I need a sensor to measure motion of small particlesSo the simplest setup would be a sensor that just sits there and gives a HIGH output if it detected motion
Let's first clarify that you actually mean motion, or do you mean presence?If a sensor like ultrasonic detects no object present, then a moment later does detect an object in range, it detected its presence not the actual motion. The motion is implied since one moment the object wasn't there, and the next moment it was, so it must have moved. But the sensor didn't see the motion: it looked and there was nothing; looked again and there was something.To detect actual motion with one sensor is different from sensing presence.....
How fast do the objects move? If they take more than 1/10th of a second to pass the visual field, then the obviouse answer is a raspberrypi + camera (you can actually get up to ~ 400 Hz framerate from the rpi camera, at least for thw first ~ 100 lines)
I need a sensor to measure motion of small particles to a close distance.