Sometimes there is a need for an independent inexpensive security alarm, for example, for protection in the country. The use of industrial designs in such cases may not be economically feasible.

What do we need from the alarm?
- Reaction to intrusion, for example with a passive IR motion sensor
- Intrusion alert by siren. The notification should work for a short time (for example, 5 minutes) and then turn off.
- After triggering, the system should return to standby mode. If necessary, it should work repeatedly.
- low current consumption for long-term (6 months) operation in standby mode.

To make such an alarm, we need:

Passive infrared motion sensor. For example, a sensor purchased from OBI is a light switch. The price is about 300r.
Siren at 12 V. In particular, a 105dB model was used, you can use any other. The price is not more than 200 rubles.
Other small things: Battery holder, 6 V relay, insulating tubes, wires.

So. We need to remake the motion sensor by switching it from 220V to 12V. A superficial analysis of the circuit showed that the circuit can operate at supply voltages from 7–8 V to 30 V. When supplying 12V, it is necessary to set the relay to 6V. (12 volt does not work). Let's open the sensor. The spherical part is removed by bending one of the supports. The halves are held on by snaps.

We take payment. As you can see, the sensor is a passive IR receiver that responds to a change in the amount of IR radiation that hits it and a simple optical system. The viewing angle of the sensor is 180 degrees.

Power must be applied to the points on the left. On the "+" positive pole and on the "-" negative from the power source. We will connect the relay winding to the points on the right. Dismantle the standard relay (black box).

Due to the lack of space inside the spherical part of the sensor, it was decided to bring the relay through the wires to the base of the case.

Power is supplied to the sensor through the switch. When the sensor is triggered, it supplies voltage to the relay coil. The relay is activated and the siren will turn on with its closing contacts. Thanks to the relay, you can connect a large number of sirens.

Bottom view. The siren and batteries are connected via terminals. Relay on the bottom left. Top right switch.

Complete system. ATTENTION! Do not turn on the siren without protecting your ears, despite its small size it is very loud and can cause hearing damage.

In the end it worked out. In accordance with the regulator on the sensor, you can set the time for the siren to work after it has been triggered. From 10 sec to 8 min. Install the sensor indoors and bring the siren outside. Unfortunately, after power is applied, the sensor is triggered, so it is reasonable to put the siren switch in a secret place and turn it on 5 minutes after the sensor is turned on. The switch can be switched by a key, like a car ignition switch.

The sensor turned out to be quite economical. According to the ammeter:
Standby current 700uA
Current in operation mode 1.1 mA
Siren current 200mA
A simple calculation suggests that 3.1 Ah is needed to work for 6 months. The capacity of an alkaline battery is about 2.5 Ah. Therefore, for the winter you need 16 mixed alkaline batteries.

Checking in the freezer showed that the system works even at -32

Added later: given our turbulent times and the autonomy of the system, it can be used to build a security perimeter in nature around a tent, for example.

Field trials showed:
1. Water glare causes false positives
2. It is possible to glue the sensor with segments of isolette so that it would form blind zones.

Schematic diagram of a homemade burglar alarm on a microcontroller:

The starting element is the motion sensor LX19B (or LX19C). These are freely sold in electrical stores and are not expensive. The security alarm sensor requires a little alteration: on its board, it is necessary to cut the tracks of the closing contacts of the relay and remove two wires from them (according to the “start” signal scheme). When a person appears in the area of ​​the sensor, the “Start” contact closes on the common wire on the circuit and the countdown starts from 9 to 0 seconds. This time is displayed on the seven-segment display. During this time, using the buttons, you must dial the correct code. Only then the alarm will turn off for 30 seconds. This time is enough to enter the room and turn off the alarm from the inside.

4 buttons are used to dial the code: Key1, Key2, Key3 and Key4 All. They are pressed in the following order: 1-2-3-1-2-1. These buttons can be located anywhere on the dial pad, but must be pressed in the correct sequence. All other buttons (Key4 All) are connected in parallel. When you press any of them, the code set is reset and everything needs to be started all over again. When the time counter displays "0", the dialing of the code is prohibited. It is necessary to move away from the door or stand still until the sensor resets the time to "nine", and then dial the code again. The more buttons in the keyboard, the less likely it is to select a code.

