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### Topic: Using only Two of three leads from a potentimiter (Read 1 time)previous topic - next topic

#### vstubbs

##### Jun 18, 2012, 06:02 pmLast Edit: Jun 18, 2012, 06:04 pm by vstubbs Reason: 1
So this may sound like a stupid question, but this a some what of an unusual need...

Ponder if you will, a potentiometer, mounted as the only sensor 35 feet on the top of a antenna tower. Only two of the three leads measure any resistance. The resistance between the two working leads is 240 ohms and 450 ohms, between these to values the antenna rotates 405 degrees. My question as I am new to programming the arduino, dose any one have an idea where to start? Please some help would be great.

#### RIDDICK

#1
##### Jun 18, 2012, 07:09 pm
do u want to measure the resitance between those 2 leads of the potentiometer?
if yes: just use a known fixed resistor (for example: 10kOhm) and put it in series between GND and +5V... then u can measure the voltage in the middle: Ux=5V*x/(10kOhm+x)...
GND----(1)potentiometer(2)----10kOhm----+5V

#### Docedison

#2
##### Jun 19, 2012, 12:12 am
Is that cable shielded? and what kind of antenna is mounted on the tower? you are placing a 35 foot long antenna on the Arduino and that is a potential for disaster.
The pot you are measuring is one leg of a wheatstone bridge, the other leg  of one half of the bridge is/was the control/direction pot. The devices were made by several different Mfrs Tenna or Allied or Blonder tongue. The motor relay drops out when the tower pot is equal in value to the control pot because the bridge is then balanced and its output is as low as it will go. The actual low depends on the equality of the other half of the bridge. For some theoretical knowledge go wiki a Wheatstone Bridge. The second issue is if there is a transmitter involved as it will induce signals into the Arduino as well. The original circuit worked very well because as you measured it is a low tech low impedance device well adapted to that limited type of control. I hope this little bit of information helped.

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

#### vstubbs

#3
##### Jun 20, 2012, 04:50 pm
Thank you for your suggestions, to answer your questions, first the arduino is not part of the RF circuit, infact it is fully shielded from any stray RF. the arduino, is intended to monitor a two lead 500 ohm potentiometer that is built into the rotor itself, and is gear driven to rotate with the antenna. Second all of the cables are shielded, and the arduino will be mounted in the same weather proof shielded box as the antenna switch is located.

#### Docedison

#4
##### Jun 21, 2012, 02:30 am
Well still doesn't change my response very much. For a 500 ohm Pot I would use a 500 ohm pot inside and use it as a rheostat so you can do a little 'calibration' +/- 50 ohms for a little swing should you need it. The mid point should be connected to an analog pin through a 1K resistor and a couple of .05Uf caps on either side of the resistor to a close ground. Move the tower to one stop and note the direction and voltage. Move the antenna and note the voltage and to the end and again monitor the voltage... IF the mid point i 1/2 of the full voltage the tower pot is fairly linear, if not You will need to figure out how to calibrate it. If it isn't really very linear then possibly it might be piece wise linear where cardinal transitions are more or less linear... All that must be discovered. You can do the calibration with a DMM and the pot. If both the tower and your pot are the same value then at max resistance (tower) the voltage 'should be 1/2 the supply or 2.5V and go to 0 when the tower is in the other direction, I don't remember If the alliance or Tenna ... had a pot that was 0 to full or +20 to +80% or some other value. I serviced one nearly 40 years ago... and my repair took less that 20 minutes. To replace a failed transformer and leave. So now for the calibration. The other reason besides the obvious for the inpuut filter is that frequently the pots are very noisy and the values I picked should roll off most of the noise.

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

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