Electronic – Vehicle buttocks detection

Consider using a Passive Infrared sensor for the purpose:

_{Images from eBay listings.}

You can find a prebuilt module like the one above on sites like eBay, for as little as US$1.50 including international shipping.

The module linked above has sensitivity adjustment, so it can be tuned to react to motion within a specific range. 10 cm may be too small a distance, beyond the lowest sensitivity setting, as per an experiment I just did. The minimum sensitivity setting seems to represent about 90 cm.

False positives are very low at the low sensitivity you would operate this module at.

Using such PIR modules is very simple: They need a DC supply of (typically) 3.3 to 5 Volts, very low current required, and the signal line goes high when motion is detected, falling back to low after an adjustable period (the second preset on the module).

Coding for it is simple too: There are several open source examples for using these modules with Arduino, MSP430 Launchpad, and various PIC, ATmega and ATtiny microcontrollers .

Trouble with ultrasound is that it doesn't stay in the box - you still have reflections from background. So for objects that are more than a few tens of cm from the sensor you need to do signal processing to find the target. In which case you might as well pick a solution using visible light to make it easier to debug. Computer vision tracking of moving objects against static background is a solved problem; see OpenCV for a free implementation. It works best against neutral backgrounds, but doesn't require that. Two cameras gives you stereo imaging and three positions.

If you just want ball speed along an axis you can get off-the-shelf radar guns for this purpose. Building your own radar system is an extremely difficult prospect.

Edit: 50m/s should have been in the original spec, that's quite fast. Ultrasonic usually works by sending out "chirps" and timing the response. Speed of sound is ~340m/s. So it takes 35ms for sound to traverse the 12m box. During that time a 50m/s target has moved 1.75m. You need to wait until your first pulse has gone out and back before sending the next, so pulse interval is at least 70ms. So you can only get one measurement every 3.5m.

You should also spec the latency: are you trying to interact with the moving object, or is it sufficient to work out where it was after is has passed? Do you care about the exact trajectory, or just speed of passing through the box (which can be found with doppler methods with ultrasound or radar).