We will be using an infrared LED - Photo-transistor pair to read the line. The sensor unit is depicted in the schematic. The plan is to have eight of these sensor units next to each other.
When taking sensor readings, a microcontroller will source the Enable pin and read the output of the ADC connected to the Vout line.
In a simple experiment we determined parameters such as minimum analog to digital conversion time and range of output through varying R2.
R2 was set to 27k Ohms, 47k Ohms, 270k Ohms, and 470k Ohms. The settling time and range of Vout was noted. The two values were measured with a white piece of paper held approx. 1 cm from the sensor unit. R1 was set to 150 Ohms. Spacing between sensors were not considered. Enable was fed a 200us 50% duty cycle square wave.
We decided to go with R2 = 470k Ohms. If we set +Vref to an appropriately low value the output voltages will be discernible. We suspect that Ts is a result of the capacitance of the diode, with R2 = 470k Ohms we need to keep the ADC conversion time to 100us or more.
The following values were 'eye balled' approximations.
27k Ohms: Ts ~ 50us, Vrange ~ 15mV
47k Ohms: Ts ~ 50us, Vrange ~ 20mV
27k Ohm and 47k Ohm
270k Ohm and 470k Ohm
