The market for systems to protect objects from burglaries and unforeseen incidents is saturated with sensors that help establish comprehensive control over housing. However, not every device is capable of providing reliable security, and connecting low-quality, cheap equipment leads to unexpected problems. As an alternative to motion sensors, a simple and trouble-free laser alarm, which is triggered when an object enters the beam spectrum.

What is the operating principle of a laser beam alarm?

Alarms with a laser beam are usually purchased in ready set, but if desired, you can make them yourself without spending a lot of effort and money. The entire operating principle of a laser alarm is associated with a special infrared beam, which is directed at a certain angle to the opposite wall of the room where the photocell is mounted.

Any object falling within a given spectrum creates refraction that can send a signal to a special detector. After reporting a violation, the built-in speaker will notify residents or security of the intrusion.

The laser detector kit includes the following construction materials:

• Relay;
• The simplest microcircuit from a flashlight;
• Photocell;
• Power unit;
• Resistor;
• Detector;
• Generator.

Due to the fact that the laser light flux does not scatter and is constantly directed in one direction, using a reflector system you can create a varied pattern that cannot be avoided. Small pieces of mirrors located at a certain angle at different ends of the room are used as reflectors.

The process of assembling laser elements and parts

The assembly principle consists of sequential soldering individual elements alarms to the board. First of all, you need to decide on the place where the laser alarm and photocell will be installed. Most often, such mechanisms are mounted in the lower part of the room at a level of 30 cm from the floor, which allows you to hide the device from prying eyes.

The video shows an experiment with laser signaling:

The installed laser on one side of the wall is connected to a relay and power supply, and in opposite place, at a distance of no more than 10 m, a photocell is attached with the expectation that the beam will fall vertically onto the lens. When an object enters the spectrum of the beam, the photocell begins to heat up, the relay transmits a signal to the resistor, and the latter to the detector.

The siren acts as a repeller, emitting a signal with a power of up to 100 dB, which can be heard at a distance of about 100 m.

A conventional lithium battery should be used as a power supply, since it will consume a minimum amount of energy and is practically necessary to issue an alarm signal.

Modern radio amateurs propose to integrate a communication module into the functionality of the system, which will make it possible to send an SMS or voice message to a specific number, which will not only scare away the robber, but also try to detain him.

In this article we will tell you how to make a laser alarm. The idea is to make such an alarm as shown in movies about superheroes.

This laser alarm simulates a tripwire when a thin wire is stretched 20 centimeters above the ground (floor). When an intruder enters the protected area and pulls a tripwire, an alarm is activated. What if you do a laser alarm and a tripwire at once? That's right, this will turn out to be quite interesting.

The alarm system discussed in the article is primarily intended for use in airsoft, but it can also be used to protect residential premises, garages, etc.

The principle of operation of a laser pointer alarm is quite simple.

The PIC16F688 microcontroller controls the laser module that sends the beam, which must be returned through the mirror. The reflected beam is received by a photoresistor. The PIC16F688 microcontroller checks the state of the photoresistor and if the laser beam is blocked, it activates a sound signal.

The laser signaling circuit is quite simple and is shown in the following figure:

To change operating modes, use switch S3 - select the operating mode: laser and / or stretching:

1. Laser + stretching.
2. Stretching.

The photoresistor must be placed inside the tube to prevent it from being exposed to sunlight or other light sources. To eliminate the possibility of accidental triggering of the laser alarm.

And the laser pointer needs to be modified by soldering the wires to the place where the batteries are installed.

The following figure shows the laser module and photoresistor tube.

To combine both elements, they must be aligned and glued together, for example, cold welding or plastic. Thus, they are assembled parallel to each other.

For the stretch version, a microswitch is used, located in the upper part of the laser alarm housing. The mikrik lever protrudes above the body, through the window, so that fishing line, thread or thin wire can be hooked onto it.

Now you can finally finish the case by making holes for the LEDs, power button, mode switches and siren.

When installing the emitter with the receiver, please note that it must be possible to adjust this part of the laser alarm.

The alarm uses a modified portable PC beeper because it is quite small and very loud. But his electronic circuit must be modified so that it can be connected to the PIC16F688 microcontroller.

Upon completion of assembly, it is necessary to check the functionality of the alarm from the laser pointer.

The scheme works as follows. When the power is turned on, the device enters setup mode, checks the laser and lets us know if the reflected beam has returned correctly to the receiver. At this point you need to adjust the mirrors. If the reflected beam is adjusted correctly, the red LED lights up.

After adjusting the beam, you need to press the button 1 time to exit the setup mode and enter the operating state.

If the laser beam is blocked, the PIC16F688 microcontroller will turn off the laser and activate the siren.
The siren will work until you press the button. Voices)

Would you like to make a laser tripwire alarm like you might have seen in spy action movies? It is quite possible to assemble it yourself from inexpensive and quite accessible components.

The circuits presented in this material will help you make a device that can detect the movement of people or other objects as they pass through the laser beam and sound an alarm if necessary.

The device consists of two circuits: a laser beam emitting circuit and a laser beam receiving circuit. The receiver circuit includes an electromagnetic relay for connecting any external electrical load, for example, a spotlight. home alarm systems, etc.

