PAGE 067 OF 150 · DRIVING WITH SWITCHES & TRANSISTORS
Pull-Up and Pull-Down Resistors
They hold an input at a known level.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
The floating input problem
An input pin connected to nothing picks up noise and reads random highs and lows. A pull-up or pull-down resistor gives it a definite default state.
- Pull-down: resistor to ground, button connects to supply. Reads low until pressed.
- Pull-up: resistor to supply, button connects to ground. Reads high until pressed.
Values
10 kilohms is a standard choice. Many microcontrollers include internal pull-ups of around 20 to 50 kilohms that you can enable in software, saving a part.
REMEMBER
A transistor lets a tiny signal control a much larger LED current.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
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