This type of alarm is one of the components of modern security systems. Their advantage is their reliability - they are practically unhackable and impossible to bypass. Thanks to laser signaling, the level of security of any object, in comparison with traditional methods and devices, increases.

Laser security systems have many advantages:

• Mobility: modules are easily transported from place to place, they can be located in different places;
• Lasers are easy to hide so that the criminal does not know about their presence until security officers appear;
• The elements that are part of the security system are easily combined with any interior and do not spoil it with their presence;
• Ability to work with sirens, with signal output to the remote control.

Their disadvantages include high cost; they are difficult to install and configure.

The basis of the alarm is a laser, which is included in the security system. The latter have a fairly high complexity, which is why they are expensive. You shouldn’t give up on them - you need to try to make a laser alarm with your own hands. As the developments of craftsmen show, this requires several devices and components that can be purchased quite cheaply. The result is an effective laser-based alarm system.

Homemade alarms use a laser and a photodetector. A beam emerges from the laser and is received by a photodetector. In this case, the resistance of the latter is close to zero. If the beam is blocked by something, the resistance of the photocell increases rapidly. This leads to an imbalance of the electronic circuit to which both devices are connected, to the activation of actuators and to the activation of an alarm.

If you want to make a laser alarm yourself, you should purchase: a laser pointer that will generate a laser beam; a photocell whose resistance changes under the influence of light flux; a relay that will connect, for example, a sound siren. A system cannot be made without tools and materials for soldering, wires, housing parts, and installation accessories.

The laser alarm circuit can be built based on the NE555 timer, which will control its operation.

The “positive” circuit from the power source is supplied to the “plus” of the sound siren; “negative” – to the 1st output of the timer. A jumper is installed between them through resistor R2 and photoresistor R3. From the jumper between the last elements there is a tap to the 6th output of the timer.

Further along the “positive” circuit, taps are arranged: through resistor R1 to the 2nd output of the timer and from it, through a breaker, to the “minus” of the siren; to the 4th, then to the 8th timer output. In addition, the 3rd output of the timer is connected to the breaker switch.

When a laser pointer beam falls on a photoresistor, its resistance is insignificant, therefore electric current flows through the first jumper of the circuit through resistor R2 and photoresistor R3. When the beam increases, the resistance of the photoresistor increases greatly and the flow of current through the specified jumper stops - it will go to the timer and from it to the siren, which with its sound will notify that someone has crossed the pointer beam.

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, a high voltage level appears at the output of the device, which can then activate a siren or some kind of 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 turn on a siren, computer or spotlight, which may deter the 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.

Fireworks to everyone! If there have been robberies in your area more than once or there is such a danger, and you want to sleep peacefully at night, then you have probably thought about the question: should I install an alarm?
But complex security systems are not always affordable, and you have to spend more and more money on installation and maintenance. True, there are also cheap alarms, but attackers have long learned to turn them off, so today I will show you how to make a simple and inexpensive laser security alarm yourself.

Laser signaling circuit

Since there are many circuits today, I showed you what I think is the most current one, using the very popular NE555 IC.

For assembly we will need the following components: piezo buzzer(which will emit a signal), two resistors(750 Ohm, 130 kOhm), microswitch, photoresistor and an integrated timer chip NE555.

A little about the NE555 timer

It was developed in 1972 by Signetics. It has a wide range of supply voltages: from 4.5 to 18 V, the output current reaches 200 mA, and the microcircuit itself does not consume much. The accuracy of the microcircuit does not depend on the supply voltage. There are a lot of elements inside the timer: about 20 transistors and many other parts.

The chip has eight legs:

1. Earth
2. Launch
3. Exit
4. Reset
5. Control
6. Discharge
7. Nutrition

It is important to remember that no more than 1/3 of the supply voltage should be supplied to the second leg (start), and 2/3 of the supply voltage to the sixth leg (stop)!

Let's return to our laser. The laser beam is directed at the photoresistor. When it is not irradiated, this leads to an increase in voltage on the sixth leg of the microcircuit, as a result of which the buzzer turns on. You can turn off the speaker by pressing the microswitch. Let's watch a short video:

The choice of resistor R1 and R2 depends on the supply voltage. For example, my supply voltage is 4.5 V, so I chose resistors R1 - 130 kOhm, R2 - 750 Ohm. Since laser batteries run out quickly, the laser can be connected to a more powerful power supply, usually 4.5 V.

With the help of several mirrors you can cover the entire room with rays, the main thing is that the last mirror directs beam directly into the center of the resistor.

The laser alarm will always warn you when you are nearby, but you can also connect a more serious scheme: for example, with SMS notification. If interested, let me know. That's all, sleep well, have good dreams!

Best regards, Edgar.

