Potentiomter and pic18f452 question...<hardware question&

KamPutty · Guest

Hi all,

Okay, I'm having major brainfarts, and I need to get moving with my project!
I want to read a value from a potentiometer, and display that value.

I can display etc., but need some telling on how to physically hook up the potentiometer to the pic (connect pinx to rez to pot etc.). I'm not a electronics person, and can handle the programming end, but may need some guidance. I currently have a pic18f454 physically hooked up and running on a 9vold, with lcd, so this should be a peace of cake...yes?!

INFO

I have currently 3 different pots I want to use, in order of importance.

#1. 100k OHM
#2. 20k OHM
#3. 200 OHM

All pots are 10 turn.
I would prefer the 100k as that yields a better resolution.

Any guidance in the hookup would be greatly appriciated!

~Kam (^8*

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

kamputty · Guest

Andrew, et al.,

The 3 pots I have are rated differently.

The pots I got are these: http://www.bourns.com/pdfs/3540.pdf

I got these 3: resistence=200, 20000 and 100000.

I would like to use the 100k as I understand, I can get better resolution for the 10 turns, yes?

Again, thanks for the reply!

~Kam (^8*

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

kamputty · Guest

Huh?

cmdrdan · Joined: 08 Apr 2005 Posts: 25 Location: Washington

Try this experiment, using your three potentiometers and a voltmeter, forgetting about your PIC....

Connect each pot to five volts and ground, and measure the voltage from the wiper to ground at each full turn, starting at zero turns and ending at 10 turns. Your readings should be the same (depending on how close to a complete turn you are) regardless of which pot you're testing. Potentiometers are ratiometric devices.

Now, back to the PIC. I'll assume that you're running at five volts and using a 10-bit ADC setup (since you stated 768 points). Zero volts will be represented by an ADC code of zero, and five volts will be represented by a code of 1023. If you were to turn the pot to the exact center of its rotation, you would see a code of 512, and a corresponding voltage of 2.5 volts if you were to measure it with your voltmeter.

The PIC works best with input resistances less than 2.5k ohms, and as the others have stated, you'd need a buffer amplifier to have success with your 20k and 100k pots. The 200-ohm pot is going to draw 25 mA (again assuming five volts) which is a little high for most practical purposes, and you may want to consider a 2k pot if this is a long-term project.

Hope this helps put your head back together!

Dan

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

kamputty · Guest

Dan, et al.,

I think I get it! I was thinking that the OHM's and resolution where related as they listed a smaller/better resolution for the higher ohm's.

So, getting a 10 turn 2k ohm pot should work for me as this is a long term project...

Now to locate a 10 turn 2k pot!

Thanks!

~Kam (^8*

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

The 200 Ohm pot would work if you can handle the power. At 5V the pot will dissapate P=E^2/R = 125mW and draw 25mA from your supply. Depending on the application that may be OK.
Otherwise if the pot is only going to turn slowly, like by a human hand, you can lower the impedance seen by the A/D by putting a capacitor on the wiper of a high impedance pot. Something like 1.0uf or 0.1uf should satisfy the A/D input. Put the cap from the A/D input to ground. If the chip has a seperate analog ground pin that is the ground you should use for both the cap and the pot.
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