Any car howler is used as an alarm sounder. The original circuit of the Okhrana was assembled on a common cathode indicator taken from some Chinese device. Even the smartest DataSheet does not know its name. Therefore, for convenient repetition, I redrawn the circuit, board and firmware on the more well-known (but not the brightest) ALS324A indicator, also with a common cathode. The board option can be used, for example, the one in the archive, and if desired, the button board can be changed.

If someone likes the circuit, but some other indicator is at hand, for example, with a common cathode or a common anode, I will change the signet, circuit and firmware according to your desire and possibilities. Archive with files and microcontroller firmware on the forum. If you have any questions, I will gladly answer them there. Good luck! Samopalkin

Discuss the article ALARM SCHEME

The article provides a diagram of a simple burglar alarm, a description of the work, resident software (firmware). The device is not difficult to assemble with your own hands. All the information you need for this is in the article.

General description of the device.

The security alarm is assembled on the PIC controller PIC12F629. This is a microcontroller with 8 pins and the price is only 0.5$. Despite its simplicity and low cost, the device provides control of two standard alarm loops. The alarm system can be used to protect large enough objects. The device is controlled by a remote control with two buttons and one LED.

Our company has moved to a new building. From the previous owners there was an old burglar alarm. It was an iron box with red LEDs and a siren above the front door and a broken electronic unit.

I installed a small circuit board in the alarm unit and turned this junk into a modern, reliable burglar alarm. At the moment it is used to protect a two-storey building with a total area of ​​250 m 2 .

So, the alarm provides:

• Control of two standard security loops with measurement of their resistance and digital filtering of signals.
• Remote control (two buttons and one LED):
  • turning on the alarm;
  • disabling the alarm through a secret code
  • setting a secret code (the code is stored in the internal non-volatile memory of the controller);
  • operation mode indication by remote control LED.
• The device generates the time delays necessary for dialing the secret code, closing the doors of the room, etc.
• When the alarm is triggered, the device turns on the sound annunciator (siren).
• The operating mode of the device is also displayed by an external light source.

The block diagram of the security alarm looks like this.

Connected to the main security alarm unit:

• 2 security loops with
  • NC - normally closed sensors;
  • NR - normally open sensors;
  • Rok - terminating resistors.
• External block of the sound notification and mode indication.
• Backup power source.
• Power supply 12 V.

Loops of the security alarm system and connection of sensors.

To control sensors (detectors), the device uses standard security loops. Loop resistance is controlled. If the circuit resistance is greater than the upper or less than the lower threshold, an alarm is generated. Normal is the loop resistance equal to the terminating resistor (2 kOhm). Thus, if an intruder breaks the wires of the loops or closes them, the alarm will go off. In this way, it will not work to disable security sensors.

In this device, the following threshold values ​​of the loop resistance are selected.

Those. loop resistance in the range of 540 ... 5900 ohms is considered normal. If the resistance value goes out of this range, an alarm will be triggered.

Scheme of connecting sensors (detectors) to the security loop.

Both normally closed security sensors (NC) and normally open (NO) can be connected to one loop. The main thing is that in the normal state the circuit has a resistance of 2 kOhm, and when any sensor is triggered, it causes an open or short circuit.

To increase the noise immunity of the system, the device digitally filters the loop signals.

In principle, everything should be clear. Connected to the PIC12F629 microcontroller:

• Two loops through RC chains R1-R6, C1, C2, providing
  • formation of power supply of the loop;
  • analog signal filtering;
  • matching with the input levels of the PIC controller inputs.

To determine the resistance of the loops, a microcontroller comparator is used. An internal reference voltage source is connected to the second input of the comparator. The values ​​of the reference voltage source (RH) for comparison with the upper and lower threshold resistance values ​​are set by software.

• Through the RC chains R7-R10, C3, C4, two remote control buttons and an LED are connected through a current-limiting resistor R11. The device provides digital filtering of button signals to eliminate chatter and improve noise immunity.

It is worth explaining the purpose of the resistor R17. The GP3 input of the microcontroller has an alternative function - a 12 V supply for programming the microcircuit. Therefore, it does not have a protective diode limiting the voltage at the level of the supply voltage. At a voltage of 12 V at this pin, the microcontroller enters the programming mode. Resistor R17 reduces the voltage at the GP3 input.

