# Mobility Scooter Potentiometer

#### BCFD546

Joined Jun 19, 2017
7
Okay guys,
I'm working on an Invacare Buzz HMV mobility scooter and found it to have a bad 5k throttle pot. I replaced it with another 5k pot and now it works. Unfortunately, the throttle wigwag only travels about 25 degrees and the pot has a 300 degree mechanical travel. Because of this, I have to manually turn the pot to get any speed. If I attach the wigwag, it doesn't turn far enough to really get any power due to the limited travel distance of the wigwag. Is there a way to tweak the new pot or is there a different type of pot or rheostat I need to replace it with? I can't find the old pot that I removed, so I don't know the model number of it. Any help would be greatly appreciated.
Thanks!

#### djsfantasi

Joined Apr 11, 2010
9,188
Someone else had a similar problem. So the "throttle wigwag mechanically restricts the rotation to 365°? I'm at the dentist, so if you can measure the rotation of your pot, thanks. It will help me answer your question.

#### BCFD546

Joined Jun 19, 2017
7
Unfortunately I'm at work right now so I can't measure. The wigwag has around a 25-30 degree rotation before it hits the physical stops on the scooter, yet the pot has a 300 degree travel. The pot works fine but has to be turned much further than the wigwag can travel before there's any real speed. I haven't been able to find a pot with a smaller mechanical travel (degrees). I'm at a loss for ideas other than tearing the pot open and drawing a trace with a conductive pen.

#### #12

Joined Nov 30, 2010
18,224
I started running the numbers and hit a limit: Is the machine looking for +/- 2.5K or is it looking for the voltage on the wiper at the alleged ends of the wiper rotation?

It's simple enough to play with some resistor values to get +/- 2.5K, but if the right answer is a voltage, that gets into a DC amplifier.

#### BCFD546

Joined Jun 19, 2017
7
Ooh. I'm not sure. With the pot in the middle position, it outputs about 2.5 v to the controller at 2.5k. A little to the negative side and we get speed in reverse, increasing to the full -5k gets us full speed in reverse. Opposite applies for forward. So in order to get full speed, the wiper had to be at its travel limit. Does this make sense?

#### #12

Joined Nov 30, 2010
18,224
Does this thread help any bit?

#### BCFD546

Joined Jun 19, 2017
7
Thanks guys! Looks like that's what I'm gonna have to do. Appreciate the help!

#### djsfantasi

Joined Apr 11, 2010
9,188
I started running the numbers and hit a limit: Is the machine looking for +/- 2.5K or is it looking for the voltage on the wiper at the alleged ends of the wiper rotation?

It's simple enough to play with some resistor values to get +/- 2.5K, but if the right answer is a voltage, that gets into a DC amplifier.
If the answer is voltage, it depends on how the pot is wired into the circuit. If wired as a variable resistor, a standard pot can be used. If wired as a voltage divider, then I agree with you.

#### #12

Joined Nov 30, 2010
18,224
If wired as a variable resistor, a standard pot can be used. If wired as a voltage divider, then I agree with you.
Knowing a little bit about cars, the usual answer is that the input to the brain is a voltage between zero and five.

#### djsfantasi

Joined Apr 11, 2010
9,188
I think I have confused the issue with imprecise terms. If the "brain" needs a single input from 0-5V, then a variable resistor is all that's needed.

The schematic would be useful.

#### BCFD546

Joined Jun 19, 2017
7
I've been looking at that throttle pot. The weird thing is, that's a completely different throttle wigwag than the Buzz has. I'm thinking the pot from it may work though.

#### Alec_t

Joined Sep 17, 2013
14,411
You could perhaps use a (good quality) standard pot plus a DC amplifier (as #12 mentioned).
Something like this should get you a 0V-5V output swing with the wiper moving over a range from 40% to 60% of its full travel V (i.e. ~30° either side of centre).

Edit: R1 and R2 should be 10k.

Last edited:

#### BCFD546

Joined Jun 19, 2017
7
That would be worth a shot! If I get the chance, I'll give it a try and see what happens.

#### Alec_t

Joined Sep 17, 2013
14,411
If you do give it a shot, the opamp needs to have a rail-to-rail output capability and be able to run from a single 5V supply. I assume the pot in the existing wigwag has 5V across its end terminals?

#### #12

Joined Nov 30, 2010
18,224
only travels about 25 degrees and the pot has a 300 degree mechanical travel.
a range from 40% to 60% of its full travel
the usual answer is that the input to the brain is a voltage between zero and five.
There are still some uncertainties.
a) +/- 25 degrees is 1/6th of a rotation, not 1/4th. Easily solved by changing R4 in the circuit of post #15.
b) potentiometers are not perfect in their centering and range. "Zero" is rarely zero and "all" is rarely what the label on the pot says, +/- 20%
c) Automotive processors discover the actual voltages of the throttle position sensor and adjust accordingly, thus rendering a and b moot, if a battery operated scooter has that kind of programming and processing power.