Light-barrier alarm that latches: LDR + 555 bistable
Open in the lab ▸Make my own copy
Shine a light on the LDR across a doorway. While the beam is there, the LDR is a few kΩ and holds the 555’s TRIG input high. Break the beam and TRIG drops below a third of the supply: the 555 sets, and stays set even when the beam comes back. The buzzer and the red LED keep going until someone presses RESET. The 555 is wired as a bistable (a flip-flop): TRIG sets it, RESET clears it, THR is tied to ground so it never clears itself. Here the beam is shown broken (the LDR is covered): press RESET, then uncover the LDR and cover it again.
What is on the bench
26 parts, 35 wires.
| Resistor | 5 |
|---|---|
| LED | 2 |
| 9 V battery | 1 |
| Photoresistor (LDR) | 1 |
| 555 timer | 1 |
| Push button | 1 |
| NPN transistor | 1 |
| Active buzzer | 1 |
Put it on your page
The project runs live inside the frame: readers can press buttons, turn knobs and read the meters.
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Link
https://zencircuitlab.com/p/zclbeamalarm
Published by its author under the lab's community terms: it may be opened, copied and changed by anyone. Simulation results are for learning; check a real build against the parts' datasheets.