Doubly twisted DPOT ganged programming

Select and set one or both channels of a dual digipot with a twist of the wrist.

Tandem potentiometers (i.e., pots that share a common control shaft) have been around for an extremely long time. That’s because they can be extremely handy. Adjusting the volume on a stereo sound system, tuning time constants in a multiple pole filter, or setting the output frequency of a Wien bridge sinewave oscillator…these are just a few obvious examples (of many) of their utility.

But of course, when mechanical pots share a common shaft, they share it forever, and therefore must turn together, likewise forever. This reality remains equally true when it suits the application at hand and when it doesn’t.

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This Design Idea (with the circuit shown in Figure 1) implements a variable frequency oscillator circuit that programs the sections of a dual dpot with easily controllable up/down steps. It generates step rates from zero to 20 Hz together with an up/down step direction signal. Both are controlled by simply (and quickly) twisting one knob on an inexpensive linear manual potentiometer a fraction of a single turn.

Three-position (center off) SPDT switch S1 controls the pots’ chip select inputs, which determines which, either individually or collectively, pot section(s) respond(s).

Figure 1 The 256-step (8-bit resolution) dual-channel DPOT U2 can be programmed up/down by a partial turn of R1, with either channel X or Y responding alone (per switch S1’s position on X or Y), or with both ganged together (per S1 in the X and Y position).

Here’s how it works.

Current to frequency converter U1 generates dpot clock pulses at 20 Hz-0-20 Hz, controlled by the sum of Q1+Q2 collector currents set ~100 nA to 5 µA by R1. The Vpp amplitude of the sawtooth waveform on C1 is set by the R7/R8 and 555 internal dividers to ~250 mV. A1 sets the up/down step direction.

When the sum of collector currents is less than 500 nA, R4 forces the step frequency to zero. This condition corresponds to R1’s wiper centered at (or near) the midpoint of travel, acting with the antilog behavior of the transistor collector current vs Vbe. Meanwhile, 5 µA yields 20 Hz. This occurs when R1 approaches either its clockwise or counterclockwise limit.

Figure 2 shows the relationship of step rate vs R1 position.

Figure 2 This graph shows the dpot step rate and direction as a function of R1.

Comparator A1 sets the dpot step direction to UP if R1’s wiper is clockwise of midpoint, DOWN for counterclockwise. Q1 sources the majority of current to U1 for UP steps, with Q2 taking over for DOWN.

Programming a desired setpoint is accomplished thusly:

Turn R1 in the desired direction until stepping begins at a rate appropriate to the size of the intended setpoint change.
As the new setpoint is approached, gradually slow the step rate to a dead stop by returning R1 toward the midpoint.

With a “bit” of practice, a U2 setpoint change of 256 steps can be accurately completed in less than 20 seconds. If you need more speed than that, the necessary modification is trivial. Just reduce C1:

Max step rate = 20Hz/C1 for C1 in µF.

If more ganged channels are needed, additional pots can obviously be added, virtually ad infinitum. Thus concluding with the highfalutin Latin phrase of the day. You’re welcome.

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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