OpAmp circuit question.

Are you serious?

pot low = VCC pot high = GND pot wiper = output (buffer as needed)

greetings, Tom

Oeps,

I incorrectly described my problem.

Background:

I have a joystick that outputs at rest a 2.5V center voltage. Moving the joystick up moves the voltage to 5V, moving it down to 0V.

I need to replace this joystick by a 'Potentiometer + Switch'

-The switch in position 'A' should add the potentiometer's output to the 2.5V

-The switch in positon 'B' should substract the potentiometer's output from the 2.5V

Quit different from what i first described.

Sorry for the confusion and thanks for any hints.

--
There seems to be something wrong with your requirement since on the
one hand you say that moving the pot toward its minimum should drive
the circuit\'s output toward Vcc,  but then when it actually _gets_
to its minimum the output should suddenly flip to GND?  That can be
done, but is it really what you want?

So, you just need a reversing switch. An ordinary DPDT switch, with the two wipers connected to the two ends of the pot, and +5 and Gnd supplied to opposite poles for the two positions.

Cheers! Rich

Sounds to me like a homework assignment where the instructor gave out what sounds like a pretty contrived excercise to invert the slope of an input signal.

About the only real application I can think of is some sort of audio application where you want a audio (log) taper and you're trying to avoid having to come up with a reverse taper pot (pretty hard to find these days)... although that wouldn't jive with Vcc/2 being at the mid-point of the pot...

I read in sci.electronics.design that snipped-for-privacy@hotmail.com wrote (in ) about 'OpAmp circuit question.', on Mon, 7 Nov 2005:

The potentiometer itself will do that for you. Just connect Vcc to the 'minimum' end of the track and GND to the 'maximum' end. If the pot resistance gives you too much voltage drop at the centre position, connect the slider to a simple op-amp buffer and feed its output to the rest of your circuit.

Of course, you my not have meant quite what you wrote, in which case disregard the above.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

In fact, i'm retrofitting an old control with something new. One constraint is the use of a momentary action SPDT switch and the use of a single tap potentiometer.

The summery:

The switch at rest : 2.5V output. The switch pushed in position A: the potentiometers position should be added to the 2.5V (for example , if the potmeter is at 50% of its value the output should be 3.75V) The switch pushed in position B: the potentiometers position should be substracted from the 2.5V (for example , if the potmeter is at 50% of its value ther output should be 1.25V)

Thx

Momentary action SPDT switch? So normally the switch is in position A, and you need to activate it to put it in position B? Or is there a third "OFF" position?

(view in fixed width font such as courier)

5V ---o \\ H POT L +----------/\\/\\/\\/\\----- 2.5V ^ 0V ---o SPDT | W | +--------- OUT

greetings, Tom

Tom,

Thx, amazing how sometimes sucs a simple circuit just won't come up in my brain...

But this is what i wanted to do.

Thanks a lot for helping my out of my to-far-away thoughts.