZ is very accurate and easy to use. Sometimes you need a load resistor to help get lower temperatures:

http://www.labjack.com/forums/index.php?showtopic=1040

To read temperatures below zero you can use a battery to get the negative supply shown on Figure 2 of the LM34 datasheet, or you can use the diode trick shown at the bottom of page 6.

Also consider the LM135 which returns Kelvin and thus is good for negative temperatures.

For current consider something that gives you a nice easy RMS value to read. We have used the ESI-695 with our U3/UE9:

http://www.amazon.com/gp/product/B000I17RAM

With the current transformer you are going to have to sample the 60 Hz waveform and then in software find the peak or RMS value.
Kelly on December 17, 2008 at 2:37 am

I do like the LM34s and didn’t realize there was a Kelvin version, which is even better. I chose the thermistors since they SEEM to be fairly accurate, cheap, and all I need is a voltage divider. I’ll probably include some LM34s or LM135s at some point however.

As far as the current sensing goes, I’ll probably build a simple rectifier circuit for each transformer to keep my costs low.

Thanks so much for the input!