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Figure 1: System Block Diagram
As can be seen from the block diagram, Bluetooth Low Energy (BLE) is being used to detect the distance between devices, it is also being used to communicate to a mobile phone. To detect whether an interaction has been made, the Bluetooth module compares the received power of a signal to the power that the signal was originally transmitted with, this gives a Received Signal Strength Indication (RSSI), which can be used to work out an approximate range. This RSSI value can then be compared to a tuned threshold value. If it is below that threshold, the device ID is stored in a flash memory, where it stays until it is sent to a mobile device. In order to ensure the device is as low power as possible, an accelerometer is being used to detect when the device is being worn. While being worn, the accelerometer detects movement and tells the whole device to be in normal operation, when the accelerometer detects that movement has stopped (i.e. the device has been removed from the user's wrist), the whole system goes into a deep sleep mode until movement is detected again.
For the device, the hardware was designed to be as small as possible, and due to this, a 4-layer PCB design was used. Figure 2 shows the schematic of the circuitry within the device.
Figure 2: Schematic of device hardware
The schematic was converted to a PCB design which could be assembled using a reflow oven. The final dimensions of the PCB were 32mm*35mm, which was small enough to be comfortable as a wearable once put into a case. The case was designed to be 3D printed into 2 parts and accepts standard quick release 22mm watch straps. A 3D render of the PCB in the case is shown in figure 3.
Figure 3: A 3D render of the complete device
The final cost of the hardware including the case and PCB is only £11, around half of the target BOM cost of £20
Software and Testing
The software was written based on the provided software for the AVR-BLE development kit. The device needs to constantly switch between scanning for local devices and being in a state where the user can connect to it with their mobile phone. To detect the approximate distance of a local device, an RSSI measurement is taken, since this value is available from an advertisement packet, there is no need to actually connect the wearables to each other, which saves on time and battery life. By reading out the RSSI measurements at 2m and 3m, figure 4 was created and was used to generate a threshold value such that 95% of devices at 2m are correctly characterised, with only 18% of devices at 3m incorrectly characterised.
Figure 4: A graph so show the RSSI values at different distances
To test the battery life of the device, the average current draw was measured in the devices worst case. Using this current draw of 7mA, the total worst-case battery life is just over 14 hours.
An app was also written to interface with the device, for the sake of this project, the app only takes data from the device, and displays it on screen. However, in a full product, this app should process the data it receives and send relevant data onto a server. Figure 5 shows the prototype app.
Figure 5: Prototype app
In conclusion, this project to make a track and trace bracelet was very successful, the device successfully detects how far away another device is, has a day-long battery and is comfortable to wear. By using a device similar to this in future, the effects of pandemics can be lessened, as the spread of the disease can be slowed without the need for major disruption such as national lockdowns. For anything trying to tackle this issue the most important factor is that as many people as possible need to take part, or the required effect won’t be achieved similar to the NHS app currently in use by a minority of the country.