Projectile speed sensor

[Mike] built a sensor rig to measure projectile speed. The setup uses a tunnel with two sensors in it. Each consists of a laser diode on one side focused on a photodiode in the other. The two are monitored by an op amp and measured by an ATmega128 microcontroller. When the beams are broken the elapsed time between the two events is measured in order to calculate speed. There is a setting to adjust the calibration for a range of speeds, which came in quite handy as [Mike] initially tested the device with rubber bands before moving on to a pellet gun and then a rifle.

It seems like he’s tempting fate by shooting a target just a few inches below his exposed circuitry but his marksmanship prevailed. We’ve seen bullet speed detectors in the past, used just for the delight of seeing how fast the projectile is moving, and also to capture an impact at just the right instant.

CNC hot wire cutter from scanners


[Raul] built a CNC hot wire cutter that he uses for cutting shapes out of foam. His device uses two flat bed scanners to provide two planes of motion. One scanner arm has the foam mounted on it and provides the Y-axis movement. The other scanner has the hot wire mounted on it and provides the X-axis movement. The cutting wire is mounted on a flexed bow made from heavy gauge coat hanger wire.

He tapped into the logic board of one scanner to gain access to the motor movements. The other is connected through a couple of H-bridges. Both are controlled by an Atmel AVR ATmega128 which in turn takes its commands from a connection with a computer printer port. A python program uses vector graphic files in SVG format and traces the outline for cutting.

We’ve got a video of this in action after the break. At our request, [Raul] took some time to post a set of pictures and make comments on them. Thanks for the hard work and great job! Continue reading “CNC hot wire cutter from scanners”