## 3 Hardware Description
### Secondary Node  
As previously explained, secondary nodes act as active IDs beacons. Each secondary node consists of two different parts: a kinetic harvester module and a transmitter module. The former is in charge of generating power produced by the animal's movement while the latter is in charge of transmitting a unique identified ID. Kinetic energy harvesting module  
The kinetic module is divided into a magnetic-coil generator and a regulator circuit. The generator is designed with a cylindric tube with two coils located at each end of the tube and a \"Neodymium-Boron-Iron\" magnet inside the tube (see Figure 2). Figure 1: Network description. When the generator is tilted, the magnet moves through the coils and it generates a voltage which follows Lenz law. As the magnet goes through one of the coils, a voltage cycle occurs. Figure 3 shows the voltage waveform at the coil output in a swing of the magnet. Notice that the magnet starts at one end of the tube, and passes through the first coil (first positive cycle), moves out of the first coil (first negative cycle), passes through the second coil (second positive cycle) and finally comes out of the second coil (second negative cycle). Once the power is available at the input of the regulator circuit, it has to be rectified and regulated before powering the transmitter. A block and schematic diagram are detailed in Figure 4.  
The energy supplied by the generator is first rectified by a diode bridge. Four Schottky diodes are used to create the rectifier bridge; a voltage drop of about 0.25 V per diode is observed. A 100 \(\mu\)F capacitor and a 10 V limiting zener diode are located at the output of the bridge to protect the next stage input. Figure 3: Waveform at the output of the magnetic kinetic generator module, after a full swing. Figure 2: The magnetic kinetic generator module. The regulator circuit adapts the input voltage to a steady stage voltage. A step-down dc-dc converter is used to stabilize the output to 3 V. The regulator charges two 100 \(\mu\)F capacitors in the short bursts that correspond to the existence of energy at the input (a magnet swing) to the desired dc voltage (3V). After the regulator, a control circuit with hysteresis is designed to ensure enough energy for a frame transmission. Two different hysteresis thresholds are obtained thanks to a Schottky diode at the output to guarantee stability. Therefore, a nano-power comparator is used to ensure no energy is supplied to the transmission module until a minimum voltage is accumulated in the storage capacitor (high level hysteresis threshold). Once the comparator enables the output, it keeps energy delivery on until the storage capacitor voltage drops below a minimum threshold (low level hysteresis threshold). The output control is managed with a pMOS transistor which acts as a switch. When the comparator allows the energy delivery, it closes the switch, and when a given threshold is reached, the comparator opens it  
Figure 4: (a) Block and (b) Schematic diagram of the kinetic harvester module. again. Duration values of power pulses are typically above 20 ms, which allows more than one frame transmission as explained in the following section. Transmission module  
The secondary node's communication is designed to operate in the 433 MHz ISM open band. The band was selected because: _i)_ it is an open band that can be used without licence, provided that some regulations are met, _ii)_ commercial low cost IC chips are available for the ISM band, _iii)_ propagation path loss is lower as the frequency decreases and _iv)_ the size of the antennas is reasonably small for a miniaturized node. The transmission module consists of a radio frequency transmitter/controller with an 8 bit ID selection switch (see Figure 5). The modulation selected is binary FSK with a frequency deviation of about 25 kHz coded in Manchester. When the power burst is available, the MCU initializes, loads the ID, turns on the RF emitter, waits for a stable signal and transmit the frame.