Wire pushbuttons in series to simplify DPOT up/down circuit

A curious connection of control buttons makes a simpler-is-better digital potentiometer interface possible.

Two recent submissions to the Design Ideas kitchen (see Related Content) have shown different basic step-per-push pushbutton interfaces for up/down digital potentiometers. Since they have the same purpose and perform the same function, they mostly can only meaningfully differ in their respective part counts. Which they did.

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But we engineers generally agree that, keeping other factors equal, simpler yet is even better yet. Therefore Figure 1, with only two resistors, one capacitor, and one logic gate, is offered as a small, but still noticeable, further improvement in the breed.


Figure 1 NC (normally closed) push-to-open momentary pushbutton switches connected in series are the basis for an unusually simple digital pot interface.

Here’s how it works.

Figure 1’s series connection of control switches lets them share a single pullup resistor. Pushing either the DOWN or UP button releases R1, pulls up R2, and begins charging C1. The resulting ~5ms low-pass bounce-filter connection to Schmitt inverter U1’s pin 1, generates a debounced, clean, and sharp pot clock edge on U1 pin 2. This will either increment or decrement the pot setting.

Which action actually happens depends, of course, on which button got pushed. Pushing UP pulls up and asserts U2’s U/-D pin 2, making the clock pulse increment the pot’s setting. Pushing DOWN leaves it low and the pot therefore decrements. The R2C1 debounce delay gives any initial pin 2 switch bounce adequate time to rattle around, settle down, set up, and stabilize before the clock drops and U2 samples it.

Releasing the button discharges C1. The associated R2C1 debounce timeconstant and Schmidt trigger action keep the discharge ramp and clock pin transition as clean and snappy as was the charge side. This is of course necessary if we’re going to avoid generating spurious trailing clock transitions that would corrupt the pot setting. Which we would never allow.

And that’s it. Which might just possibly be as simple as it can get. But I wouldn’t bet on it.

Stephen Woodward‘s relationship with EDN’s DI column goes back quite a long way. Over 200 submissions have been accepted since his first contribution back in 1974.  They have included best Design Idea of the year in 1974 and 2001.

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