Read multiple switches and a potentiometer setting with one microcontroller input pin

Use a single ADC input pin to sense pushbuttons as well as a potentiometer.

Kevin Fodor, Palatine, IL; Edited by Martin Rowe and Fran Granville -- EDN, April 22, 2010

Read More Design Ideas

Selecting the resistor values is a multistep process, and a spreadsheet aids in performing the calculations. Say, for example, that you want 5-kΩ potentiometer R_ADJ to produce a 0 to 100% value into the microcontroller. Typically, you would map the sampled value of 0 to 255 into a 0 to 100 value to represent a percentage. However, by selecting the values of bias resistor R_BIAS, you arrive at a direct analog input centered on the 0 to 255 range of the ADC-for example, 78 to 178.

To compute the appropriate high- and low-side bias-resistor values, the following equations solve this circuit as a simple voltage divider:


Substituting and solving for R_BIAS and given that the maximum voltage reports a value of 255, the maximum low voltage reports a value of 78, the maximum high voltage value reported is 178, and R_ADJ has a value of 5 kΩ yield the following equation:


The computed value of R_BIAS is 3875Ω. Using a standard value of 3.3 kΩ, the potentiometer's input ranges from 73 to 182. This range yields a larger dynamic range than you need but allows for a guard range between the potentiometer's values and the pushbuttons' values. Because the position of R_ADJ affects the overall resistance the circuit sees when you press either switch, the microcontroller must interpret a range of values for each switch. To determine the switch resistance, R_SW, for either S₁ or S₂, you use a parallel-resistor network at both extremes of the potentiometer's position.

When you press S₁ and R_ADJ is at the maximum position, the effective resistance of the bottom leg of the divider is R_SW in parallel with the series combination of R_ADJ and R_BIAS. At the minimum position, the effective resistance is R_SW in parallel with R_BIAS:


You determine the value when you press S₁ by evaluating the voltage divider that R_BIAS and R_RFFMAX form:


Observe that when R_ADJ is at its maximum value and you press S₁, it must produce a value less than the smallest value R_ADJ produces by itself to uniquely determine that you have pressed the switch. So the maximum effective resistance, R_EFFMAX, must produce a value less than the maximum low voltage, as the following equation shows:


Substituting and solving this equation for the switch resistance yields:


Using the spreadsheet to compute the switch resistance yields 1558Ω, and you can choose a nominal 1.5-kΩ resistor. This selection causes S₁ to produce a range of 28 to 71 when you press it, depending on the potentiometer's position. Likewise, choosing the same value for S₂ produces a range of 184 to 227. These ranges are bands of values that you can use to determine which switch you pressed regardless of the potentiometer's position. Although selecting symmetrical resistor values is not necessary, it minimizes the number of calculations you need to perform and simplifies the design. Furthermore, selecting smaller series switch resistors opens the guard range between them and the potentiometer, which may be desirable if the resulting values are too close together. The microcontroller uses a small subroutine, Listing 1, to determine both switch positions and the potentiometer's setting.


Using this technique with four pushbuttons and one potentiometer is well within reason. Experimenting with the spreadsheet helps make quick work of determining just the right series-resistor values for each switch and its output range.