A Microwave Oven Helps Rotate Indoor OTA Antenna

Pressing on in my quest to be cable free for O.T.A. television viewing, I have found that my antenna must be rotated to 3 different points for effective reception of all the channels I can receive. The entire range is less than 180 degrees, so I was desirous of fabricating a controllable rotate platform that could be operated with my T.V.'s own remote, and a salvaged microwave oven turntable motor turned out to answer that need perfectly.

Step 1: Safety First

Line voltage may be present during construction, act accordingly.

Step 2: Turntable Motor Theory

It seems one of life's mysteries is just why and how a microwave oven's turntable motor changes direction; is it because it is designed to do so to promote even cooking, is it to prevent harm to the oven, or is it a closely guarded secret known only to the president and his advisers? Actually the random rotation is a beneficial inbred side effect of the motor design, not an engineering specification. The typical turntable motor is a synchronous type, about as inexpensive a motor as you can build and still have it rotate. Being inherent in their design, they reverse their direction of rotation if a load greater than their output torque is encountered or when stopped and started, and this effect is exploited to switch turntable direction in a non- destructive manner. As it so happens, this utility is also a very handy feature for an antenna rotators' platform too.

Step 3: Survey the Application

The motors I have encountered during microwave oven scrapping have been of two types: 21- 24 volts a.c. and 120 volts a.c., typically between 2.5 and 3 watts power. To determine if it would provide enough torque to drive my antenna platform, I constructed a test bed and loaded it beyond the antenna's weight. Both types seemed capable, but I chose to use the 120 version for simplicity's sake as the low voltage type would have required a step down transformer.

Step 4: Construct a Base for the Mechanics- Stock Preparation

I had a good supply of bed frame wood slats that were of a useable width; cutting overlong lengths for an octagon shape gave me the 8 pieces of clear stock to build with. Kerfing for the support panel came next, two passes gave me the stopped depth and width for it's insertion. After cutting a 22-1/2 degree bevel on each end to proper length, I constructed a fixture to hold the sections for slotting with a biscuit joiner. It is simply a ramp with a side stop, again at 22-1/2 degrees of bevel so that reference for the slot is on the outside face of the joint, exactly where I want it to be. The resultant joint will be easy to clamp without slippage, and be immensely strong as well.

Step 5: Construct a Base for the Mechanics- Assembly

I did a dry fit to ensure an easy assembly, and also used two overlapping scraps as a story stick to get the final inside dimension for the support panel. After it's fabrication I drilled the motor mounting holes before assembling it into the band- clamped and glued frame, and finally dead-weighted the assembly to ensure flatness while it dried. A black aerosol spray finish was applied for aesthetics.

Step 6: The Rotate Table

A round was cut from scrap ply to nest within the perimeter of the wheeled plate roller ring that was also salvaged with the motor. The drive hub mounting was machined in the center before it was painted black and hot glued to the ring.

Step 7: Setting the Limits

Two plastic squares 180 degrees apart established the sweep arc that would reverse the rotate platform, which had a corresponding pin that would contact the stops during operation. This arrangement would bring about the antenna returning to “home” position or any point in between.

Step 8: Electronics and Electrical

Given that limit stops were used to control the extreme positions, I wanted a means to start/ stop on command, and a “learning” infrared module was the simple answer. It was a latching type, which meant it would conduct power until the next signal deactivated or switched it off. It has the ability to read a signal from any remote's unused button, store and use it for processing, and thus freed me from having to use a separate unit for this task. It's IR receiver is edge located, so I mounted the device on a Sponson, then fabricated and heat- bent a clear plastic cover so as to celebrate it's presence and maximize operating angles. The module required 5 volts d.c., so I used a wall wart transformer of about 7 volts and conditioned it through a 7805 regulator before hooking it up. The 120 volt input was divided and made common to one side of the motor and routed through the module's relay on the common/ normally open contacts, then returning to the motor's other pole.

Step 9: Parting Thoughts

Now, from the comfort of my bed, I can swing my antenna around to my favorite stations while surfing for something to watch, or I could just get off my backside and go into the shop and make another Instructable.