While most of my home is made smart using ESP8266s, I plan to dive into MySensors for more energy critical stuff. The first project is a keyfob.
Attached is a first design for a 8-button keyfob mit hardware HMAC authentication via an ATSHA204, running on a CR2032 coin cell and fitting into a commercially available keyfob enclosure (see README on github repository).
The ATMEGA is supposed to run in power-down mode (400nA with disabled ADC, internal oszillator @ 8MHz), the powered down RFM69HCW should take 100nA in sleep mode and the ATSHA <150nA in sleep mode, so the overall standby consumption should be well below the self-discharge rate of the coin cell.
In standby, the buttons will have interrupts attached via PCINTx_vect, so that a button press will wake the ATMEGA.
Before I send out the PCB order, perhaps somebody is interested in having a closer look ... this is both my first MySensors project and my first low power project. Also, it is probably the most compact thing I have ever clicked together. (I am so glad about KiCAD's push and shove router ...)
Any comments? Tips? Ideas? Errors?
I would hate to send this to production and end up with 80 unusable PCB (Taking about PCBs, I will have a lot of spare ones in Germany, is anybody is interested ...)
The major issues I could currently think of are:
- Does MySensors implement the sleep mode of the ATSHA? Is it even possible in that package? I don't really know much about the chip (only followed the MySensors tutorial) and it would be a bugger if it sucked my battery dry ... - Answered: Yes it does.
- Will the coin cell provide enough current for transmission? I added a 22uF tantal, should I include more?
KiCAD project on github: https://github.com/kvoit/MySensors_SecureKeyfob/commits/master