The talisman will make use of BlueTooth for both communication bridged to the internet and also to detect the proximity of the talisman to beacon units.
To confirm the use of the RSSI (Radio Spectrum Signal Strength) and determine the available accuracy and range a device was built.
The Bluetooth Proximity device has an internal microprocessor, BlueTooth Module and a 2 line LCD.
After starting, each row in the LCD shows the signal strength reading for each of two example beacons. This shows that it is possible to detect when the talisman is within proximity of one or more beacon units.
The beacon units are very simple and contain a low cost bluetooth module that is simply permanently powered via a wall plug pack or other USB power source. There is no requirement for the beacon to contain any microprocessor or to connect to the internet.
To test the proof of concept the two beacon should be plugged into a USB power source, and placed in two rooms around a house. Then the LCD display unit should also be powered via the USB cable and moved around to allow observing the possible detection range and accuracy.
The various tools for developing the firmware were installed on the mac, and a very simple basic program was coded to achieve the simplest test of being able to upload source code onto the device.
The prototype was successfully programmed to simply flash through the colors of the left hand side LED.
The flashing LED shows that the development tool, the compiler, and the programming dongle are working.
An error was found and fixed with two fine copper ‘bodge’ wires. The analog voltage supply and ground was left off. A future revision of the PCB would include these connections and remove the need for the manual addition of the two small fince copper bodge wires.
The fine pitch 32bit microprocessor was placed on the prototype device.
The testing was done to ensure that there were no bridges between the pins.
In the photo a tall bolt is visible. The bolts in the photo will not be present in the end user devices. The bolts are added on the development unit to allow permanent connection of power onto the device. On this development device long extension cables are added to allow the rechargeable battery to be located further from the PCB.
The end user device will not have any of the wires, and the battery will be placed on the top of the two nuts.
The components for the power management were added and tested successfully.
The reverse protection allows the user to apply charging power to the two screws in the wrong orientation without any damage.
The battery charger IC charges the onboard Li-Po battery when power is applied to the two nuts. The charge current was measured at 45mA which could be reduced very easily by using a smaller configuration resistor.
The voltage regulator was tested and ensures a consistent 3.3 volt is supplied to the circuitry.
A second attempt was made to pour a silicon mold around the shaped resin form.
The pour was not the best, and the liquid resin did not flow correctly. An ugly bumpy lump of silicon can be seen in one corner. Normally the silicon would flow and form a very flat level on each half of the mold.
The first attempt failed when attempting to degas the liquid silicon. In the first attempt the liquid silicon immediately cast inside the vacuum chamber into a spongy foam. The silicon resin was 12 months old, and has a recommended shelf life of 6 months.
It remains to see if this 2 part mold will be able to retain the resin without leaking!
The parts to be used in the dongle that plugs into a computer arrived. The particular USB Module chosen was selected to work out of the box without requiring hunting for USB drivers.
This module will be in a small enclosure along with a wireless Bluetooth module.
Together the USB module and the Bluetooth Module will allow a program running on a computer to bridge from the server, over the internet, through the USB module, into the Bluetooth module, over the RF airwaves, into the Talisman Bluetooth Module and finally onto the Talisman device.