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Drifting potentiometer readings


chuckuntulis
(@chuckuntulis)
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I am new to using Pico. I am trying to use a Parallax Joystick because I do not have the joystick mentioned in the video https://dronebotworkshop.com/pi-pico-circuitpython/&source=gmail&ust=1640809254528000&usg=AOvVaw04ANGYplMjLJkrekEAQ43 C"> https://dronebotworkshop.com/pi-pico-circuitpython/ . Even when the joystick is not moving, the X and Y values change on every reading. I have 
L/R+ and U/D+ connected to 3V3 (Pin 9)
L/R connected to pin 32 ADC1
U/D connected to pin 31 ADC0
GND connected to pin 23
 
I labeled the two axis pot as X,Y.
I added another pot and labeled it Z ,connected it to ADC2 and I see the same behavior
Each time through the loop, I increment the count and print that count as first element on the line

15 X = 1.86592 Y = 1.64436 Z = 0.758935

16 X = 1.85222 Y = 1.62825 Z = 0.896704

17 X = 1.86753 Y = 1.6355 Z = 0.796802

18 X = 1.85625 Y = 1.62905 Z = 0.713012

19 X = 1.86108 Y = 1.63469 Z = 0.833056

20 X = 1.85464 Y = 1.62502 Z = 0.88784

 
I am not sure what is happening and what if anything to do.
 
I am not sure why there is a tag of "No more attachments are allowed today" with no explanation why.

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DroneBot Workshop
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Posted by: @chuckuntulis

I am not sure why there is a tag of "No more attachments are allowed today" with no explanation why.

Hi Chuck

This is because it's your first post, after you make a few more the restriction will be lifted.

In regard to your joystick, are you certain that it is an analog joystick (i.e. two potentiometers) as opposed to a digital one?  

😎

Bill

"Never trust a computer you can’t throw out a window." — Steve Wozniak


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DroneBot Workshop
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One other thought - you say that your readings are erratic even when substituting potentiometers for the joystick? 

Did you try measuring the voltage being presented to the analog pins as you moved the pots? It should change from zero to 3.3-volts as you move them.

The results you are getting sound to me like the analog inputs are floating, or that you might have a grounding issue.

😎

Bill

"Never trust a computer you can’t throw out a window." — Steve Wozniak


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DaveE
(@davee)
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Posts: 407
 

Hi @chuckuntulism,

You say "L/R+ and U/D+ connected to 3V3 (Pin 9)" ... was that a typo? Bill showed ..

image

A quick Google suggests Pin 23 for ground maybe ok, but pin 9 for 3v3 looks wrong.

Best wishes, Dave


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chuckuntulis
(@chuckuntulis)
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Joined: 6 months ago
Posts: 3
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Very strange. I disconnected all the wires from the joystick but the

x = get_voltage(x_axis) and

y = get_voltage(y_axis)

still get varying values

and the other pot is also still registering variable values

13 X = 2.53542 Y = 1.76199 Z = 0.879785

14 X = 0.511596 Y = 0.468896 Z = 0.664673

15 X = 0.4552 Y = 0.474536 Z = 0.617138

16 X = 2.37912 Y = 1.67336 Z = 0.851587

17 X = 1.62261 Y = 1.1521 Z = 0.80083

18 X = 0.086206 Y = 0.219946 Z = 0.596191

19 X = 1.51545 Y = 1.16016 Z = 0.742016

20 X = 2.38557 Y = 1.65886 Z = 0.878979

I will disconnect all of the joystick and the additional pot and start from scratch. The buttons all behave as expected. I think I may have another pico.

If I turn the extra pot all the way counterclockwise, it reads 3.29919 as do the X and  Y pins that are not connected to anything now. If it is turned slightly or full clockwise all three get_voltage shows the variable values.

I will do some more experiments later.


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chuckuntulis
(@chuckuntulis)
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Joined: 6 months ago
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  • Thanks Bill and DaveE. I have removed all connections except for the ones to the pot -
  • power from 3V3 pin 36
  • GND to pin 38
  • data connected to ADC0 pin 31

And the data looks good and changes as I turn the pot and is in the range of 0 to 3.29. The second digit to the right of the decimal point wanders around a bit.

Thanks for your help getting me to a known good state.

I will now reconstruct the circuit based on the video and then start adding other stuff.

I have added a reset button connected between GND and RUN/RESET so I do not have to pull out the USB to reset it.

Is there an easy way to identify pins without counting pins on the top of the board? I have a pinout diagram but I still have to count pins.

I have been using Mu (not my favorite editor) because I can see the output on the Serial display.

Another issue: I would prefer to use VSCode but I can't get the CircuitPython extension working again. I stumbled onto getting it working once but I have not been able to get it working again. I think the issue is related to setting up a Workspace which I don't seem to be able to do using the CIRCUITPY drive. It won't accept CIRCUITPY drive as the starting point for a Workspace. 

Thanks

Chuck


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DaveE
(@davee)
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Posts: 407
 

Hi @chuckuntulis,

  Great to hear that you have fixed your initial problem. Getting started always seems difficult. Once you pass the initial hurdles, you can usually move in small steps which are easier to retrace when things stop working.

  I don't have an R-Pi Pico, so I am relying on the data sheet, etc. for these comments.

  The ADC is specified as 12-bit, i.e. 4096 values, so with a 3.3V ref, it should resolve to ~1 mV.

Fluctuations corresponding to about 1 mV are inevitable. However, 2nd decimal place variations corresponds to roughly 10mV fluctuations, which may be due to noise on the power rail. This may not matter for your application, but perhaps the following suggestions will help if they are an annoyance.

A no-cost option to try is using the ADC-VREF pin 35 instead of the 3.3V on pin 36 to power the potentiometer. This has a simple resistor-capacitor filter, and corresponds to the ADC reference. The absolute ADC values will change by a small amount, but this can usually be allowed for in the software conversion.

Similarly, I would try using pin 33 for ground to the potentiometer, as the data sheet suggests it is intended to be the analogue ground. I also note Bill uses it on his sketch, which is usually a good hint!

As a further measure, the data sheet also suggests adding 3.0V reference like an LM4040, but that may not be necessary .. and obviously requires an extra part.

------

For pin identification, if you have a printer, perhaps you can print some strips to fix next to the board?

-----

Sorry, I have not tried CircuitPython, so perhaps others can help. Maybe start a new thread with a direct question?

Good luck with the rest of your project!


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