Introduction: A Safer Polarity Reversal Switch

This accessory project may seem too basic at first glance, but it really incorporates some features that will benefit the experimenter or tinkerer. First and most important is the choice of switch action. This uses a double- pole double- throw center- off device, which means that before a load is switched it is put in a neutral status before becoming energized once again. Many similar looking projects use a simpler D.P.D.T. switch which may well be of a make- before- break action with no neutral safeguard at all, possibly causing circuit damage. Connection to a circuit is made very quick and easy by the use of 4 spring loaded terminals that require no tools for hookup. This combination is mounted on a framed panel for durability and easy handling, and can take considerable abuse yet still reliably perform it's job.

Step 1: The Construction

A typical sub-miniature switch is employed, and in this build a salvaged speaker terminal bank was also used. Wiring is a straightforward crossover type as is found in plenty of sites on the internet, I used this one here as a guide:

http://rollertrol.com/images/schematics/switches/DC-motor-reversing-switch-schematic-wiring-diagram-285x275.jpg

A scrap piece of ABS plastic and wood completes the kit. I applied a boiled linseed oil finish just to help repel hand goo, but a simple wax job or no finish at all will suffice too. When finished it's hardly bigger than a typical computer mouse and kind of feels the same too.

Step 2: The Results

In this case it was interesting to note the slight value changes as the gearmotor was first driven right, then left, indicating slightly more internal friction in that direction.

Step 3: Parting Thoughts

Back when I was a young junior engineering technician, a prototype electronic project we were working on was giving us fits; as we switched modes it inexplicably threw out weird results, and we weren't making much progress because of the malfunction. It just happened that I slowly moved the scaling range switch just slow enough that it induced a break before make connection and thus we found the culprit: a make- before break switch was used early on, causing the erratic behavior. I was a darling with the project manager for oh, about an hour before I was once again the shop lab rat, but it taught me to take nothing for granted in my projects from then on, and so I hope this builds saves someone out there a bit of grief.

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