Over the past weekend and earlier this week, I had a chance to characterize the low current capabilities of the mesh boards. I was particularly interested in seeing if I could run a Xenon off a single cell battery for extended deployments.
The idea of my project is simple. Create a motion sensor board that can last years on a single CR2032 battery. It would detect motion, send a message over mesh and go back to sleep. The device would spend >95% of it’s life in sleep mode. Unfortunately that 95% has to be optimized or it’ll never laster more than a few months at best.
I measured the devices in the two different sleep modes. Unfortunately it was not as promising as I had hoped:
|Li @ 3.4V (mA)||Deep Sleep (uA)||Stop Mode (uA)|
Side note this is extremely dependent on hardware and the firmware involved. I used DeviceOS 1.4.0 for this particular experiment. I also know that this hardware is extremely capable of getting down in the µA range. It just takes some elbow grease and time.
I understand that these boards are not meant for low power deployments through convention means. There is a way around it though…
Bypassing all the power circuitry.
At this point you may think I’m wacko, but the fact of the matter is, most of the chips on a Xenon can actually tolerate a wider voltage range. Officially, using data sheets, it’s 2.7V to 3.6V.
This means, for the most part, a CR2032 can be connected directly to the 3.3V signal. Testing in the same modes as above, I found the Xenon went from 846µA to 49µA. That’s a 17x improvement.
Since making that decision, I’ve had my freshly assembled motion sensor spamming me with Pushover messages. There’s still some more optimization to do but it’s getting there.
I wrote up a much more lengthy guide on the subject. For folks wanting to eke out every single µA in their batteries you should check it out.
Important note: this should go without saying but using a Xenon outside it’s normal operating mode could prove disastrous if you’re not careful. I’ve bottled my years of experience building embedded hardware into this guide. If you decide to do it yourself proceed with caution! (and make sure you never put more than 3.6V on the 3.3V rail!)
Here are a few more pictures of the “naked” assembly: