Potentiometer 5 or 6 detents

[Adam Shapiro]

Originally Posted by Travis Covington

The rotary switch seems like a good solution, if it does in fact have solid feeling stops, so we could easily differentiate between the 6 positions.

The only problem is, I would be the one wiring it up and have practically zero electronics knowledge and would need some help selecting resistors and wiring it up to specific pins on the switch and the RC. If you guys could help me out there, It would be HUGELY appreciated.

The idea here isn't as much the actual resistance as the concept. The current through the circuit I is defined as I=V/Rtotal where Rtotal is the sum of the two resistors: the initial one connecting from 5V to the common lead, and the secondary resistor from the switch to ground. Then the voltage measured at the common lead can be calculated as V=IRswitch (the resistance from the switch to ground). The actual R value will cancel out.

----- Rc ----- Rswitch <--the dashes are just spacers here...
5V--^v^v--|--^v^v--gnd
________Input
Rc=R, Rswitch=#R
I=(5V)/(Rc+Rswitch)
Vinput=I(Rswitch)=(5V)(#R)/(R+#R)=(5V)(#)/(1+#)

Hope this helps.

[Travis Covington]

Originally Posted by Adam Shapiro

Hope this helps.

Sort of... In reality, I still have no idea where to start. I am an ME/MFE with very little electronics experience... so please bear with me.

[Adam Shapiro]

Originally Posted by Travis Covington

Sort of... In reality, I still have no idea where to start. I am an ME/MFE with very little electronics experience... so please bear with me.

Heh, not a problem. You should probably choose some value of R in the 10000s to protect the input (I believe).

EDIT: It seems the OI manual suggests 100k ohms for a value. This seems resonable as it will only allow for a very small amount of current between 5V and gnd (I=V/R=5/(100000)=50uA). This is for safety as well as to prevent against large flows of current when using the OI. Don't forget the OI has its own built in fuses to protect itself, but you still want to be careful.

[Travis Covington]

My plan was to mount it on the RC.

[Adam Shapiro]

Originally Posted by Travis Covington

My plan was to mount it on the RC.

The RC has the same thing going for it. The only thing I would be concerned with however is that using a rig like this might be a bit cumbersome. Personally I'd use a 100k ohm potentiometer, rather than a rotary switch, and just set specific ranges as certain programs. This could also be reprogrammed to allow for smaller ranges -> more programs. That's up to you though. The math/physics behind the voltage divider idea would stay the same though, you'd just hook it to an analog input on the RC rather than one on the OI.

[kaszeta]

Originally Posted by Adam Shapiro

The RC has the same thing going for it. The only thing I would be concerned with however is that using a rig like this might be a bit cumbersome. Personally I'd use a 100k ohm potentiometer, rather than a rotary switch, and just set specific ranges as certain programs. This could also be reprogrammed to allow for smaller ranges -> more programs. That's up to you though. The math/physics behind the voltage divider idea would stay the same though, you'd just hook it to an analog input on the RC rather than one on the OI.

That's what Team 95 has done several times. We just had a pot on the robot, no detents, and ranges marked by the knob corresponding to each strategy. With the little knob we used, this allowed us to do up to 12 positions[1] without worrying about ambiguity, although after BAE we realized that we were only really using 4 of these positions, so we reprogrammed the switches.

I think next year I'll encourage the team to make something for the OI, however.


[1] Which meant that the switch went from 0 to 11. One team member asked "can't it just to to 10 instead." Of course, I had to answer "no, these go to 11." Only one person got the joke.

[Adam Shapiro]

Originally Posted by kaszeta

I think next year I'll encourage the team to make something for the OI, however.

Personally I like having the ability to switch the program at the last minute from the OI. We normally do a binary selector of some sort mounted on our controls.

Originally Posted by kaszeta

One team member asked "can't it just to to 10 instead." Of course, I had to answer "no, these go to 11." Only one person got the joke.

How could they not get that?? More people need to see that movie!

