Circular Soft-Pot - Changing Floating Pin Value

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thechill
 
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Circular Soft-Pot - Changing Floating Pin Value

Post by thechill »

Hi Everyone,

I have the circular soft-pot, set up as described:
Pin 1: 10K resistor -> Ground
Pin 2: 100K resistor -> A9 Flora
Pin 3: 10k resistor -> 3.3V
(https://www.adafruit.com/products/1069)

Without any pressure, my floating pin values read about 445 (the reading range from top to bottom of the pot is 350 to 650). Is there a good way to reduce/increase the floating pin value to > 650 or < 350 so it doesn't interfere with my normal reading range?

Hypothesis is to increase resistance on Pin 2 (?)

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adafruit_support_mike
 
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Re: Circular Soft-Pot - Changing Floating Pin Value

Post by adafruit_support_mike »

Use smaller resistors between the pot and Vcc/GND.

You need at least one fixed resistor in series with the Soft Pot because it's possible to short the high and low terminals together. If you connect those directly to Vcc and GND, enough current will flow through the short to damage the Soft Pot, and possibly the rest of the circuit connected to it.

You don't necessarily need 20k of series resistance though. A single 1k to either Vcc or GND will limit the short-circuit current to a few milliamps.

The Soft Pot has a nominal end-to-end resistance of 10k, and any pot is basically an adjustable voltage divider. If you put the Soft Pot in series with two other 10k resistors, the voltage across the ends of the pot will be about 1/3 of the supply voltage.

If you put the Soft Pot in series with a single 1k resistor, the voltage across the Soft Pot will be about 90% of Vcc and will extend all the way to the rail that doesn't have the fixed resistor.

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thechill
 
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Re: Circular Soft-Pot - Changing Floating Pin Value

Post by thechill »

Thanks for getting back so quickly, I'll give that a shot!

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thechill
 
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Re: Circular Soft-Pot - Changing Floating Pin Value

Post by thechill »

Hey Mike,

Pin 1: 1K resistor -> Ground
Pin 2: A9 Flora
Pin 3: 3.3V
I now read 120 to 1020 (bottom to top of the pot), with the center pin floating at 800. Previously, it was 350 to 650, center reading 445.

While it did give me more range (as expected), the "not in use" output by the center pin remains in the the middle of my sensor range. Perhaps this "not in use" value is just the pot as the divider and can't be changed?

If so, is there a way to hack this to get it to baseline later down the pot (i.e., at a higher or lower value)?

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adafruit_support_bill
 
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Re: Circular Soft-Pot - Changing Floating Pin Value

Post by adafruit_support_bill »

If you want to keep the center pin from floating when it is not active, you can use a 'weak' pulldown from A9 to ground. If you use a high-enough resistor value (say 200K or more) it will have minimal effect on your readings when pressed.

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