Input Devices

For this week the intent was to read a signal from a sensor or input device using a microcontroller platform. In a previous assignment the PSoC platform had been used so we decided to use this once again. The input device selected was first a simple potentiometer (10K Ohms) set up as a linear slider and later the infrared distance sensor GP2D12 from Sharp. The datasheet can be found in www.sharpsma.com.

PSoC and the Linear Slider

The PSoC platform has the advantage of including analog configurable hardware blocks that can be used as analog-to-digital converters (ADC) that are very useful in reading an analog signal from one of its pins.

Although the programming environment is based on C and includes a wiring interface in which you drag and drop predefined modules it was necessary to configure the input pin as an analog input, set the clock signal for the conversion rate of the ADC, set a gain block that act as a buffer to have better impedance at the entrance of the signal and have a cleaner signal. All those configurations were done in the PSoC Designer interface with the guidance of professor John Jairo Cabrera here at UAO.

The C code is fairly simple, it consists of declaring variables to store the reading (ADCData) and converting it into a floating value representative of the voltage (voltios) of the signal and later the distance according to the sensor. The floating value requires conversion into a string to be able to output the value in the LCD.

Part of the processing necessary to display the value is first to understand the equivalent of the reported value, in this case the ADC has 12-bits and the entire range cover signals from 0-5V, therefore the convertion to volts implies multiplying by 5 and dividing by 4096 which corresponds to the scaling factor 0.00122. This was tested with the potentiometer / linear slider.

The LEDs underneath the LCD blink according to the routine defined in a previous homework. Notice also the value reported with respect to the position of the slider.

Infrared Distance Sensor

Once the data was being displayed correctly on the LCD, the work was to correlate that electric signal with the distance of the object. According to the datasheet referenced before, the sensor responds aproximately linear to the inverse of the distance (iD) in a range from 9-80 cm. The equation extracted from the graph was iD=0.04167*V-0.0075, and then distance L=(1/iD)-0.42. This was coded in the program but the system return double the distance, a factor of 2 was incorporated and the system performed as expected.