Like I said in my last post I’ve decided to rethink my wireless temperature sensor. Instead of using a local wireless protocol like ZigBee or the Nordic Semiconductor 2.4GHz wireless. I want to investigate using WiFi instead. After doing a bit or research I found the Electric Imp WiFi modules (Electric Imp) look like they might be a good fit. They should have a low sleep current (<10uA) and seem to have a lot of support for the software side which is where I usually am lacking in my projects; I’m fine designing hardware but get frustrated with software development easy.
With some very quick back of the napkin calculations I figure that I can have one Electric Imp Running off 2 x AA batteries that logs the temperature and sends it out to the internet every hour and have it last at least a year on one set of batteries. Yesterday I bought and Electric Imp IMP001 module and a breakout board. I powered it with 2 x AA batteries in series and set it up to log the temperature of a thermistor and the battery voltage. With a little bit of code I have it sending the readings out to Xively; I used them when they were called Cosm on a previous post.
Here are the live feeds from Xivley for the past 24 hours for both the thermistor temperature and battery voltage (Sorry to all future readers if these go down eventually)
Now this isn’t a perfect test since the Wifi on the Electric Imp module with cut out when the voltage reaches 2.5V or about 1.25V per cell and Alkaline AA batteries are fully drained when they get to 0.8V per cell. So at some point I will need to design a board with a boot converter so I can utilize the full capacity of my batteries.
I did some testing last night to measure the current draw of the Imp module and found that my sleep current was a bit higher than what is specified in the datasheet; I was getting around 800uA but that’s most likely I have it setup wring and not going to sleep fully. I did make sure that I am controlling the thermistor properly which a GPIO pin so that isn’t wasting current when the device is asleep. I also measured the current when it wakes up and transmits.
This was measured through a 10 ohm resistor so we have a peak current of about 130mA and an average current for the whole transmit pulse of about 50mA for 75ms. So that’s not bad as long as it’s not waking up very often I should get pretty decent battery life. I think when I have the whole thing setup I will probably have it text or e-mail me when the batteries get low.
Going forward on this project I think I am going to have to get another electric imp module so I can do some more development while I leave this one running and I will work on my own board with a boost converter and maybe some other sensors to connect to the Imp.
Andrew