Electronics > Projects, Designs, and Technical Stuff
Two potentiometers seemingly connected together by their wipers!
grumpydoc:
--- Quote from: ejeffrey on August 25, 2020, 04:04:52 pm ---Try multiplexing. Apply v+ and GND to pins 1 and 3 with pins 2 and 4 as analog inputs. Then excite 2 and 4 and measure 1 and 3.
No idea why you would build something like that and there is probably a better way to measure but if you aren't super concerned about update rate this should work.
Edit fixed pinout.
--- End quote ---
If I understand your suggestion this means reading one pot wiper voltage "through" the other pot - so you will have 0-25k between the voltage that you want and the A2D (?) input. Wouldn't matter if the analogue input had a suitably high resistance but it sounds potentially (sorry) a bit high to get an accurate reading from a typical microcontroller A2D input. Just looking at a random data sheet suggests an analogue source resistance in the order of 2.5k is the spec.
FenTiger:
(Hmm. This might be a bit easier to explain if I'd numbered the four sense resistors.)
The idea is that the amplifier inputs are high impedance, and so the current entering at the top, the current that flows between the two wipers, and the current leaving to ground at the bottom are all the same.
Looking at the top half: the total current flowing in the top two sense resistors is constant, regardless of the position of the wiper. If it's all the way to the left then most of it flows through the left hand resistor; if it's all the way to the right, most of it flows through the right hand resistor. The position of the pot thus shows up as the difference between the voltages at the upper amp's inputs.
The bottom half works the same way: the sum of the currents is equal, and independent of the position of the upper pot, but their ratio depends on the position of the lower pot.
Perhaps the op-amp symbol is a bit misleading; they're meant to be differential amplifiers, and you won't want an op-amp's full open loop gain in practice, so you'll want to add some resistors there to reduce their gain to something realistic.
The outputs of the two amps would connect to two analogue inputs on your Arduino.
I don't actually know if it would work. It was just a quick thought.
mikerj:
--- Quote from: grumpydoc on August 26, 2020, 04:56:44 pm ---If I understand your suggestion this means reading one pot wiper voltage "through" the other pot - so you will have 0-25k between the voltage that you want and the A2D (?) input. Wouldn't matter if the analogue input had a suitably high resistance but it sounds potentially (sorry) a bit high to get an accurate reading from a typical microcontroller A2D input. Just looking at a random data sheet suggests an analogue source resistance in the order of 2.5k is the spec.
--- End quote ---
The Atmega328 has a very high ADC input impedance (~100Mohms) but it will take a gulp of current during acquisition to charge the internal sample cap. A low value capacitor on the ADC pin can help with this, provided a suitable delay is used to ensure the cap is charged. Since the cap will have to be charged/discharged by the GPIOs when powering one of the pots for measurement it should kept low in value, maybe a few nF.
ledtester:
If you don't care about compatibility, just break the connection between the two middle pins and bring out your own connector. You can also break the connection with a switch to retain compatibility with the original Logitech steering wheel.
ejeffery's idea would also work.
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