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Button with a pull-up resistor

Released, R1 pulls the output up (LED on); pressed, S1 takes it to ground (LED off)

Button with a pull-up resistor, simulated on a breadboard

Tap a part to see its voltages and currents. It is a model, not a measurement.

Predict, measure, explain

  1. Predict The LED is on before you press. What happens when you press S1?
  2. Measure Press S1 and watch the LED. Then change R1 to 10 kΩ without pressing.
  3. Explain
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    It goes out: the button joins the LED's side to ground and the current takes that path. With 10 kΩ the LED only gets 0.31 mA: the pull-up is weak on purpose.

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