[ZZII 527]

Here's a quick way to turn a DP6T switch into a 6-position binary encoder that only needs three pins:

POS   A   B   C
1     0   0   0
2     1   0   0
3     0   1   0
4     1   1   0
5     0   0   1
6     1   0   1

[Joe Ross]

I found the following 11 detent potentiometer that is cheap: http://jameco.com/webapp/wcs/stores/...uc tId=286512

[Al Skierkiewicz]

There are pots that exist that have detents built in but there are generally used for volume controls (are log taper, 10K) and have 10-12 detent positions. A better solution is to pick up a six position switch at Radio Shack.
http://www.radioshack.com/search/ind...ar y%20switch

This switch is easily modified to become a twelve position switch by bending one of the stopper tabs out of the way and cutting one of the contact springs. Wire 9.1K ohm resistors from each terminal to the one next to it to form a series string. When you are done, you have a stepped 100K pot. Wire as you would any pot to an analog input on the OI and then set your programmers to work. RS has some nice shiny knobs as well. If you would like to see one in action, stop by our pit in Atlanta.

[Travis Covington]

Are you saying to use the 9.1k resistors if I wanted to have 12 positions? Would I still use the 15k resistors if I were to only use the 6 positions?

[Tom Bottiglieri]

Originally Posted by Travis Covington

Are you saying to use the 9.1k resistors if I wanted to have 12 positions? Would I still use the 15k resistors if I were to only use the 6 positions?

You can use either. If you use the 6 position switch, 15k resistors will give your programmers a little more leeway to distinguish one "step" from another.

[Alan Anderson]

Originally Posted by Travis Covington

Are you saying to use the 9.1k resistors if I wanted to have 12 positions? Would I still use the 15k resistors if I were to only use the 6 positions?

For an OI-connected selector, you definitely want the maximum resistance to be just under 100k, and the minimum resistance should probably be at least 2k to keep from overloading the +5 source. That's why I suggested 15k for six positions. 9.1k will be fine for 11 steps, but I think it's a little too high for comfort if you want 12.

The total series value of an RC-connected "stepped potentiometer" is not that critical, but 100k is still a good target.

[Gary Bonner]

Here is a photo of our switch from last year, which was connected to the RC. It’s a six position switch with five 22k resistors (100k / (# positions - 1) = 20k, 22k was the closest we could find at Radio Shack). The switch positions were then 0k, 22k, 44k, 66k, 88k, & 110k. This would give analog values of about 0, 205, 410, 615, 820, and 1024. In the autonomous code, we looked for a range of 205, centered on each of those values.

void User_Autonomous_Code(void)
{
		.
		.
		.

	switch (((Get_Analog_Value( rc_ana_in01 )+100)/205)+1)	//read autonomous switch
	{  
		case 1:	//switch position 1 
	
			while (autonomous_mode)   /* DO NOT CHANGE! */
			{
				if (statusflag.NEW_SPI_DATA)      /* 26.2ms loop area */
				{
					Getdata(&rxdata);   /* DO NOT DELETE, or you will be stuck here forever! */

					/* Add your own autonomous code here. */
					
					Putdata(&txdata);   /* DO NOT DELETE, or you will get no PWM outputs! */
				}
			}
			break; //end of case 1
	
		case 2:	//switch position 2
	
			.
			.
			.

		default:
		{
			// something's wrong - do nothing
		}
	}  //switch
}  //User_Autonomous_Code

[Alan Anderson]

Originally Posted by Adam Shapiro

The idea here isn't as much the actual resistance as the concept...

If the switch is going on the OI, the actual resistance is rather important. The OI analog inputs want to see a variable resistance between ~2k and 100k to give the full range of values.

For a six-position rotary switch, I'd use six 15k resistors wired like this:

+5 <----/\/\/--*--/\/\/--*--/\/\/--*--/\/\/--*--/\/\/--*--/\/\/--*
                         ^
 OI                      |
analog input <-----------'

The * characters represent the six positions of the rotary switch, and the ^ is the common connection.

What you'll get from this is the equivalent of a joystick with six discrete resistance values: 15k, 30k, 45k, 60k, 75k, and 90k. You should use a dashboard program to determine what the actual numbers end up being, and choose appropriate ranges in the software to decide which of the six positions is chosen.