Circuits / Getting started
Button with a pull-up resistor
Released, R1 pulls the output up (LED on); pressed, S1 takes it to ground (LED off)
Tap a part to see its voltages and currents. It is a model, not a measurement.
Predict, measure, explain
- Predict The LED is on before you press. What happens when you press S1?
- Measure Press S1 and watch the LED. Then change R1 to 10 kΩ without pressing.
- Explain
Show the answerHide the answer
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.
What do you need?
- Resistor 1 kΩSearch on Amazon
- Push buttonSearch on Amazon
- Resistor 330 ΩSearch on Amazon
- Green LEDSearch on Amazon
- Jumper wires × 2Search on Amazon
- BreadboardSearch on Amazon
What does a pull-up resistor do?
A pull-up resistor holds a point at the supply voltage until something pulls it down. Here R1 (1 kΩ) connects the output to 5 V, and the button S1 connects it to ground. Released, the output sits high and lights the green LED through R2 (330 Ω) at about 2.3 mA. Pressed, the button ties the output to 0 V: the LED goes off and 5 mA flows from R1 straight into ground. That current is the price of the pull-up, which is why real ones are usually 10 kΩ or more. Try R1 at 10 kΩ: the waste while pressed drops to 0.5 mA, but the LED gets only 0.31 mA, because a weak pull-up can hold a pin but cannot feed a load. This is the wiring behind INPUT_PULLUP on a microcontroller: a pressed button reads LOW.
Values from the Protoboard ISO simulation: a model, not a bench measurement.
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