Solar Power Measurement System Using ARM Cortex

A solar measurement system built on an ARM Cortex controller. It measures the voltage, current, temperature and light intensity of a solar panel and displays them.

Solar panel and 12 V power adapter / step-down transformer supply the circuit; a regulator gives the controller and sensors a steady 5 V. The LDR light sensor senses whether it is light or dark so lights can switch automatically. The Temperature sensor measures the temperature that the system monitors or controls. The Current sensor measures the current drawn by the load.

It runs at 72 MHz with more memory and hardware peripherals, so control loops, filtering and communication run faster and more precisely than on an 8-bit controller.

An LDR is made of cadmium sulphide. In light its resistance drops to a few hundred ohms, and in darkness it rises to about a megaohm. Used in a voltage divider, it gives a voltage that depends on light.

LM35 gives an analog output of 10 mV per °C, so 250 mV means 25 °C. The controller reads it with the ADC and converts the value to degrees.

It uses the Hall effect: the current through the chip's conductor creates a magnetic field, and the Hall sensor converts it to a proportional voltage, isolated from the main circuit.

Without it the input pin floats and reads random values. The pull-up keeps it HIGH until the button connects it to ground.

The input section senses the real-world quantity, the STM32 board processes it according to the program, and the output section displays the result and drives the actuators. The power supply section feeds every block.

I2C for the display, RTC or motion sensor (only SDA and SCL lines), a single-wire digital protocol for the temperature sensor and analog inputs read by the ADC for the analog sensors. Each protocol was chosen because the module supports it and it uses the fewest pins.

The controller and most sensors work at 5 V (ESP boards and some modules at 3.3 V). The battery powers the board, and a regulator steps it down. Motors and relays get their own supply line so they do not reset the controller.

Using P = V × I for each load, adding a margin of 20 to 25%. The solar panel is sized from the daily energy need divided by about 5 peak sun hours in India. Battery capacity is the energy needed per day divided by the battery voltage and allowable depth of discharge.

Accuracy is how close a reading is to the true value. Precision is how close repeated readings are to each other. A sensor can be precise but inaccurate if it is not calibrated.