The EOPD or Electro-Optical Proximity Detector uses an internal light source to detect the presence of a target or determine changes in distance to a target. By generating its own light source, the EOPD is able to filter out all external light signals so as a robot moves from bright areas to dimly lit areas or through shadows, the EOPD automatically compensates and only returns a signal based on its own light source. Targets can be accurately detected over distances up to about 8 inches (20 cm), depending on the target size, shape and reflective qualities.
The HiTechnic EOPD Sensor is an Electro-Optical Proximity Detector. Electro-Optical? This means an electronic device which uses visible light as part of its operation.
The HiTechnic EOPD is similar to a standard light sensor except that it uses pulsed light to effectively eliminate the interference of ambient or background light from the reading. By using pulsed light, the brightness during the pulse can be relatively bright but remain safe because the average power output is very low. The sensor reads the difference between the light measurement at the detector before the pulse is emitted and the light measurement during the pulse. It then subtracts the difference so the reading is a direct measurement of the reflected light energy after removing any dependence on ambient light.
The EOPD sensor has two modes of operation, x1 sensitivity and x4 sensitivity. In x4 sensitivity mode, the HiTechnic EOPD can easily detect the red ball which comes with the LEGO MINDSTORMS NXT set at a range of at least 15 cm (approx. 6 inches). Since the sensor can be overloaded by close white targets, the x1 sensitivity mode can be used for detecting the position of LEGO pieces, such as a small white part attached to a black beam, used as a mechanism home detection device.
The HiTechnic EOPD can be read at up to 300 samples per second. Internally, the EOPD sensor actually runs at between 350 to 400 samples per second to ensure there is always a new reading for the NXT at 300 samples per second.