# Cardboard and Paper Gun Shows Off Clever Construction

This project by [blackfish] shows off a cardboard lookalike of an MP5 that loads from a working magazine, has a functional charging handle, and flings paper projectiles with at least enough accuracy to plink some red party cups. It was made entirely from corrugated cardboard, paper, rubber bands, and toothpicks.

In the video (embedded below) you can see some clever construction techniques. For example, using a cyanoacrylate adhesive to saturate areas of wood, cardboard, or paper to give them added strength and rigidity. The video is well-edited and worth a watch to see the whole process; [blackfish] even uses a peeled piece of cardboard — exposing the corrugated part — as a set of detents (6:56) to retain the magazine.

# Op Amps Combine Into Virtual Ball In A Box

What happens when you throw a ball into a box? In the real world, the answer is simple – the ball bounces between the walls and the floor until it eventually loses energy and comes to rest. What happens when you throw a virtual ball into a virtual box? Sounds like something you might need a program running on a digital computer to answer. But an analog computer built with a handful of op amps can model a ball in a box pretty handily too.

OK, it takes quite a large handful of op amps and considerable cleverness to model everything in this simple system, as [Glen Kleinschmidt] discovered when he undertook to recreate a four-decade-old demonstration project from AEG-Telefunken. Plotting the position of an object bouncing around inside the virtual box is the job of two separate circuits, one to determine the Y-coordinate and bouncing off the floor, and one to calculate the X-coordinate relative to the walls. Those circuits are superimposed by a high-frequency sine-cosine pair generator that creates the ball, and everything is mixed together into separate outputs for an X-Y oscilloscope to display. The resulting simulation is pretty convincing, with the added bonus of the slowly decaying clicks of the relay used to change the X direction each time a wall is hit.

There’s not much practical use, but it’s instructional for sure, and an impressive display of what’s possible with op amps. For more on using op amps as analog computers, check out [Bil Herd]’s “Computing with Analog” article.

# Stretchable Traces for Flexible Circuits

Electronic components are getting smaller and smaller, but the printed circuit boards we usually mount them on haven’t changed much. Stiff glass-epoxy boards can be a limiting factor in designing for environments where flexibility is a requirement, but a new elastic substrate with stretchable conductive traces might be a game changer for wearable and even implantable circuits.

Researchers at the Center for Neuroprosthetics at the École Polytechnique Fédérale de Lausanne are in the business of engineering the interface between electronics and the human nervous system, and so have to overcome the mismatch between the hardware and wetware. To that end, [Prof. Dr. Stéphanie P. Lacour]’s lab has developed a way to apply a liquid metal to polymer substrates, with the resulting traces capable of stretching up to four times in length without cracking or breaking. They describe the metal as a partially liquid and partially solid alloy of gallium, with a gold added to prevent the alloy from beading up on the substrate. The applications are endless – wearable circuits, sensors, implantable electrostimulation, even microactuators.

Looks like progress with flexibles is starting to pick up, what with the conductive silicone and flexible phototransistors we’ve covered recently. We’re excited to see where work like this leads.

# Gently stroke this drum

The silent drum is played with your hands. It acts as a midi device by analyzing the movement of the rubbery black drum head. As you can see in the photo, one side of the body is clear and the other is white. A light shines up into it to boost the contrast and a camera picks up the black head as it moves past the white side of the shell. [Jaime Oliver] has provided an interesting look at the analysis method used with this instrument and there’s also a system of notating a composition for future performance. See and hear it played in the demo after the break.