Before smart cameras, wireless sensors, and connected home security systems became commonplace, protecting a room could be as simple as a laser beam and a clever analog circuit. This classic Elektor project shows how a handful of components can be combined into a reliable beam-break alarm.

The Circuit

The design uses a laser pointer aimed at a BPW34 photodiode. As long as the photodiode receives the laser beam, the circuit remains idle. When the beam is interrupted, the photodiode's resistance changes, causing a TL072 configured as a comparator to switch state and activate an alarm. An adjustable threshold lets the circuit be tuned for different lighting conditions, while built-in hysteresis and filtering help prevent false triggering from electrical noise or brief interruptions.
 
The laser alarm circuit is simple and could be assembled on a piece of breadboard.
The circuit is simple and could be assembled on a piece of breadboard.
Although the original design is over two decades old, its engineering principles have stood the test of time. Modern optical sensors used in industrial automation, robotics, and laboratory equipment still rely on the same concepts of light detection, threshold comparison, and noise immunity. Today's engineers might replace the comparator with a dedicated device or add a microcontroller for networking and advanced features, but the underlying circuit remains an excellent example of elegant analog design. Revisiting projects like this is a reminder that while technology advances, solid engineering fundamentals never go out of style.

“After assembling the circuit and testing it, you should mount it in a black box that has just a small hole,” the designer, Dimitris Kouzis-Loukas, explained. You may decide to put the laser in the same box but only if you are sure there is no way the photodiode can ‘see’ the laser beam directly. The small hole should be filled with a black drinking straw so that only light from the direction of the laser beam can enter. With the appropriate setup of the box and the mirrors, the laser beam is so intense that even direct sunlight cannot affect the operation of the photodiode.
 
Laser alarm 2
When the beam gets interrupted by an object, the resistance of a
photodiode increases, and an alarm is activated.

The Laser Alarm Project

The original article, “Laser Alarm,” appeared in Elektor July/August 2006. Take a look at the article.
Editor's Note: The article first appeared in a 2006 edition of Elektor Magazine. Some parts and components might not be available at this time. Still, we think the article and the project are interesting and informative.

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