Introduction

This project will present the possibility of designing a simple self-powered solar tracking sensor circuit using a Dialog Semiconductor SLG88103 operational amplifier. The complete circuitry is self-powered, since it uses only the power obtained from the photodetectors. There is no need for an additional external power supply. The photodetectors are arranged so that they can sense the direction of solar irradiance. Based upon the signal processing circuitry at the outputs, two highly sensitive voltage signals are generated. These signals correspond to the pitch and roll angles of the sun with respect to the photodiodes. The circuitry has an inherent automatic gain control. Therefore, the output signals are only proportional to these angles and not to the solar irradiation level.

The Sensor Circuitry

The schematic of the proposed simple self-powered solar tracking sensor circuitry is presented in Figure 1. Processing the signals from the photodetectors requires only two operational amplifiers and several resistors. Additionally, as the photodetectors (photodiodes PDYU1, PDYU2, PDYD1, PDYD2, PDXR1, PDXR2, PDXL1, and PDXL2) work in the photovoltaic mode the generated power is significant enough to power the op-amps. Both the X and Y direction use four photodiodes (PDYU1, PDYU2, PDYD1, and PDYD2) connected in series to power on the operational amplifiers OP1 and OP2. The corresponding voltages, obtained at the photodiodes when in photovoltaic mode, are given by:

where 𝑉𝑖 is the corresponding i-th (i = YU1, YU2, YD1, YD2, XR1, XR2, XL1, and XL2) photodiode voltage, 𝑉𝑇 is the thermal voltage given by 𝑉𝑇 = 𝑘B𝑇⁄𝑞 where 𝑘B = 1.38×10-23 J/K is the Boltzmann constant, 𝑇 is the absolute temperature, 𝑞 = 1.602×10-19 C is the elementary charge, ℜ is the photodiode responsivity, 𝑃𝑖 is the i-th photodiode captured optical power, and 𝐼S is the photodiode saturation current.

To keep photodiodes in the photovoltaic mode they must be connected to the high impedance nodes, thus requiring high values of the resistance RL. The corresponding photodiode-captured optical power depends upon the shadow position within the enclosure, i.e. it depends on the shadow distribution over the active photodiode surface. This is presented in Figure 1. The actively illuminated area on the photodiodes’ surface depends upon the pitch and roll angles of the sun with respect to the photodiodes, as presented in Figure 2. This is, naturally, valid only for the photodiodes that are shadowed by the enclosure. For example, if the Sun illuminates the sensor from the first quadrant, shown in Figure 2, only photodiodes PDYU2 and PDXR2 will be in the shadow and their corresponding illuminated area will be:

where small pitch ξ and roll ψ angles were assumed (ξ, ψ ≪ 1) thus giving, in the first approximation, the linear dependence of the illuminated photodiode area with respect to the corresponding angles, where A is the area of the photodiode active surface, K is the positive proportionality constant that depends on the sensor geometry, and where A ≫ Kξ, Kψ is also valid.

The corresponding photodiode voltages 𝑉𝑖 according to equation (1) are given by:

where E is the solar irradiance. The output voltages VX and VY are given as:

where RF is the feedback resistor resistance. Equations (2), (3), and (4) give:

In the first approximation the output voltage signals VX and VY are directly proportional to the pitch and roll angles with sensor sensitivity S. As the output signals are independent of the solar irradiance the circuitry has an inherent automatic gain control.

Realization with SLG88103 Operational Amplifiers

The realization of a simple self-powered solar tracking sensor circuitry will be based on the extremely low-power characteristics of the SLG88103 operational amplifiers [4]....

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