Practical case: Series resistor for GPIO input protection

Materials

• 1 × Development board with microcontroller (for example, Arduino Uno).
• 1 × Push button (4-pin or 2-pin type).
• 1 × 10 kΩ resistor (R1) in series with the GPIO input.
• 1 × 10 kΩ resistor (R2) for pull-down (reference to GND).
• 1 × 5 V supply (can be the same from the Arduino, +5V pin).
• 1 × Breadboard.
• 6–8 × Male-to-male jumper wires.

Wiring guide

• Connect the +5V pin of the microcontroller (for example, Arduino Uno) to the positive power rail of the breadboard (label it as +5V).
• Connect the GND pin of the microcontroller to the ground rail of the breadboard (label it as GND).
• Choose a digital pin on the microcontroller, for example GPIO D2; this will be the GPIO node.
• Connect one side of the push button to +5V.
• Connect the other side of the push button to the VA node (this will be the intermediate node between the push button and the series resistor).
• Connect resistor R1 = 10 kΩ between the VA node and the GPIO node (input protection series resistor).
• Connect resistor R2 = 10 kΩ between the GPIO node and GND (pull-down so that the pin is at 0 V when the button is not pressed).
• Verify that there is no direct connection (just a wire) between +5V and GND.
• In the microcontroller code, configure pin D2 as a standard digital input (INPUT, without internal pull-up).

Schematic

            +5V
            |
            |
          [PULS] Pulsador
            |
            o VA node
            |
          [R1] 10kΩ  (serie protección)
            |
            o GPIO node (pin D2 MCU)
            |
          [R2] 10kΩ  (pull-down)
            |
           GND

Measurements and tests

• Basic check with multimeter:

  • Measure continuity of the circuit with the multimeter in continuity mode between +5V and GND: it should not beep (there must be no direct short-circuit).
  • Check the approximate value of R1 and R2 by measuring their resistance in ohms before assembling them (around 10 kΩ each).

• Voltage measurement at the GPIO pin (V_GPIO):

  • V_GPIO means “voltage at the GPIO pin” relative to GND.
  • Place the black probe of the multimeter on GND and the red probe on the GPIO node.
  • With the button RELEASED: you should measure approximately 0 V (thanks to resistor R2 as pull-down).
  • With the button PRESSED: you should measure approximately 5 V (the voltage is transferred through R1 to the GPIO pin).

• Current measurement through the series resistor (I_R1):

  • I_R1 means “current flowing through resistor R1”.
  • To measure current, the multimeter must be placed in series with the component:
    • Disconnect the end of R1 that goes to the VA node.
    • Connect the red probe of the multimeter to the VA node and the black probe to the free end of R1 (so the current passes through the multimeter).
  • Press the button and read the current: you should get a value on the order of 0.5 mA (5 V / 10 kΩ = about 0.5 mA).

• Functional test with the microcontroller:

  • Upload a simple program that reads pin D2 and sends via the serial port the text “BOTON: PULSADO” or “BOTON: SUELTO”.
  • Observe on the serial monitor that:
    • Without pressing the button: a low state (logical 0) is reported stably.
    • Pressing the button: a high state (logical 1) is clearly reported.
  • If the microcontroller resets or hangs when pressing the button, check the connections and the values of R1 and R2.

• Success criteria:

  • V_GPIO ≈ 0 V with the button released and V_GPIO ≈ 5 V when pressed.
  • I_R1 ≈ 0.5 mA (less than 1 mA) when the button is pressed.
  • The microcontroller correctly detects the state changes without erratic behavior.
  • There is no noticeable heating of the resistors or the microcontroller to the touch.

Common mistakes

• Connecting the push button directly between +5V and the GPIO pin without a series resistor:
• This often works, but reduces protection of the pin: if it is accidentally configured as a low output, the current can be very high (limited only by internal resistance).
• Forgetting the pull-down resistor (R2):
• The pin is left “floating” when the button is not pressed, causing random readings (spurious jumps between HIGH and LOW).
• Miswiring the 4-pin button:
• In many buttons the pins are paired internally; if it is wired in parallel instead of in series, the button may remain always active.
• Placing resistor R1 in the wrong place:
• R1 must go between the push button and the GPIO pin (VA node to GPIO node), not between +5V and the button, nor between GPIO and GND (that is the function of R2).

Safety and good practices

• Always work with a supply of 5 V or less for these basic exercises.
• Do not change the wiring with the circuit powered: first unplug the USB or the external power supply.
• Use resistor values of at least 1 kΩ when putting something in series with a GPIO pin, unless you have a clear reason to use less.
• Read in your microcontroller’s datasheet the maximum recommended current per pin and per port to know how much protection you need.

Possible improvements

• Add a small capacitor (for example, 100 nF) between the GPIO node and GND to further reduce button bounce (simple RC filter).
• Implement in the firmware a software debounce algorithm (for example, ignore changes within a 20 ms window).
• Replace R2 with a pull-up (between GPIO and +5V) and connect the other side of the push button to GND, with the same idea of adding R1 in series for protection.
• Try different values of R1 (4.7 kΩ, 22 kΩ, 47 kΩ) and observe how the current I_R1 and the noise picked up on the GPIO pin change.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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