Laser Tripwire Alarm with One 555 Timer

Laser Tripwire Alarm with One 555 Timer (It Won’t Stop Until You Reset It!) Break the beam and this alarm goes off, and it keeps going even after you step away. In this video we build a laser tripwire using a single 555 timer in bistable mode. First we simulate it in Falstad to see exactly how the voltages change, then we demo the real circuit on a breadboard. You’ll learn: How an LDR voltage divider detects a broken laser beam Why the 555 triggers below one-third of the supply (1.67V on 5V) How grounding pin 6 turns the 555 into a latch, or one-bit memory How the reset pin works, and why it can’t be silenced while the beam is blocked A simple rule of thumb for setting the alarm’s sensitivity ⏱️ Chapters 0:00 Break the beam 0:24 How it works in Falstad 3:18 The real circuit on the breadboard 5:20 Wrap-up 🔧 Components used NE555 timer IC LDR (light-dependent resistor) in a small black tube 1k resistor + 10k trimpot (103) 10k resistor (reset pull-up) + push button 10nF capacitor (pin 5) LED + 330Ω resistor Active piezo buzzer 650nm 5V laser dot module 5V supply, breadboard and jumper wires 📐 Key numbers Beam on: LDR about 500Ω, pin 2 at about 4.6V, so the alarm stays quiet Beam blocked: LDR about 38k, pin 2 drops to about 0.7V, so the alarm triggers Trigger point: pin 2 below 1.67V (one-third of 5V) Rule of thumb: the alarm trips when the LDR rises above about twice the bottom resistance ⚠️ Safety: never look into a laser beam or point it at anyone’s eyes, even a low-power module. Falstad circuit simulator: www.falstad.com/circuit/ 💬 Where would you put a laser tripwire? Tell us in the comments! ▶️ Next video: LDR Light Theremin, turning light into sound. #electronics #555timer #lasertripwire #securityalarm #LDR #bistable #diyelectronics #falstad #breadboard #electronicsforbeginners