Electronic Design

Manually Operated Digital Pot Doesn't Need A Microprocessor

Digital potentiometer ICs such as the MAX5160 and MAX5161 are good replacements for mechanical types. But these ICs are designed to work with microcontrollers. In this manually operated digital potentiometer, two pushbuttons control the wiper resistance without a microcontroller (Fig. 1).

To mask the effect of contact bounce in mechanical switches S1 and S2, a dual switch-debouncer (IC1) is used. A microprocessor-supervisor device configured as an oscillator (IC2) provides a clock signal to the digital potentiometer. Initially, OUT1, OUT2, and RESET are high. If S2 is momentarily de-pressed and released, OUT2 causes the RESET input of IC2 to go low. When S2 is released, OUT2 and RESET return high. The wiper position is controlled by a 5-bit resistive ladder internal to IC3 (similar to that of a 5-bit digital-to-analog converter). Therefore, this single cycle of oscillation lowers the wiper resistance by 1/32 of the total resistance range of the potentiometer.

IC2 continues to oscillate if S2 is depressed and held. As a result, the resistance is lowered in a series of steps (Fig. 2). Similarly, depressing S1 increases the potentiometer's resistance. Capacitors C1 and C2 set the frequency of oscillation at approximately 3 Hz.

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