In previous materials, we looked at many ways to make various alarms, but have not yet talked about making the most effective type of such security systems - laser. We hasten to correct the mistake and present a review of a video on making a homemade laser alarm.

What do we need:
- thyristor BT169;
- capacitor;
- 47k resistors;
- photoresistor or LDR;
- LED light bulb;
- laser.

First of all, we present a laser signaling circuit, according to which we will assemble it on the Breadboard.

Let's start the assembly with a thyristor, which we connect to the breadboard. On the thyristor there is a cathode on the left, an anode on the right, and a control electrode in the center. The diagram shows that the plus does not go directly to the thyristor, but necessarily passes through what we want to turn on. In this case, through an LED light bulb.

Therefore, the next step is to take the plus and apply it somewhere near the thyristor.

Then we feed this plus through the LED to the anode.

Let's look at the diagram. The cathode is immediately connected to negative. The cathode is on the left, so we connect the left leg of the thyristor to minus.

You also need to connect a photoresistor and a capacitor to minus. The author connects the capacitor to minus and to line 45 on the breadboard.

We connect the photoresistor to minus and to the same line.

Now we connect a resistor to the same line, but from the positive side.

Now these three need to be applied to the control electrode of the thyristor. To do this, we connect one contact of the wire to line 45, and connect the second to the central contact of the thyristor.

Let's test the alarm. To do this, you need to turn on the laser and point it at the photoresistor. After turning on the power to the breadboard, you can see that the LED is not lit. As soon as you move your finger between the laser and the photoresistor, the LED light will immediately light up. After this, the alarm will turn off only when the power is turned off.

The alarm works according to the following principle. Once the light coming from the laser is blocked, the photoresistor activates the entire circuit. The thyristor, in turn, turns on the buzzer or LED, which we used in this case, and the alarm goes off. Note that even when using a tweeter, you should not remove the LED light bulb, since in this case the alarm will turn on when the object blocking the laser is removed and the laser begins to shine on the photoresistor.

The proposed design may be useful for protecting non-permanent openings - windows, passage doors - or installed along the perimeter of an open object. The operating principle is triggered when the laser beam is interrupted by an intruder. Despite its simplicity, the system turned out to be quite reliable and economical, and the red laser operating in short pulse mode is practically invisible to the intruder.

Figure 1. Laser security system transmitter diagram

The transmitter, the diagram of which is shown above, consists of a short pulse generator and a current amplifier loaded onto a laser pointer, which is easy to find in almost any stall. The generator is assembled using elements DD1.1, DD1.2 and, with the ratings of the frequency-setting circuit indicated in the diagram, operates at a frequency of about 5 Hz. Next, the signal goes to the differentiating circuit C2R3, which generates short pulses with a duration of about 10 μs. This not only makes the device economical (one six-volt battery type 476 is enough for more than a year of continuous operation of the transmitter), but also invisible to the intruder.

Next, the pulses are equalized in shape and amplitude by elements DD1.3, DD1.4 and are sent to an amplifier assembled on transistor VT1. The amplifier is loaded onto a laser pointer, which is modified - the batteries are removed and the cone-shaped tip is removed. Resistor R7, connected in series with a resistor “imprinted” into the laser flashlight board itself (its nominal value is about 50 Ohms), is current-limiting for the laser LED, toggle switch SA1 turns on the continuous operating mode of the emitter, necessary for adjusting the transmitter-receiver system.

For greater economy and frequency stability, the DD1 microcircuit is powered by a voltage reduced to 3-4 V, the excess is suppressed by resistor R6. The average current consumption by the transmitter does not exceed 10 μA; the LED consumes about 20 mA per pulse, so there is no power switch. The transmitter remains operational (of course, with a decrease in range) when the supply voltage is reduced to 4.5 V.

The receiver, the circuit of which is shown in Figure 2, is assembled on an integrated circuit DA1, the sensitive element is a photodiode FD263-01. When replacing it, you need to take into account the length of the illumination pulses - the response time of the LED to illumination should be 5-10 times lower than the laser pulse duration.

In its place, for example, FD320, FD-11K, FD-K-142, KOF122 (A, B) and many others will be able to work. In response to each transmitter flash, the receiver generates a high-level CMOS amplitude pulse at the output. It can be used for further processing. To exclude external illumination, the photodiode must be installed in an opaque tube that acts as a hood.

Setting up the system comes down to its alignment. This is done visually, aiming the laser beam at the photodetector as accurately as possible. To do this, switch SA1 to switch the transmitter to continuous radiation. After completing the adjustment, both the receiver and the transmitter must be firmly secured. In principle, such a system does not require “micron” adjustment. During the experiments, it worked reliably when the photodetector, spaced 50 m from the transmitter, was located in a circle of radiation scatter with a diameter of 30 cm.

Based on materials from “Radio” No. 7, 2002.