Friday, October 9, 2009

Windmeter / Anemometer Circuit

The circuit for the Windmeter is self powered. A 12v solar cell array provides power to the circuit and charges the battery during the day. At night the 12v battery is used to power the circuit. Operation for many days without sunlight is required, so the circuit operates at 3.3v and uses a high efficiency switching power supply. The circuit draws 1.35 mA at 12v. The circuit should operate for over 50 days without sunlight from a fully charged battery. The solar cell will take 3 days to fully charge a dead battery. The circuit should never loose power.

Power enters the circuit through L7 and D1 to prevent reverse battery connection. C10 is used as a input filtering capacitor while C11 is used as a input de-coupling capacitor. U3, L1, D2, and C12 form the switching power supply. R3, R6 and R7 form the low battery power down.

A magnet and Hall effect sensor are used to measure RPM of the rotating cups. One pulse per revolution is produced. The sensor is on the back of the PCB and the magnet is attached to the rotating shaft. Microcontroller U2 is used to calculate wind speed from the pulses produced by the Hall sensor. The micro also records the varying speeds and provides a interface to a Laptop / computer. U1, a low power RS232 1 channel driver, is used to join the PC serial port and the micro's UART. C2 - C5 are used by the RS232 Driver IC to form charge pumps. C1, C6, C7, C13 are used as bypass / de-coupling capacitors throughout the circuit. C8, C9 are used to help start oscillation. The crystal of 4.096 Mhz was selected for easy and accurate time measurement. R1, R2 are used to form a voltage divider which feeds ADC channel 0 to measure Supply voltage. Q1 and Q2 control the power applied to R1 and R2 to improve power consumption.

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