Monday, April 8, 2013

Useless Box

I've been wanting to make a useless box since I saw several postings on Make Magazine's website.  I decided to make two for an auction for a choir that I sing in called the Northwest Chamber Chorus.  I based the design completely off one found on Thingiverse, using the author's .stl files to generate gcode for our Replicator 1.  Thanks so much jamesarm97!!!  The only thing I modified was the arm (dxf file here).  I made one in white and one in black.

Parts List:

This is how to wire the DPDT
switch.  Looking at the bottom.
The wiring that makes this work is fascinating and really clever.  The DPDT reverses the polarity of the battery that the motor sees when the switch is thrown.  In addition, one leg of the circuit has a momentary switch in series with the motor.  The momentary switch is soldered so that it is normally closed, but opens when the switch is pressed.

Picture taken from Pombo on
In the "all off" position shown at right, the momentary switch is pressed so no current flows (top illustration).  When the DPDT switch is thrown, the motor is now in series with the battery and swings the arm counterclockwise (middle illustration).   As soon as the arm begins to move, the momentary switch is now unclicked and so it is shorted.  

When the arm  hits the DPDT switch and pushes it "off", the motor is now in series with the battery but with the opposite polarity, so it begins to rotate in a clockwise direction.  This happens until it contacts the momentary switch, which then opens the circuit.  This loop repeats every time the power button is pressed.

Once all the parts were printed, it was basically just a matter of assembly.  I had to try several designs for the arm until I found one which worked, but it was not very difficult.

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