Practical case: Pull-up with button and resistor

Materials

• 1 × Resistor 10 kΩ, 1/4 W (R1, pull-up)
• 1 × Momentary pushbutton, normally open (S1)
• 1 × 5 V DC supply (e.g., USB 5 V or bench supply)
• 1 × Breadboard
• 6 × Jumper wires
• 1 × Multimeter (DC volts mode)
• Optional: 1 × Microcontroller board (to read the input)

Wiring guide

• Abbreviations used in the schematic:
• V_IN: voltage at the node where R1, S1, and the MCU input meet. Measure here with the red probe.
• COM: common reference for measurements at ground. Connect the black probe here.
• Connect the power supply +5 V to the breadboard’s positive rail and its ground to the ground rail.
• Place R1 so that one end goes to +5 V and the other end creates a node on an empty row (this node will be V_IN).
• Place S1 so that one side connects to the V_IN node and the other side to the ground rail (GND).
• If using a microcontroller, connect the input pin to the V_IN node and the MCU ground to the same ground rail.
• For measurements, connect the multimeter black lead to the COM dot (ground) and touch the red lead to the V_IN dot.

Schematic

          +5 V (o +3.3 V)
               │
               │
              ┌┴┐
              │ │        R1 = 10 kΩ (pull-up)
              │ │
              └┬┘
               │● Vn
               │──────────────► U1 pin digital (entrada)
               │
              ┌┴┐
              │ │        S1 = Pulsador NA
              │ │
              └┬┘
               │
              ┌┴┐
              │ │        R2 = 100 Ω (sensor de corriente, opcional)
              │ │
              └┬┘
               │● VRS−
               │
              GND
               ▲
               ● VRS+

Measurements and tests

• Before powering:

  • Verify R1 is between +V and the V_IN node, not to GND.
  • Verify S1 is between the V_IN node and GND, not to +V.
  • Confirm the supply polarity (+V to positive rail, GND to ground rail).

• Voltage at V_IN (idle, button released):

  • Set the multimeter to DC volts.
  • Black probe to ●COM (GND), red probe to ●V_IN.
  • With S1 not pressed, V_IN should read close to +5 V (within a few tens of millivolts of the supply).

• Voltage at V_IN (button pressed):

  • Keep probes as above.
  • Press and hold S1. V_IN should drop to about 0 V.
  • Release S1; V_IN should return to +5 V immediately.

• Current check (sanity estimate):

  • When S1 is pressed, current through R1 ≈ 5 V / 10 kΩ = 0.5 mA (well within safe limits for a 1/4 W resistor).

• If using a microcontroller:

  • Configure the input pin without internal pull-up/pull-down first.
  • Read the pin: HIGH when S1 released, LOW when S1 pressed.
  • Enable the MCU’s internal pull-up and repeat tests (you may remove R1 to compare behavior).

Common mistakes

• Using too small a resistor (e.g., 220 Ω), which wastes current and may stress components when the button is pressed.
• Wiring the button across +V and GND directly; this shorts the supply when pressed.
• Forgetting the ground reference for the multimeter, leading to erratic readings.
• Not sharing grounds between the microcontroller and the external circuit.

Safety notes

• This is a low-voltage circuit, but avoid shorting +5 V to GND.
• If using a bench supply, set current limit to around 100–200 mA.

Improvements and variations

• Debounce: add a small capacitor (e.g., 100 nF) from V_IN to GND to reduce contact bounce, or implement software debouncing.
• Pull-down variant: swap positions so R1 goes from V_IN to GND and the button connects V_IN to +V, inverting logic.
• EMI robustness: add a series resistor (e.g., 100 Ω) between the node and the MCU input to limit spikes.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are