# Gesture Recognition of Sign Language Alphabet Using a Magnetic Positioning System
## 2 Materials and Methods
### Magnetic Positioning System (MPS)
#### 2.1.2 Transmitting Nodes  
Each transmitting node can be programmed to generate a unique coil excitation frequency in a predetermined range. A ready-to-use integrated solution based on the PRoC (Programmable Radio-on-Chip) CYBLE-222014 by Cypress Semiconductor [40] was selected to minimize the overall node size. The device is based on an ARM Cortex-M0 processor and integrates into a small form factor of 10 \(\times\) 10 mm a number of components needed for its operations like crystal oscillators, antenna chip and passive elements. CYBLE-222014-01 supports several peripheral functions, such as analog-to-digital converters, timers, counters, and Pulse Width Modulators (PWM), as well as serial communication protocols (I2C, UART, and SPI) through its programmable architecture. CYBLE-222014-01 includes a royalty-free BLE (Bluetooth low Energy) stack compatible with Bluetooth 4.2 and provides up to 16 general purpose input/outputs (GPIOs) and has 256 KB of memory. The peripherals used for the expected functionality of the transmitting node are: BLE stack, PWM, timers, clock dividers and GPIO. Figure 5 shows the block diagram of the transmitting node. The first block is the power supply section. The wearable device has been designed to be battery powered; however, multiple power sources are supported due to the presence of a fixed 3.3 V voltage regulator (TSP71533 by Texas Instruments) that permits obtaining a stable supply for a wide range of input voltage: 3.75-24 V. The second block is the core of the device represented by CYBLE-222014 with its internal used peripherals. The PWM peripheral is configured to produce an output voltage signal with a frequency in the range 149.47-210.53 kHz with 500 Hz steps, the duty-cycle being fixed at 50%. The output of the PWM device is connected directly to the coil to set the square wave, at the desired frequency. The printed circuit board (PCB) has been designed using EAGLE Autodesk software for schematic, routing and Gerber extraction (Figure 6a). The result is a 2-layer PCB whose dimensions are 25 \(\times\) 15 mm (Figure 6b). In Figure 7, the PCB implementation of the transmitting node is represented with the elements described above. In addition, the wearable device is equipped with both a signaling LED to assess its status and a programmable SMD (Surface Mounting Device) low-profile tactile switch (SW) as a simple user interface. To further reduce the dimensions, the programming interface is obtained through a series of pads to whom the programming probes will be connected. Figure 4: Rear view of the \(yz\) plane of the magnetic positioning system. Receiving coils inserted into carved slots are visible, together with the microcontroller unit acquiring the signal on four receivers. The wearable nature of the device requires no wired connections for data exchange to increase its portability. In this case, the BLE wireless interface was chosen as it is a widely used standard. Therefore, data exchange does not require ad hoc programs but can be used to provide a good performance. Figure 5: Block diagram of the transmitting node. Figure 6: Printed circuit board design of the transmitting node. Representation of the routing result (**a**) and fabricated PCB without components (**b**). Figure 7: Photo of the realized PCB implementation of the transmitting node with parts populated. managed through the many generic apps available for both PC and mobile devices. Using the wireless interface, it is possible to set the frequency of the square wave generated by the PWM peripheral. The wireless communication channel is provided by the on-board BLE facility, which guarantees low-power communication. The GATT profile (Generic Attribute Profile) is used to exchange data according to the BLE protocol [41], and the transmitting node acts as a Bluetooth server. One client-writable custom characteristic is defined to store the desired PWM frequency. The transmitting node, after the power up sequence, starts advertising its presence and accepts client connections.