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I'm looking for a very simple circuit, but for the life of me (and my old age!) I cannot find something that I'm able to make work for this project and would like some input from you folks.

I'm looking to build an antenna rotator direction circuit, one that is able to tell me which way the antenna is pointing (for an amateur radio antenna/mast). I know this should be easy enough to do with a circular pad at the base of the rotator, with a wiper arm and multiple surface mount resistors in series that will light up a ring of LEDs to show the rotator's position. I do not want to use any sort of arduino appliance, simply because of the RF involved at the antenna's base, just trying to keepthis as simple as possible.

So, what I'd like to know is what type of components are necessary to light up either one led at a time depending on resistance, or in succession as resistance is added. I'd like to have at least 8 segments (N-S-E-W, as well as NE-SE-SW-NW), or 16 if possible without a lot of extra components and/or wiring for better detail of the position. To add to a bit of complication to this circuit, I'll be separating the two parts by 50-100 feet using something like Cat5 cable between the two "units". I'm hoping that, at the mast, I can just have two wires running back to a box that shows the position of the arm that's contacting the wiper pad by "reading" the resistance it sees. Obviously, it will have to be relatively large resistance separation between the segments to keep the extra Cat5 cable from throwing off the resistance readings.

A quick image of what I'm thinking of:

layout

What type of circuit would be simple, using only a few items and be low powered, up to 12 V DC (which is what the rotator is using now) and be capable of utilizing only a few wires from one point to the other.

I'm also open to reading voltage too if that makes it a bit easier to design the circuit, and at the control box I can have a regulated power supply running to the mast's wiper pad.

For those interested, this is for a Mastwerks 10 meter rotatable mast, and I've already built a wireless motor system that works quite well, but one has to have view of the antenna to see which way its pointed! Not a huge issue during the day and out in the field, but if the antenna is 100' away from the operating point and blocked by trees or other things, the operator has to walk out to point the antenna, then walk back and see if that's the correct direction.

There is a rotator system coming out some day by the manufacturer, but for now this would be a great (and economical) alternative to having a commercial system for the mast.

Thanks! Chris

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    \$\begingroup\$ More like 5 wires but a synchro-T- to synchro-R would be nice and robust - but they've gotten pretty $$$ these days (also need a 400Hz supply) \$\endgroup\$ Jul 9, 2023 at 19:26
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    \$\begingroup\$ Yes. I only knew them as selsyns. I guess they are called synchro-T and -R now? The ones I have were used for the WW II radar mast used with the 1944 200-mile range radar system I acquired many years ago. It's the way I'd go look, if the price isn't insane. \$\endgroup\$ Jul 9, 2023 at 20:19
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    \$\begingroup\$ @periblepsis yes, synchro-transmitter and synchro-receiver. For anyone curious, you just connect them together and supply 400HZ low voltage single phase AC and one shaft follows the other in position with no electronics at all. In the case of an antenna mast, the risk of lightning strikes should probably be considered. \$\endgroup\$ Jul 9, 2023 at 21:21
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    \$\begingroup\$ @SpehroPefhany Of all the options out there the only one that really fits this situation best for robustness in this purpose (nearby presence of high power RF, avoidance of 'arduino-like appliances', Outdoor, etc) it is a selsyn pair. Used onboard ships in demanding and critical applications not unlike this. Everything else pales in comparison. \$\endgroup\$ Jul 10, 2023 at 3:00
  • \$\begingroup\$ @SpehroPefhany I found this paper that seems really nicely written (to my eye.) I know that the number of Hams -- serious ones spending real money -- are a small market already. And those with rotating antenna masts... well, probably a small percent of that already small percent. Commercial consumers can buy the expensive spread or use phased arrays. But I can't imagine how anyone would want to sell into such a small non-commercial Ham radio market. Chris is a unicorn of sorts I fear. \$\endgroup\$ Jul 10, 2023 at 4:46

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If I understand your question correctly, what you have at the mast is a stepped voltage divider - the further the rotation, the higher the voltage in identical, fixed increments. To turn this into something to switch LEDs on and off requires an A/D - analog to digital converter. It doesn't have to be a true A/D IC. It could be a string of comparators, each driving one LED. Two quad comparators (LM339) gets you 8 LEDs. Lotsa resistors, but they are cheap. No calibration needed for 8 positions with 5% resistors. Change to 1% resistors for 16 positions.

It can be done with only two wires running out to the mast, but with three wires all of the resistors out at the mast would be the same value. There is a way to so that with only 2 wires, but now you need a small constant-current circuit to drive the mast circuit. What you are building is basically a digital ohmmeter with a coarse analog display. Not difficult at all, and lotsa ways to get there.

Now for the fun part. It just so happens that National Semiconductor has been building such a chip since the 70's:

https://www.ti.com/product/LM3914?keyMatch=LM3914

This is a very well-known part, and there are tons of modules on ebay with the chip and LEDs. If you want to build your own circular display, then just the chip will work. The circuits are in the datasheet. Nice thing about this part is that you can change it from traveling dot mode to bar-graph mode (all lights below the current position stay on) by switching one pin.

Given the long wire run, low-value resistors out at the mast will improve noise immunity, at the cost of operating current. Eight 470 ohm resistors would draw 32 mA from the 12 V source. Is that a problem?

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I would build a 10mA current source and supply your resistive ring with it (higher current is better because of noise).
Once the resistance increase the voltage on it increase also (linearly).
(There are also more precise versions of current sources)

Regarding the indicator, simplest option is just put an analog voltmeter parallel to resistive ring and mark the south-north directions on it. Also they sell and different voltmeters, battery level indicators which have A/D inside on the inputs.

schematic

simulate this circuit – Schematic created using CircuitLab

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Rather than getting into ADCs, DACs and voltage comparators, I would use the segmented slip ring as a simple switch to supply power to the individual LEDs depending on the switch position - that may require a few more wires between the mast base and the indicator display, but it avoids any electronics.

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Consider this:

  1. Use a CAT5/6 cable between the mast and the receiver.
  2. At the mast afix 4 magnetically operated reed switches vertically at a reference position.
  3. On the rotating mast afix 1 to 4 magnets vertically so that they rotate past the reed switches.

The magnets operate the reed switches in a binary fashion. Position 1 would have 1 magnet operating the bottom switch. Position 2 would have 1 magnet operating the second switch. Position 3 would have a 2 magnets operating the 1st and 2nd switch and so on.

Connect one pair of the CAT 5 cable to each switch.

This would allow 15 preprogrammed positions. A binary zero would indicate the antenna is between two positions.

At the receiver, the binary number is easily displayed using a binary to 7 segment display or other method.

There are other creative ways to use magnets and switches. This is not the only one.

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The old and gold LM3914 is an LED bar/dot graph display driver, which lights an LED (or group of LEDs) depending on the level of some input potential compared to a user-definable range. It is usually employed in audio level meters, but it's good for any application that requires some kind of level or position indication using LEDs.

It has 10 LED outputs, but you can use only 8 of them, as long as you define the lower and upper "reference" potentials appropriately, and you can even join two such units to indicate up to 20 discrete levels. In any case, you really need to read and understand the datasheet to get the full picture.

Your "position" sensor idea is already compatible with such a device. It can be built to provide some position-dependent potential, that the LM3914 senses and uses to select a single LED to illuminate. In its simplest form, the circuit might look like this:

schematic

simulate this circuit – Schematic created using CircuitLab

This should work with the resistance values I've shown, but R11 (and perhaps R8) will need adjustment to make sure the first and last LEDs light at the correct rotary selector position.

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