Belinonda 4 is a deviation from my Microchip mainstream. Searching on the net for something more powerful i have found Lietuvos company 8devices producing Carambola 1 and Carambola 2 modules. They are mini modules built around a processor with on-chip Wi-Fi adapter, a boot flash and some megabytes of ram for OpenWrt, decompressed from flash at boot time. Peripherals include a 3.3 volt serial, Ethernet without Phy, USB, SPI and some GPIO pins. RF output is on a pin and, in parallel, on a UFL connecor. Carambola 1 is based on Ralink 3050 and seems to be phased out. Carambola 2 (picture) is Atheros 9331 based and seems to be the ultimate board.
It is supplied with preinstalled OpenWrt. There nothing to hack, system is not locked and completely programmable from user. Price on the net is less than 25 euro. For a slightly highe price you can have a development system, a board with the module, voltage regulator, chip antenna, USB-serial converter, Ethernet Phy and all connectors.
Carambola 2 lookeed great for a Wi-Fi web server and then for a wearable Internet site. Starting from development system, and dropping all the unnecessary, has been designed Belinonda 4. Here can be seen both sides separated,
Finished PCB, both sides, as usual built from Millennium Dataware, is shown in pictures. Eagle CAD files are available here.
First of all Ethernet has been dropped, because can not be used on a portable and wearable system. Next a microSD slot has been added, but to use it installed operating system has to be sysupgraded. For details please look on OpenWrt site. USB female-A connector has been placed in such a way that a small pendrive remains within PCB border. This is the basic position for site data. A webcam can be added to the site, to stream video data, but it requires the connector. Then data must then be kept from microSD in the slot. It is possible connect pendrive and webcam to a hub. It is supported, but in my opinion is too bulky. Next, USB-serial converter for the console has been removed from the board, using instead an external one. Today on chinese sites, like DealExtreme or AliExpress, converters are for sale for few euros. They are small boards with USB male connector, circuitry and cables, in general breadboard wires. Basic version has 5 pins: | Gnd | +3.3V ! +5V | Txd | Rxd |. More sophisticated versions include Rxd Txd activity led, and sometimes control lines DTR RTS. Usually the chip is not the traditional FTDI, but a Silabs 2102. Anyway drivers for Windows and Linux are easily availables, and often pre-installed in recent releases. This way a circuitry necessary only fon configuration and settings is no more on the board, saving space and energy. A configuration option using web interface remains still available. Power supply can come from serial four pin connector or from micro USB, connected in parallel. Micro-USB data lines are not connected. In the pictures a Belinonda 4 fully assembled can be seen. This version has an external antenna attached to the UFL connector and the USB-serial converter attached to four pin, 0.1" pitch connector.
An essential item for Belinonda 4 is antenna. Carambola 2 basic module is supplied without antenna. Development system, instead, has a chip antenna. Model is unknown, but in general all antennas of this kind have an omnidirectional emission pattern. Moreover in general they are tuned for work with some clearance around. It would be nice, for a wearable system, to have an half-space emission pattern instead. RF power is low. A cell phone is more powerful and, in general, nobody cares, but, even without paranoia, an inexpensive prevention is possible. Molex, a company well known for automotive and electronics connectors, has in production an helical chip antenna. She is omnidirectional too, but according to Molex is not de-tuned above to a ground plane. Then ground plane can be inserted to act as a shield to human body. Moreover no tuning circuitry is necessary, and antenna can be connected directly to a single-ended 50 Ohm RF output. Obviusly if connecting trace is long it must be a 50 Ohm transmission line. Antenna soldered on circuit can be seen in the picture.
Antenna is for sale on RS site. Part number is 759-5325. It has been the basic choice for Belinonda 4. Results are been good, emission is good for size, and no interference with computer circuitry has been observed. Transmission line has been calculated using a free program, available here. Main dilemma was the dielectric permittivity of PCB. FR4 laminated is not for microwaves, and tolerance is quite wide, from 4 to 4.8 . Calculation has been done with average value of 4.4, and worked. Using FR4 from a different producer, an adjustement could be necessary. To wear conveniently Belinonda 4 i have designed a small box (it is not a masterpiece !). In the pictures MeshLab renderings for box
MeshLab is free, and can be downloaded here. STL files of box and cover are available here for 3D printing. Cover, box and the finished PCB can be seen in the following pictures.
