Figure 1: 2.4 GHz communications board, shown inside of homemade protection sleeve, with female header ends affixed by conductive glue. This is done because the end of the PCB contains the antenna, and the enclosure is aluminum. The PVC is affixed with a screw and tap hole underneath it (click to enlarge).
Note: Never use an aluminum enclosure to contain batteries (especially lithium batteries). Aluminum makes nasty shrapnel.
The Pi2 powered tablet has a few things now … a seven inch monitor screen, a GPS receiver, an atmospheric pressure and temperature sensor, a nice 192 KHz audio board (for weather fax reception), a built-in battery, a built-in charger, a colorful stoplight membrane keypad interfaced to GPIO, a sidecar SDR shortwave receiver, and a lot of great software. I’m still adding stuff to the little box though.
More than one way to communicate …
To communicate between tablets, one can use standard WiFi, but devices for it typically have a pretty high power consumption rate. I found some micro powered communication boards that operate in the unlicensed 2.4 GHz band (this is the same frequency range used by WiFi). These devices can run on as little as 30 milliwatts. I’ll describe my experiments to use these guys in the next few pages. The idea is to make the Odroid powered tablet and the Pi2 powered tablet communicate with one another, without adding very much extra power drain. Theoretically, the connection could be continuous, and used for swapping data of various sorts, move charts and weather faxes around, run TCP/IP links for web based data distribution, and other things without significantly decreasing battery time.
The “lilliput power level” communications devices are controlled via the SPI bus. The standard arrangement calls for a five pin connection from the comm board to the pi2 (or Odroid) – but thanks to another hobbyist (listed below), I’ve found a way to make the connection with only three wires between the boards. I’ll post more info about the two different wiring schemes after I have wired them into the respective Odroid and Pi2 GPIO interfaces.
Note: the author does not have a recent, applicable background in circuit building, or battery related issues, so this is presented as the work of a hobbyist, and is not meant for duplication by others. Readers should look elsewhere for design advice and info.
Note: This author and site is not affiliated with the Raspberry Pi or Pi2 in any way. For information about those projects visit http://www.raspberrypi.org. “Raspberry Pi” is a trademark of the Raspberry Pi Foundation. The Odroid is a SoC based SBC available from hardkernel.com, and is not affiliated with this author or site in any way.