The OLinuXino Micro is EUR25+shipping retail, and has pin holes for multiple UARTs available, and a USB port that can take a TP-LINK WR721N or similar USB Wi-Fi dongle.
This board has 64MB RAM, which is probably enough, and a microSD card slot from which a bootable Linux (not Android) image can be used.
There is an option including Wi-Fi:
But the Wi-Fi is the wrong chipset (lacks simultaneous ad-hoc + AP mode of operation). Also, the MINI has much higher power consumption, around 2W - 4W, due to the inclusion of the USB + ethernet controller IC.
Power consumption is very low:
Without a USB peripheral, power consumption is between 4.2V*75mA = 0.32W and 4.2V*120mA = 0.51W.
I will try to determine the power consumption of the WN721N so that we can estimate a total power budget, but it is clear that it isn't going to be worse than the MK808B, and the Wi-Fi performance will be much better than the MK808B, and will properly support simultaneous AP+ad-hoc mode as we are accustomed.
We will assume that a 50Wh battery will still be sufficient for ~24 hours operation, plus enough spare capacity to recharge a typical smart-phone once in that time. We have ordered the following as a test battery:
The board has extra UART pins in addition to the serial console, which makes attaching the radio easy. The extra UART is already available as the /dev/ttySP1 device on their stock Linux distribution:
This would be mated with an RFD900 or similar radio, as with our other designs.
We have also had initial conversations with the people at olimex about making a version with the AR9721 Wi-Fi chip (the same one that is in the WN721N) built into a modified OLinuXino board, so that we would have just one PCB + radio.
In the meantime, we can buy the WN721N for AUD$10 + postage locally in Australia – which is cheaper than any current offers on AliExpress.com:
Another option is to use one of the following AR9271-based modules:
An interesting discovery with these (thanks to bitmonkey) is that there is a low-power version of the AR9271 called the AR9271L. It trades off maximum transmit power (13dB - 17dB depending on bitrate) for lower power consumption. Given that the AR9271 consumes 0.68W when listening, any reduction would be welcome.
These modules could potentially connect directly to the pins that the USB connector on the MICRO uses, by first de-soldering the USB connector and making a short adapter cable/connector.
Using the AR9271L would be a little more involved as an additional 3.3V regulator would be required to supplement the 300mA maximum of the regulator on the MICRO. Specifically, the AR9271L can draw upto 500mA @ 3.3V according to the following datasheet. Frustratingly, the listen current of the AR9271L is not listed, only the max TX current draw.
The WUBA-* devices are about US$25 in small quantities.
We have since found the following, that costs US$6 in small quantities:
This is AR9271L based, so we would still need a 500mA 3.3V regulator. There are plenty of 800mA 5v → 3.3V regulators out there, e.g., the following two options for the same LM1117-based device:
Price is US$1.13 ea for 20 units or US$0.91 ea for 100 units.
The efficiency of these regulators is not ideal, however, as they are linear regulators, and thus increase the effective power consumption of the AR7291 Wi-Fi module to effectively be that of the AR9271 5V version. Basically they chop the 5V down to 3.3V and that 1.7V just goes out as heat. The end result is only 66% efficient, i.e., power consumption of the Wi-Fi module will effectively be 100/66 = 1.5x higher than it needs to be.
In that case, we might as well just use the AR7291 5V version, and get the extra Wi-Fi TX power, and reduce the complexity of the design a little.
So this means we have a potential solution with the following components:
For battery powered operation
Total cost in small quantities around AUD$123 plus case and misc wiring (if using an Wi-Fi dongle instead of having it integrated, plus some shipping costs.
The power budget is similar to the MK808B-based solution, so a 50Wh battery for about AUD$40 will be fine. And for $37 more we should be able to get a box for the electronics, connecting cables, and some material for the tool-pouch style package for a total prototyping BoM of around AUD$200 per unit.
We might even think about getting some 1.5W or so solar panels to make a solar powered version.
See https://www.olimex.com/Products/OLinuXino/iMX233/_resources/OPEN-EMBEDDED-README.TXT for URLs to download a rootfs and kernel for this board, and instructions to make a bootable microSD card.
There are also instructions for building the above directly from source.