An inexpensive way to interface to potentiometer


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Here's how to read a pot, using only two resistors and a capacitor:


It is based around the technique used for the joystick interface on the Apple II.


Only downside is that you need to calibrate the the limits - but perfect as a paddle interface for a game of Pong, where you could just remember the max & min readings.

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I've done this using a 555 timer, which I notice is how The Woz did it (in his case with a 558 quad timer).  I think you get better stability with a 555 rather than depending on the FPGA's input threshold voltage.  You can get a low-voltage 555 these days, or use a 74VHC123A dual one-shot.


If you really want to avoid external chips, you might get better results with a precision FPGA input standard that uses a Vref pin or use differential inputs comparing to a common Vref.

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