Why every button needs this resistor
Why a button input needs a pull-up: a 10k resistor holds the input high until you press, and an LED shows the state.
Free build sheet
The parts, every wire with its length and bend points, a bend guide, both sides of the board and checkboxes to track your progress. Print it or keep it open at the bench.
Open the build sheetWhat you need
9 parts on a perfboard 16 × 28 holes, plus hookup wire.
| Qty | Part | Where to buy |
|---|---|---|
| 1 | Ceramic Cap100nF C1 | Buy Ceramic Cap on Amazon |
| 1 | Power Input (2-pin) J1 | |
| 1 | Signal+Power Header (3-pin) J2 | |
| 1 | Screw Terminal 2P (5.08mm) J3 | |
| 1 | LED LED1 | Buy LED on Amazon |
| 1 | NPN Transistor (TO-92) Q1 | Buy NPN Transistor (TO-92) on Amazon |
| 2 | Resistor (axial)10k R1, R2 | Buy Resistor (axial) on Amazon |
| 1 | Resistor (axial)330 R3 | Buy Resistor (axial) on Amazon |
Check every pin against the datasheet of the exact part in your hands before wiring.
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Cut list
12 wires, 312 mm (12.3 in) in total, in the order the build sheet cuts them. Wire numbers match the build sheet. Each length includes 5 mm per end to go through the hole.
| Wire | Colour and net | Length | ||
|---|---|---|---|---|
| 6 | BlackIN E12 → G9 | E12 → G9 | 23mm | 13mm (90°) |
| 12 | BlackGND M11 → G21 | M11 → G21 | 51mm | 30mm (90°) |
| 11 | BlueGND K9 → L10 | K9 → L10 | 14mm | Straight |
| 5 | Blue5V J5 → K14 | J5 → K14 | 35mm | 8mm (90°) |
| 4 | Green5V C3 → J3 | C3 → J3 | 28mm | Straight |
| 10 | GreenGND K2 → J8 | K2 → J8 | 33mm | 10mm (90°), 25mm (90°) |
| 1 | RedCOL K24 → H24 | K24 → H24 | 18mm | Straight |
| 7 | RedIN H8 → M8 | H8 → M8 | 23mm | Straight |
| 3 | White5V C8 → C5 | C8 → C5 | 18mm | Straight |
| 9 | WhiteGND D3 → K3 | D3 → K3 | 28mm | Straight |
| 8 | YellowIN L9 → L5 | L9 → L5 | 20mm | Straight |
| 2 | YellowBASE E19 → F23 | E19 → F23 | 23mm | 15mm (90°) |
About this build
A pushbutton input done right. With the button open and no resistor, an input isn't high or low. It floats and picks up noise. A 10k pull-up holds the input at a solid high, and pressing the button ties it to ground through that same resistor, at 0.5 mA.
A 100nF ceramic on the input smooths out contact bounce. A PN2222A NPN transistor and an LED with a 330 ohm resistor show the state right on the board, and the input also leaves on a 3 pin header so a microcontroller can read it.
The button itself is off the board, on a 2 pin screw terminal. The transistor is a TO-92 part and lead orders vary, so wire it from the build sheet and the PN2222A datasheet.
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