Remote Control Switch

Over this summer I have been to some decent garage sales, although my subdivision’s garage sale has been moved to September (when I will be about 500 miles away at Michigan Tech).  One of the things I usually think about when deciding to purchase items is ‘how does it work?’.  This is probably pretty common among makers and hardware hackers, but it has always been a sticking point for me because I rarely want to use a device for it’s intended purpose.  One of the things that caught my eye was an old radio shack remote controlled coax A – B switch.  In my experience consumer electronics from this era (don’t get all pedantic on me, I can’t tell you a year, but in my experience things from a certain era have similar external properties, construction techniques, and internal properties) are constructed internally in a nice, easy to understand manner that operate how I would expect.  For example They tend to do things like use a linear voltage regulator to step the voltage down to what the chips require, rather than a convoluted set of diodes in series and resistors that are carefully selected to allow the exact voltage and amperage that the circuit requires.

Once open I found what I had assumed there to be, a voltage regulator, a couple of ICs whose datasheets were readily available, and a relay that is either switched on or let relax to form the two switch positions.  An interesting thing I found is that the reason this device is rated to work at 12 volts is that the relay is actuated by a transistor that directly switches the unregulated input on and off.  If you were to replace the power supply with one of any voltage from 7 to whatever voltage the 7805 can take you could replace the relay to match that (assuming the transistor that switched it could take it as well).  While I was trying to pin out the existing relay I eventually gave up and replaced it with a 12 volt relay I already had on hand.

I then ran that to a speaker cable spring clip thing that came off of an old amp some time ago, thus making it A Com Com B.  I set it up so when the light for A is on the red connector on the A side of the device was connected to common and vice versa.  I have no normally open and normally closed, I have A, B, and common.

This design could do with a latching relay so the coil does not have to be drawing power the entire time one of the two settings is actuated.  That, however is another project I would like to try which is a three way switch (or an n-way switch) that is just a button that switches the current position of the switch regardless of the current position.

The pictures that would allow you to build the entire device from scratch (re-construct the schematic from the board layout) are available at this link:


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