Potentiometer with ADC

[stevethetinker]

Does anyone know what resistance potentiometer to use?

[Alan Anderson]

Anything from 2k to 100k ought to work. Higher resistances will be slightly more susceptible to noise. We've used 10k with good results.

[Danny Diaz]

Originally Posted by stevethetinker

Does anyone know what resistance potentiometer to use?

This year we're using 5k ohm pots with great success.

-Danny

[Shinigami2057]

I have a dumb question - I Just hooked up a 100K pot to analog input 01, but Get_Analog_Value(rc_ana_in01) is only returning values from 0-9, and it definitely is not linear. This is the (very simple) code we're using just to test:

printf("%u\n", Get_Analog_Value(rc_ana_in01));

Am I missing something important here? Is there something wrong with the pot? (It's wired correctly)

Thanks

[Ken Streeter]

Originally Posted by Shinigami2057

I have a dumb question - I Just hooked up a 100K pot to analog input 01, but Get_Analog_Value(rc_ana_in01) is only returning values from 0-9, and it definitely is not linear. This is the (very simple) code we're using just to test:

printf("%u\n", Get_Analog_Value(rc_ana_in01));

Am I missing something important here? Is there something wrong with the pot? (It's wired correctly)

Thanks

The problem is that Get_Analog_Value returns a 10-bit value, rather than an 8-bit value. In general, we discarded the lowest 2 bits of the potentiometer value as "noise" and just used the top 8 bits via the following code:

 int Get_Robot_Pot(void) {
    unsigned char robot_pot_8bits;
 
    // analog inputs on the robot are returned as 10 bit values, the line below
    // drops off the bottom two bits to give a one-byte value for the elevation
    robot_pot_8bits = (unsigned char) (Get_Analog_Value( rc_ana_in01 ) >> 2);
    printf("Pot Value: %3d\r\n", (int) robot_pot_8bits);
    return ((int) robot_pot_8bits);
 }

[Shinigami2057]

How is that a problem when analog inputs return an unsigned int (16-bits on the PIC18, more than enough for the 10-bit ADC conversion)? I'm assuming %d is expecting an int, so it should be fine. Even if the compiler was lopping of the 2 MSbs of the result, it should still return a value from 0-255.

Thanks for the help, though. We're going to try some new linear taper 10k/50k pots tomorrow. We really weren't sure exactly what kind of pots the old ones were, only that they were 100k ohm.

[Alan Anderson]

Originally Posted by Shinigami2057

Is there something wrong with the pot? (It's wired correctly)

Define "correctly".

Use a voltmeter to look at the signal (white) wire. It should vary from 0 volts to 5 volts as you turn the pot. If it only gets up to a few millivolts, it's possible that you have the ground and wiper connections swapped.

[Shinigami2057]

I'll definitely check that, thank you. We currently have the pot wired so that the signal wire is in the center, which I assumed to be the wiper based on a previous topic here. Do all pots use the center connection for the wiper?

[Alan Anderson]

Originally Posted by Shinigami2057

Do all pots use the center connection for the wiper?

That's true of all of the generic single-turn pots I'm familiar with. However, the five- and ten-turn pots are different, with the wiper at one end and a little offset from the other two connections. The linear "slide" pots don't seem to have a consistent pattern.