The outputs are all independent, but the LEDs are arranged in pairs to halve current consumption. The LEDs are controlled by a MOSFET with a gate normally pulled high, allowing the LEDs to be turned off by setting the MOSFET gate to a low voltage. Turning the LEDs off might be advantageous for limiting power consumption when the sensors are not in use or for varying the effective brightness of the LEDs through PWM control.
The LED current-limiting resistors for 5V operation are arranged in two stages; this allows a simple bypass of one stage to enable operation at 3.3V. The LED current is approximately 20-25mA, making the total board consumption just under 100mA.
Interfacing with the QTR-8A Outputs:
There are several ways you can interface with the QTR-8A outputs:
* Use a microcontroller's analog-to-digital converter (ADC) to measure the voltages.
* Use a comparator with an adjustable threshold to convert each analog voltage into a digital (i.e. black/white) signal that can be read by the digital I/O line of a microcontroller.
* Connect each output directly to a digital I/O line of a microcontroller and rely on its internal comparator.
This last method will work if you are able to get high reflectance from your white surface as depicted in the left image, but will probably fail if you have a lower-reflectance signal profile like the one on the right.