## 3 Technical Background
### Wireless Charging Implementation Circuit  
The purpose behind launching this kit is to allow researchers to evaluate and study the wireless power transfer. It consists of six different components, which include: RF transmitter, energy harvesting receiver boards, patch and dipole antennas, wireless sensor boards, a PIC MCU-based development board, and a programming tool. All the values of the parameters used in the experiments are according to the Powercast kit data sheets [14]. 1. Transmitter (TX91501-3W-ID): The transmitter shown in Figure 2a uses the spread spectrum technique to send energy at a frequency of 915 MHz in the Industrial, Scientific, and Medical (ISM) band. The power transmitted is modulated using Direct Sequence Spread spectrum (DSS) which gives the advantage of less interference as we are sending the narrow band on a wider band, which in turn decreases the noise interference. It can move along with the power broadcast, transmit data, which is Amplitude Shift Keying (ASK)-modulated, and sends an 8-bit ID at random intervals at a frequency of 2.4 GHz. There are two different transmitters, one is able to send 1W Equivalent Isotopically Radiated Power (EIRP) and the other one can send 3W EIRP. In this paper, we used the 3W transmitter, which is supported by the utilizedkit [14]. EIRP is the maximum power that is radiated from the transmitter in the direction where the antenna exhibits maximum gain in relation to the isotropic antenna. The antenna integrated in the transmitter has a gain of 8 dBi and is 60 degrees beam patterned, and vertically polarized. The transmitter can provide energy for unlimited harvesters;
2. Wireless sensor board (WSN-EVAL-01): This board, which is shown in Figure 2d is able to sense humidity, light, temperature. Furthermore, it gives the option for user to add another sensor-measuring device, which can be connected to the external sensor board found on a terminal block (J3) if needed. The external sensor jack can only supply 3 Volts. The sensor board also has the ability of measuring the Received Signal Strength Indicator (RSSI) of the received power. The sensor information along with the RSSI measurement and node ID of the transmitter that is captured are sent as a packet via a Microchip MiWi P2P low power protocol [34]. The transmission is conducted using a frequency signal centered at 2.4 GHz to the Microchip access point together with the sensor ID that a user can set using the ID selection bits on the sensor board. This information can be viewed on a computer using the hyper terminal utility. The sensor board can be powered using the harvester board if attached on top of the breadboard connection on the harvester board. It has a PIC kit programming header that is used to program the PIC MCU on the sensor board;
3. Harvester receiver P2110 evaluation board (P2110-EVB): There is an SMA connector on the harvester board as shown in Figure 2c where the receiving antenna is connected and matched to 50 \(\Omega\). As depicted in Figure 3, the receiver board has a P2110 harvester circuit which consists of an RF-DC converter that is implemented using the Microchip pcc110 chip and a voltage booster using the Microchip pcc210 chip. There is also a V\({}_{\text{CAP}}\) pin that can be connected to an external capacitor of the users' choice or to the capacitors that are already available (C3 of 10 mF or C5 of 50 mF). Furthermore, the user can adjust the S2 switch available on the board to either operate the sensor board or use the harvested power to light an LED, which can be easily seen as the LED blinks, or to indicate the arrival of a packet at the access point. As depicted on Figure 3, after the RF signal is received via the antenna and fed to the harvester RF\({}_{\text{IN}}\) terminal, it is converted to a DC signal via the RF to DC converter circuit, which thereafter can be either stored in the capacitor connected to the pin V\({}_{\text{CAP}}\) (e.g., C3). Obviously, a smaller capacitor will charge much faster than a larger one but can provide power only for a shorter period.