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
D1 - D3 5 mm blue LED 3
For D1 - D3 5 mm LED bezel 3
M1 Servo motor 1 Dynam B2232
SEN1 - SEN3 Ultrasonic distance sensor 3 HC-SR04
Circuit Board 5 cm x 7 cm perfboard 1
PARTS
LIST
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