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# Help Appreciated With RC Servo Control Circuit

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#### Russell Hayes

##### New Member
Hi Guys,
This is my first post here although I have been lurking around here and other electronics forums for a while now.

I am in the middle of a small project to build a simple wired control device to remotely control the focus ring of a camera lens and need a little help. The device consists of a hand held box with a ‘control knob/wheel’ that, when rotated, proportionally drives a standard RC servo with a gear attached. The gear then engages to another gear fixed to the lenses focus ring, thus, focus can be controlled remotely. There are similar commercial products available like the images below:

The heart of the device is a small commercially available servo controller, which comes shipped with a 10K potentiometer. The servo is connected to this controller; also 4x AA batteries are connected to power the system.

3 wires from the controller are then connected to the 10k potentiometer and, as the pot is rotated, the servo turns proportionally and in a linear fashion to the control knob pot. So far so good! Below shows how this is wired up:

The servo can rotate 180 degrees. When the main control knob is turned fully left, the servo is at 0 degrees. When the main control knob is turned fully right, the servo is at 180 degrees. Perfect!

Now, my goal (and where I’ve hit a hurdle) is to add two ‘end point limiters’ into the mix. By this I mean 2 extra control knobs/potentiometers, which reduce the travel of each end of the servo. So when these ‘limiters’ are both set to ‘full’ the servo rotates its full 180 degree travel when the ‘main knob/wheel’ is rotated from end to end.

If the ‘left limiter’ is turned, the servo only travels from 0 degrees to 100 degrees (depending on setting), from example. Or, if the ‘right limiter’ is turned, the servo only travels from 80 degrees to 180 degrees, from example. If both are turned, the servo only travels between 80 to 100 degrees, for example.

I have tried a simple wiring of 3 potentiometers, like below:

This works but the problem is that the servo no longer turns proportionally to the control knob. When the knob is turned, it starts slow and then accelerates as the pot is turned. It is not linear to the control knob.

Does anyone know why this would be or how to solve it so that the servo rotates proportionally and in a linear fashion to the control knob pot?

The yellow wire still needs to be connected to the wiper of the middle pot.

How would you recommend I wire this up to give the results I am aiming for?

The two end pots are wired as rehostats (wiper and one end); the middle pot is wired as a potentiometer (wiper to yellow wire to middle input of controller, ends to the two rehostats on either side of it). If you run both rehostats to zero resistance, you are where you started; as you add resistance in either rehostat, you reduce the travel in that direction...

Hi,

Ok, I wired it up like how I think you mean:

What happens now is, that the left limiter is working as an 'additive' regardless of the position of the 'main knob'. The right limiter has no effect what so ever.

So if the 'main knob' is fully left, the left limiter move the position, but also when the 'main knob' is on the right.

Now, looking at the PCB of the 'circuit controller', there are indeed 3 separate soldering tags for red yellow and green, but, the yellow and green seem to be connected by a pcb track?

Ok, then you are just building a two wire network like this:

The lower limit is defined by one rehostat to define the minimum resistance, the upper limit is defined by shunting the main control pot to define the maximum resistance. The resulting network has only two connections...

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I would be curious to know what the voltage is on the orange and green wires are.

If I understand this I would run the orange and green wires to the end terminals on the two new pots.

The use one wiper in place of the green and the other wiper in place of the orange. The two new pots will set the min and max voltage. The old pot selects a voltage in that range.

3v0

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Thanks again, really appreciate your help.

OK, wired as below:

The result is:

Lower Limit: Turning this changes the servo position when the control pot is at either end. I only want the lower limit to change the left/lower limit of the control pot.

Control Pot: Again, this is not turning the servo in a linear fashion, is is slow on one end, but fast on the other.

Upper limit: Does seem to limit one end of the pot, although wrong end?

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I thought you just wanted to reduce the range of the main Control Pot by setting the two end pots once, and never touching them again.

It seems like you want three independent controls which do not interact; so why not use a three position switch to select the three desired ranges?

Ok, lets back track to clarify want I want to achieve.

I want the lower limit control to set the servos 'left' position and the upper control to set the servos 'right' position. The control knob should mix linearly between these two positions.

With both upper and lower limit set to full, the servo linearly travels from 0 to 180 degrees when the control knob is turned fully from end to end.

If we set the lower limit control to set the servo to 20 degrees and the upper limit to 120 degres, the servo servo linearly travels from 20 to 120 degrees when the control knob and turned fully from end to end.

So the main control knob should linearly mix between the 2 limiter settings.

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So, the only issue you have is that the control pot setting is nonlinear? Are you using a "linear" pot or is the pot "audio taper"???

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Hi,

You are correct. In my second diagram, I am getting what I want but the control is not linear. I am using a linear pot, yes. That is the odd thing...

Ok, then this is getting harder. Go back to just a single pot. Hook up only two wires, one end and wiper.
Set the Pot to the minimum desired position of the focus ring, disconnect and measure the resistance.
Set the Pot to middle position (center of travel) of the focus ring, disconnect and measure the resistance.
Set the Pot to the maximum position of the focus ring, disconnect and measure the resistance.

Report the three resistances...

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Sure thing, will do.

I will get my ammeter from work tomorrow and attempt to get them readings.

But for what its worth, with the one pot wired as in the first diagram, the control IS linear and how I want it. Like this, it makes no difference if the green wire is attached or not.

Not sure if that info helps?

Note that I modified the previous posting about the resistance measurement.

I expect the yellow wire provides a voltage to the controller used to set the PWM duty cycle signal for the servo.

The original pot is a voltage divider with one end tied to gnd and the other V+ (not too sure what voltage is used).

In regards to my previous post.

My method simply allows you to set the end voltages for the original pot using two additional pots as voltage dividers.

As long as the controller does not draw a significant current on the yellow it shou1d work like a charm.

Set center pot to left max, then set left pot to limit left movement.
Set center pot to right max, then set right pot to limit right movement.
The center pot will provide linear movement between the two set points.
Full left will be the left limit, full right will be the right limit.

If this is wrong I want to know why.

3v0

....

If this is wrong I want to know why.

3v0

The OP specifically said that the original pot is NOT a voltage divider; i.e. two of the wires are shorted at the PCB. I was also under the impression that the original pot was a divider where all three wires are significant...

The OP specifically said that the original pot is NOT a voltage divider; i.e. two of the wires are shorted at the PCB. I was also under the impression that the original pot was a divider where all three wires are significant...

3v0

Hi Mike,

I have not manuaged to get any readings. I am a bit of a numpty with this so I am not sure how to make the measurements.

Here is a link to a scan of the instructions provided with the servo controller (look at the pot solder tags, you can see the yellow and green tabs are linked:

http://www.sgpro.co.uk/servocontrollerinstructions.jpg

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Hello All

I am also looking to accomplish this as well...what was the outcome here???

Erik Anderson

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