Nuts and Volts - July 2014

would use some T-slot connections that would allow nuts and bolts to hold everything together. I could just imagine the language I would need to use when assembling the thing, though. I am sure the three side pieces full of electronics would not behave while I was trying to align and screw down the top and bottom of the housing.

I remember reading some articles about a laser cutting plywood in such a way to allow it to bend. Google “laser cut plywood bendable” for more information on this technique. I had some 3 mm mahogany and some 3 mm baltic birch plywood, so I gave the process a try. The results with the mahogany were not great. It seems inherently stiffer than the birch. After two or three design attempts, I

had the birch bending like a limp noodle. The box design ended up being a long 39 cm strip that had three bendable joints cut into it so that it could be formed into a three-sided box. The back side of the box comes together in the center and is held tight using a small plywood brace and four nuts and bolts. The only holes that are cut into the long strip (other than 10 small bolt holes) that makes the walls are six holes for the three ultrasonic sensors to poke out of, and two holes for the power input jack and alarm output jack. The HC-SR04 ultrasonic sensors have four very tiny 1.4 mm mounting holes in the corner of the module. I didn’t have any nuts and bolts this small, so I opted to simply use double-sided foam tape to hold them in place. This tape is crazy sticky, so it won’t release unless you are deliberately trying to remove a sensor. I think if I was going to spend a bit more time on the design, I would have built a support bracket that would hold the sensor board from the rear. The front has a crystal oscillator near the top, and the back is full of surface-mount electronics along with a 90 degree angled header, so foam tape would still be required as a spacer. The outer sticky portion could be left covered so it would be easier to work with, and then the sensor would not be able to be pushed in even with lots of force.

It took quite a bit of trial and error to get the perfect radius and triangle size for the upper and lower panels. After the first two or three didn’t quite fit, I changed from plywood to cardboard until it finally came out, matching the curves of the bent corners exactly. The top and bottom

ITEM DESCRIPTION QTY MODEL #/NOTES U1 Five volt regulator 1 LM7805

U2 Arduino Pro Mini 1 Five volt 16 MHz version For U2 24-pin wide DIP socket 1 Optional if Arduino is to be socketed SW1 On/Off switch 1 SW2 Momentary N/O pushbutton 1 C1, C2 47 µF 25 volt capacitor 2 C3 0.1 µF 50 volt capacitor 1 Any voltage rating over 10 volts is fine R1 - R3 1/4 watt 470 ohm resistor 3 R4 1/4 watt 1K ohm resistor 1 Q1 NPN transistor 1 2N4401