A laser tripwire alarm, like in the movies
Break the laser beam and a red LED and buzzer latch on until you press reset. A photoresistor and one 555 timer.
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
14 parts on a perfboard 15 × 29 holes, plus hookup wire.
| Qty | Part | Where to buy |
|---|---|---|
| 1 | Ceramic Cap100nF C1 | Buy Ceramic Cap on Amazon |
| 1 | Ceramic Cap1uF C2 | Buy Ceramic Cap on Amazon |
| 1 | Power Input (2-pin) J1 | |
| 2 | LED LED1, LED2 | Buy LED on Amazon |
| 1 | Potentiometer Q1 | |
| 1 | Resistor (axial)1k R1 | Buy Resistor (axial) on Amazon |
| 1 | Resistor (axial)100k R2 | Buy Resistor (axial) on Amazon |
| 1 | Resistor (axial)560 R3 | Buy Resistor (axial) on Amazon |
| 1 | Resistor (axial)680 R4 | Buy Resistor (axial) on Amazon |
| 1 | Photoresistor (LDR) U1 | |
| 1 | DIP-8 U2 | |
| 1 | Tactile Button 6mm U3 | |
| 1 | Piezo Buzzer U4 |
Check every pin against the datasheet of the exact part in your hands before wiring.
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Cut list
22 wires, 517 mm (20.4 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 | ||
|---|---|---|---|---|
| 18 | BlackGND C22 → E25 | C22 → E25 | 23mm | 13mm (90°) |
| 6 | BlackARM E15 → H11 | E15 → H11 | 28mm | 15mm (90°) |
| 12 | BlackVCC G15 → K12 | G15 → K12 | 28mm | 15mm (90°) |
| 11 | BlueVCC D12 → F14 | D12 → F14 | 17mm | Straight |
| 17 | BlueGND D21 → D17 | D21 → D17 | 20mm | Straight |
| 5 | BlueSENSE G9 → I15 | G9 → I15 | 35mm | 8mm (90°), 23mm (90°) |
| 4 | GreenSENSE G7 → I7 | G7 → I7 | 15mm | Straight |
| 22 | GreenGND K6 → I5 | K6 → I5 | 18mm | 10mm (90°) |
| 10 | GreenVCC B6 → B12 | B6 → B12 | 25mm | Straight |
| 16 | GreenVCC H3 → M6 | H3 → M6 | 30mm | 18mm (90°) |
| 1 | RedTRIM D10 → D8 | D10 → D8 | 15mm | Straight |
| 13 | RedVCC C5 → H5 | C5 → H5 | 23mm | Straight |
| 19 | RedGND F17 → F12 | F17 → F12 | 23mm | Straight |
| 7 | RedALARM E23 → J25 | E23 → J25 | 28mm | 18mm (90°) |
| 21 | WhiteGND L7 → L8 | L7 → L8 | 13mm | Straight |
| 3 | WhiteRED L27 → L24 | L27 → L24 | 18mm | Straight |
| 15 | WhiteVCC J20 → L23 | J20 → L23 | 23mm | 10mm (90°) |
| 9 | WhiteALARM M18 → I11 | M18 → I11 | 38mm | 23mm (90°) |
| 2 | YellowGREEN E27 → E24 | E27 → E24 | 18mm | Straight |
| 20 | YellowGND G11 → K8 | G11 → K8 | 23mm | 16mm (45°) |
| 8 | YellowALARM L25 → L19 | L25 → L19 | 25mm | Straight |
| 14 | YellowVCC J11 → K19 | J11 → K19 | 33mm | 8mm (90°) |
About this build
A laser tripwire alarm. A laser pointer keeps a photoresistor lit, which holds the sense voltage low. Break the beam and the voltage climbs past two thirds of the supply, which flips a 555 wired as a latch.
Once it trips, the red LED and the piezo buzzer stay on even after the beam comes back, until you press the reset button. A green LED shows it's armed, and a trim knob sets the trip point.
The parts library has no buzzer, so the video draws the buzzer footprint in the app first. It runs on 9V on a 15 by 29 perfboard.
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