Front-panel push button controls can conveniently adjust a PCB-mounted internal potentiometer’s resistance.
Digital potentiometers in conjunction with MCUs find common use in analog circuits. The processor can adjust the gain of an amplifier, frequency of an oscillator, time delays, etc. using a digital potentiometer whose resistance is controlled via software. However, plenty of MCU-less applications also exist; in some cases, they implement conventional potentiometers mounted in system front panels for manual resistance control purposes.
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Imagine the convenience, however, if two push buttons are alternatively provided in the front panel to increase or decrease the resistance of each potentiometer, which can then be alternatively located on the PCB within the system. Such an approach may, for example, be a welcome capability in audio, high frequency, automotive, and other applications. This is exactly what the circuit shown in Figure 1 does.
Figure 1 Each press of the “INC” (increment) push button increases resistance by one step. Similarly, each “DEC” (decrement) push button press decreases resistance by one step. The power supply and ground pins of the various ICs are not shown in this schematic.
This simple design uses only two ICs to adjust the digital potentiometer, shown here as an example 128-step 10K ohm AD5220BN10 (PDF) and labeled U3. A more complex implementation can include multiple versions of the circuit, each based on a different-value potentiometer, and each separately managed via unique control of the CS, CLK and U/D pins.
U3 pins A and B are resistor ends, and W is the “wiper”. How does the circuit work? When you momentarily push the “INC” (increment) push button, a pulse is generated which sets flipflop U2A. Its Q output is connected to the U/D pin of the digital potentiometer. Hence U3 is set to the “up” tap direction and the wiper advances by one position.
For every subsequent “INC” push button press, the wiper further advances by one step. Similarly, when you momentarily push the “DEC” (decrement) push button, flipflop U2A is reset, causing U3 to subsequently reverse course and adjust resistance in the “down” direction. Additional “INC” push button presses further decrease the wiper, by one step each time.
And how can you test the circuit? Connect U3 pin A1 to a 5V supply and B1 to ground. Now connect a digital voltmeter to wiper W1. If you press the “INC” push button, the voltage should increase, and if you push the “DEC” push button, the voltage will decrease, if the circuit is functioning correctly. The following video shows a simulation of the circuit, complete with my narration.
Jayapal Ramalingam has over three decades of experience in designing electronics systems for power & process industries and is presently a freelance automation consultant.
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