Created: Aug 17, 2015
No description available.
A windshield wiper is a device found in vehicles that removes rain drops or dirt from a windscreen or windshield. It is made up of a metal arm and a long rubber blade that is attached to the metal arm. An electric motor is attached to the arm of the wiper to make the wiper swing back and forth over the windshield. Usually the speed of the motor is controlled by a manual rotary switch located near at the car steering wheel. Sometimes, when the rain is so heavy, the driver is having a hard time driving and adjusting the speed of the windshield wiper simultaneously. In this reference design, the wiper speed is being adjusted automatically by an electronic circuit with respect to the condition of the rain.
The whole circuit is composed of a rain sensor, a comparator, a microcontroller and a motor driver. The rain sensor used in this circuit is a PCB with tracks arranged in a particular position with a specific dimension and length acting as a resistor. Initially, the rain sensor acts as an open resistor if there is no rain drops passing on its surface. The resistance of the rain sensor depends on the PCB copper resistivity, length and area. When the speed of the rain drops passing on the surface of the sensor is slow, it only shorts small number of PCB tracks at a time which in result, the PCB will have a longer length and smaller area resulting into a larger resistance from an initial open or infinite resistance. When there is a heavy rain and the speed of the rain drops passing on the sensor’s surface is very fast, it shorts almost all the PCB tracks of the sensor which results into a large area and a short length resulting into a small resistance.
The speed of the motor depends on the PWM signal output coming from the microcontroller. These PWM signals decide the duty cycle of the motor driver. The duty cycle ratio of the PWM signal coming from the microcontroller is dependent on the voltage read by the analog-to-digital converter (ADC) PC0 pin of the microcontroller across the terminals of the rain sensor. The larger the resistance of the rain sensor, the bigger the voltage it gets from the 5V supply. If the resistance of the sensor is infinite, the ADC reads a voltage approximately 5V and doesn’t give a signal to the motor driver. If the resistance of the sensor decreases as the speed of the rain increases, the microcontroller ADC will read a decreasing amount of voltage drop across the terminal of the sensor and increases the duty cycle ratio of the PWM signal to rotate the motor faster. The motor is driven by two PE014005 general purpose relays of TE Connectivity. The use of two relays enables the motor driver to rotate the motor in forward and reverse direction. The purpose of the two transistors is to energize the coil of the relays since the microcontroller pin can only supply small amount of current not enough to energize the coil of the relay directly.