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Our pals at Design News have a new "Gadget Freak" on building your own velocity monitor for paintballs - "Looking to paint the town red (or his buddies at any rate), Nathan Loden devised this simple device to detect if the paintballs are traveling at a safe speed (less than 300 ft/sec). Consisting of sensor circuitry and a microcontroller with a three digit, seven segment display driver, it detects the presence or absence of a ball by measuring the amount of IR energy striking the sensor, causing a voltage swing detectable by the micro. A timer is set to overload if the number of instruction cycles exceeds a limit equal to 300 ft/sec, indicating safe shooting." - Link.