Looking more in detail the electric schema (to be seen directly in Eagle CAD), there is first the Carambola module. Power supply circuitry has a 3.3 volt regulator. It is a ST L1117, with a maximum output of 800 mA, more powerful than Microchip TC1262, used in other projects of this site. BEWARE ! They are both in SOT223 package, but pinouts are different ! Next are stabilization capacitors. To basic schema a third capacitor, in the bord point more far from regulator, has been added. It is ceramic, 4.7 microfarad, and is available on RS site, part number 741-6821. Being ceramic and big, it is not inductive and filters radiofrequency very well. I like a lot this unusual three capacitors system, copied from a japanese schema. Main external power is standard 5 volt USB, and can be supplied from a power bank. 5 volt entry points are two in parallel: when the board runs normally, in wearable configuraion, power supply comes from micro USB connector, placed only for that, and having only power pins connected. 5 volt in input are redirected to USB-A female connector, allowing the board to work as host, powering the device. They are also redirected to a 3.3 volt regulator, feeding Carambola module. During programming and cofiguration from root user, 5V power comes from a 4 way connector, attached to an external USB-serial converter and next follows the same path. Three other pins of this connector are ground, transmitted data and received data of a 3.3 volt serial line. In this way the serial console can power the module like a USB (but beware ! at this point it is no more a USB !). Many different models of USB-serial converters ara available . My preferred one is built around Silabs 2102 chip. Levels of TxD and RxD signals are 3.3 volt. In general converters have a minimum of five output pins : ground, TxD, RxD, +5volt e +3.3volt. 5 volt comes directly from USB host. 3.3 volt comes from on board internal regulator and available current is limited. Belinonda 4 is designed for a 5 volt power supply, and 3.3 volt line remains unused. On the board exist a 3.3 volt regulator, and it has a voltage drop. Powering the board with 3.3 regulator output will be 2.8 - 2.9 volt, insufficient for Carambola. In the circuitry a protection diode between 5 volt input and voltage regulator has been inserted. USB connector is polarized and voltage inversion is impossible, but powering the board from serial converter such a mistake can be done very easily. Without diode it is enough to be a little bit careless and Carambola will be destroyed. Beware ! Carambola is quite sensitive to power supply fluctuations. Using a low cost SMD diode (with a measured voltage drop of about .85 volt) the module boots normally, but when Wi-Fi starts, increasin current absorption, sometimes it resets. The problem has been solved using a low drop Schottky diode (0.38 volt measured). The model tested is available from RS and part number is 638-7915. Finally on the board you can find microSD slot, hardware reset button, two user programmable buttons, three user programmable LEDs, and a 8 way connector with ground, 3.3 volt output and the remaining 6 GPIO unused pins. Three holes in the PCB are for board fixing, and in the plastic box there are three corrisponding pins. MicroUSB power input makes possible the direct use of a power bank, now widespread for powering cellphones and portable electronics. They are in general supplied with a cable, used for power the device and, when inverted, to recharge the power bank itself. Observing the finished work Belinonda 4 looks a little bigger than expected. But it works, and a more compact version is not pressing. We can wait for the evolution of USB connectors in portable electronics. The European Community forced cellphones producers to unify recharge using mini USB connectors. Subsequently chinese producers switched to micro USB and the EC decision was completely inattended and forgotten. In fact most bulky part is USB-A female connector, and would be nice to use another female micro USB. Webcam are not a problem problema because they have a cable, and an adapter, well known as OTG cable, can be used. In present configuration a pendrive can remain within board boundaries, and the use of an OTG cable for him would destroy system compactness. Pendrives with micro USB connector, designed to be directly conected to cellphones, are just hitting the market, Right now they are too bulky and too expensives and an evolutional improvement is expected. Overall we can say that present configuration is already a good compromise, and wearability is quite good. Normally only one cable is necessary, power supply, and, even using a webcam, this further cable is very near and parallel. No porcupine configuration, as can be too often seen in boards designed only to optimize themselves.
Final result can be seen in picture.
Belinonda 4, closed in his box, is connected to a neck cable, together with a standard badge used in meeting identification. Wearability target is achieved.
Software is completely OpenWrt based. This is not a tutorial, OpenWrt site has a lot of them. Anyway it is an operating system for users with some experience, not for absolute beginners. The 8devices site has a page full of modules recompiled for AR9331 and tested. Installed Carambola 2 flash filesystem has many preloaded modules, among them the uhttpd web server. Other modules can be downloaded from 8devices site and installed. For Belinonda 4 i have chosen the more powerful lighttpd web server, supporting more connections. Apache would have been the best, but for AR9331 it was not yet available. Using development system installations are very easy. It is enough connect it via USB to a PC, and connecti it to Internet via Ethernet (the development system, not the PC) and from 8devices site the desired software can be downloaded. Installation program, as usual in Linux, goes directly from Internet to system disk. Dependencies are handled. Belinonda 4 does not have Ethernet. Connection can be done using Wi-Fi, but i want to have a local copy of everithing downloaded from Internet. Then my preferred solution is download modules from Internet using a PC and save them on a pendrive. Next pendrive is connected to Belinonda 4 USB, and installation can be done locally. With development system connected to Internet commands to install lighttpd are:
opkg install lighttpd
Local installation is a little bit more complex. First task is install USB for pendrive. This task has to be executed only once. Drivers are already here, only peripheral has to be logically created, but this has to be done anyway, if you want to use it. Command is:
mkdir -p /mnt/usb
Every time you insert the pendrive you should mount it (it is standard Linux !)
mount /dev/sda1 /mnt/usb
If modules are in the "mods" directory of the pendrive you should enter in it
If name of module to be installed is mod.ipk command is:
opkg install mod.ipk
This has to be done for every required module. Dependencies are no more automatically handled, but every miss is warned. This item seem very confused in Internet, but it is easy. If you give to opkg installation program the name of an existing file, the file is used, otherwise it is searched in the net. And remember: .ipk files are zip files, rename them as .zip and a PC opens them without problem. And for Android .apk files it is exactly the same.
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