The Pololu QTR-8A and QTR-8RC reflectance sensor arrays are intended as line sensors, but they can be used as a general-purpose proximity or reflectance sensors. Each module is a convenient carrier for eight IR emitter and receiver (phototransistor) pairs evenly spaced at intervals of 0.375" (9.525 mm). 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 5 V operation are arranged in two stages; this allows a simple bypass of one stage to enable operation at 3.3 V. The LED current is approximately 20-25 mA, making the total board consumption just under 100 mA. The schematic diagram of the module is shown below:
Specifications for the QTR-8x Reflectance Sensor Array
- Dimensions: 2.95" x 0.5"
- Operating voltage: 3.3-5.0 V
- Supply current: 100 mA
- Output format for the QTR-8A: 8 analog voltages ranging from 0 V to supplied voltage
- Output format for the QTR-8RC: 8 digital I/O-compatible signals that can be read as a timed high pulse
- Optimal sensing distance: 0.125" (3 mm)
- Maximum recommended sensing distance for the QTR-8A: 0.25" (6 mm)
- Maximum recommended sensing distance for the QTR-8RC: 0.375" (9.5 mm)
- Weight without header pins: 0.11 oz (3.1 g)