Laser beam radiation scheme

This circuit is based on a standard red laser LED with a wavelength of 650 nm and a power of 5 mW. The laser diode is powered by a source voltage of 5 V. Two auxiliary components are connected in series with it: diode D1 (1N4007) and resistor R1 with a resistance of 62 Ohms. To obtain a laser diode, you can disassemble an unnecessary but working laser pointer if you do not want to go to an electronic components store to purchase a separate laser diode.

Laser beam reception circuit

The basis of the laser beam reception circuit is a 5 mm LDR photoresistor. This component is used to control a relay circuit, which is activated by silicon thyristor T1 (BT169). Diode D2 (1N4007), connected in antiparallel to electromagnetic relay RL1, acts as a conventional diode to protect the electronics from damage caused by the back-EMF pulse of the relay coil when thyristor T1 turns off. Note that the relay must be energized, that is, its contact must be closed when the laser beam is not shining on the photoresistor. You can use the S1 power switch to turn your laser alarm on or off.

Installation of laser tripwire alarm

If you are planning to secure the path on your stairs, then it is better to install a laser alarm as shown in the picture below.

First of all, try to place the emitter module and the receiver module at the top of the stairs with a small distance between them. Then direct the laser beam from the emitter to the reflector located at the bottom of the stairs and align it with the light sensor of the laser beam receiver. On the other hand, if you want to protect a wide area, it will be better to use a series of reflectors or mirrors along with a laser alarm system.

Using a toy with a laser, which, as you know, costs a penny, you can create an alarm and install it at the entrance to your apartment, garage, or yard. There are almost no expenses, and the benefits are disproportionately large.

To assemble the structure, you will need a laser pointer and several radio components. The operating principle of the alarm is based on the sensitivity of a photoresistor that reacts to a laser beam.

This video shows how to assemble a laser alarm. To do this you will need a pointer and a few parts. The device circuit is assembled on a 555 timer. To detect laser radiation, we need a photoresistor. It is connected to a second resistor to form a voltage divider. The resistance of the second resistor should be comparable to the photoresistor. In our case it is equal to 100 ohms. When a photoresistor is not irradiated, its resistance increases. This leads to an increase in voltage on 6 pins of the microcircuit. As a result, a logical zero appears at the output of the microcircuit and the beeper turns on.

You can turn off the speaker and reset the system by switching the logical analysis of the speaker to trigger. Switching back, we return the circuit to ready mode.

To check, we will assemble the circuit on a breadboard. If everything works correctly, we will assemble it on the board. Let's place the photoresistor on long legs to be able to adjust the position after installation. Attach the battery housing to the board with a glue gun. We secure the loose wires around the board with an elastic band. It's time to install the system. In the simplest case, it would be better to be like a stretcher located on one side of the door. Located opposite each other. First, let's fix the alarm. We use adhesive tape to secure the pointer button in the on state. We will mount the pointer in place. It is better to set the settings exactly to the center of the photo resistor. After this, turn on the system. Anyone entering will activate the alarm. A single stretch works great. With the help of several mirrors you can cover the entire room with rays. Let's fix the pointer on one of the surfaces. The beam is directed at one of the walls. Continue adding the mirror. The main thing is that the latter directs the beam to the photoresistor.

Since the system consists of a single continuous laser, any obstacle in the way will trigger the alarm.

A pleasant advantage of such an alarm is the ability to cover a significant space if supplemented with a system of mirrors. The beam will cross space through many channels, controlling the smallest areas of the site.

To increase operating time, replace the batteries with more powerful ones or rechargeable batteries.

Maybe you would like to learn how to understand the principle of operation of electrical circuits using an example?

This laser pointer alarm, which you can assemble with your own hands, is similar to the one we can see in various films. The alarm uses a laser beam to protect your valuables and property.

Essentially, when any obstacle (person or animal) appears between the beam and the sensor, the resistance of the photodiode increases and, as a result, the output of the device appears high level voltage, which can then activate a siren or some actuator.

The receiver current consumption is about 10 mA. The laser pointer and receiver can be placed in a common housing, and the laser beam can be directed to a photodiode using a mirror.

Description of laser alarm

In the diagram we see the operational amplifier TL072 (IC1.A) configured as a voltage comparator. It compares the reference voltage at the inverting input of the op-amp (pin 3), coming from the adjustable resistor divider on P1, R4, and the voltage supplied to the direct input of the op-amp (pin 2) from the divider, consisting of photodiode D1 and constant resistor R3.

When the laser beam is interrupted, the voltage at pin 2 of the comparator drops below the reference voltage at pin 3. This results in the op amp output 1 being high. As mentioned above, this signal can be used to activate a siren, computer, or spotlight that may deter an intruder.

Resistor R2 provides hysteresis to prevent circuit instability when the voltages at both inputs of the comparator are equal. Capacitor C1 is designed to ignore short-term interruptions of the beam, for example, by flying insects. If you want the signaling sensitivity to be higher, you can reduce the capacitance of capacitor C1 to 1 µF.

The circuit is simple and can be assembled on a small piece of breadboard. Once the circuit is assembled and tested, you must place it in a suitable housing that has a hole for the photodiode. It is advisable to first install the photodiode in a black tube in order to prevent the entry of an extraneous light source.