I am looking for an inexpensive battery-powered z-wave relay that I can use to control low-voltage devices. Does such a device exist? If not, is there information about how I can “roll my own”? I am willing to build the controller myself if someone can point me in the right direction.
There aren’t any devices that I know of, which would check all your boxes. There are options if you just want to control low-voltage devices through a relay, but none of those operate on battery, as far as I know. Maybe someone else can jump in here. There could be something that I just don’t know about.
As for building the controller yourself, it will also be difficult to satisfy all your requirements: Your options for zwave control a limited, due to licensing requirements. Even the ThingShield (SmartThings Arduino shield) speaks zigbee rather than zwave, afaik. Getting any kind of reasonable battery life from an Arduino is notoriously difficult. You’ll certainly have to by-pass the linear voltage regulator, and replace it with some kind of switching regulator with low quiescent voltage draw. Also, you’d have to put the Atmega to sleep liberally. I am not sure if the ThingShield supports any kind of low voltage mode, or interrupt driven wakeup. The “ThingModule” certainly has these capabilities, but they may not be exposed. The shield may also have another inefficient voltage regulator, which may be harder to bypass. I think it’s definitely possibly, but may not be as plug-and-play as you’d like it to be.
If I were you, I’d choose a different microcontroller. Something that runs at the 3.3v logic level, and which you can run off of a LiPoly, and also avoid the ThingShield altogether. You should look into the SparkCore (www.spark.io). For $39 you’ll get an ARM chip and wifi, as well as access to their cloud infrastructure, i.e. you can control the SparkCore from a SmartThings device handler through simple http GET and POST commands. You could easily add a relay, MOSFET, TRIAC or whatever works best in your case, to control some low-voltage load. Depending on the type of battery you use, you’d still have to think a bit about power management, as well as come up with clever software solutions to maximize your batter life. This might be a better starting point, though.
Lastly, check this out for some inspiration:
Maybe you can cram a bare-bones arduino on the prototyping section of the ThingShield. You’d still have to find a way to put the ThingModule to sleep without missing any incoming communication packets, though.
Thanks for all the information. I’m aware of the SparkCore, which is certainly a very interesting device at a reasonable price. I’m afraid that, it too, will have battery life problems based on its specs. It should be possible to turn off its LEDs completely and to do timed wakeups, but I am concerned that every wakeup will require renegotiating the wireless link, which will use a lot of power and take quite some time. I am rethinking my requirements to see if I can use a power supply rather than batteries.
Thanks again for the info.
@anotherron, that sounds good. As for the SparkCore, the hardware is technically better suited for low power operation, but I agree with you that it probably won’t be working out of the box. I was thinking that it might be less work to get low power consumption than using an Arduino, but maybe I am wrong.
The CC3000 wifi module has some nifty tricks to achieve low power consumption. There is a low power (but still too much for long battery life) listen mode, but you can put the chip into very low power sleep mode(s). It does cache network specific auth data, which allows it to reconnect to your wifi in a matter of some milliseconds. Here is further info in case you are still interested: http://e2e.ti.com/support/low_power_rf/f/851/t/162231.aspx
All the better if you can just use a power supply, though. If that’s the case you can probably get something going really quickly.