• Through two transistor switches VT1, VT2, the microcontroller controls the siren and external LED indication. Because these elements can be connected with a long cable, the transistors are protected from line surges by VD4-VD7 diodes. Transistor switches allow switching current up to 2 A.
• A voltage of 5 V to power the PIC controller is generated by the stabilizer D2. Do not ignore the VD8 LED. Its functions include not only power indication, but also the creation of a minimum load for the microcontroller. If the PIC controller consumes current less than 2-3 mA (for example, in reset mode), then the voltage of 12 V through resistors R8, R10 can raise the microcontroller supply voltage above the allowable one.
• The inputs for the 12 V power supply and the backup power supply are separated by diodes VD2, VD3. A Schottky diode is used as a VD2 diode in order to provide priority to the power supply when the voltages are equal to the backup power source.

I assembled the device on a 54 x 45 mm board.

Installed it in the body of the old alarm. Left only the power supply.

The remote control is made in a plastic case with dimensions of 65 x 40 mm.

Software.

The resident software is developed in assembler. The program cyclically resets all variables and registers. The program cannot hang.

You can download firmware for PIC12F629 in HEX format.

Management of the security alarm system from the panel.

The remote control is a small box with two buttons and an LED.

It is better to install it indoors near the front door. With the help of the remote control, the alarm is turned on and off, the secret code is changed.

Modes and control.

When power is first applied, the device enters ALARM OFF mode. The LED is not lit. In this mode, the device is during the working day.

To turn on the alarm (ARMED mode), you must press two buttons at once. The LED will begin to blink rapidly, and after 20 seconds the device will switch to ARMED mode, i.e. will start monitoring the state of the sensors. This is the time it takes to leave the room and close the front door.

If during this period of time (20 seconds) any button is pressed, the device will cancel the armed mode and return to the ALARM DISABLED mode. Often people remember something just before leaving the building.

20 seconds after switching on, the device will switch to the ARMED mode. In this mode, the LEDs of the remote control and the external indication unit blink approximately once per second. In the ARMED mode, the state of the sensors is monitored.

When any security sensor is triggered, the LEDs begin to flash frequently, and the alarm counts down the time after which the siren will sound. This time (30 seconds) is necessary in order to have time to turn off the alarm by typing the secret code on the remote control buttons.

There are 2 buttons on the remote control. Therefore, the code looks like a number from the numbers 1 and 2. For example, the code 121112 means that you need to press the buttons 1, 2, three times 1 and 2 in sequence. The code can have from 1 to 8 digits.

If the code is entered incorrectly or incompletely, you can press two buttons at the same time and repeat the code entry.

If the correct code is entered, the device switches to the ALARM OFF mode.

If within 30 seconds after the sensor was triggered, the correct code was not dialed, then the siren turns on. You can disable it by typing the correct code. Otherwise, the siren will sound for 33 seconds and then the unit will turn off (go into ALARM OFF mode).

It remains to explain how to set the secret code. This can only be done from the ALARM OFF mode.

Both buttons must be held down for 6 seconds. Release when the remote control LED lights up. This will mean that the device has entered the passcode setting mode.

Then wait until the LED goes out (5 sec). The device will enter the ALARM OFF mode, and the new code will be stored in the internal non-volatile memory of the microcontroller.

Because Since the microcontroller of the device is clocked from an internal generator of low accuracy, the specified time parameters may differ by ±10%.

Alarm states.

In practice, work with signaling comes down to actions.

• Leaving the room. Press two buttons at the same time and close the door within 20 seconds.
• Upon entering the room. Within 30 seconds, dial the secret code.

Drawbacks, possible improvements.

The device can be easily modified for its specific conditions. All improvements relate only to the hardware. They do not affect software.

• It is advisable to install two sirens. One in the outdoor indication and warning block, the other in a hard-to-reach place. The current of the transistor key (2 A) allows this to be done.
• It would be necessary to protect the siren wires from short circuit with a transistor current stabilizer. In the presented version of the scheme, an attacker can close the wires of the siren and, when the alarm is triggered, a short circuit of the power source will occur.
• If desired, you can connect powerful and high-voltage sources of light, sound, etc. through electromagnetic relays. The allowable current of the keys allows this, and the keys are protected against surges when switching the relay winding.
• A battery can be used as a backup power by adding a simple charge circuit to the circuit.

Appearance of the installed alarm system.

Now only the front door sensor is connected to the device. I plan, over time, to add security sensors. Two loops are quite enough to guard our two-story building.

By the way, if only one loop is used, then a 2 kΩ resistor must be connected to the second.

There are other software options for the device on the site's forum. There you can discuss, ask questions about this project.

Enough simple alarm to protect your car, you can make do it yourself. To do this, you need quite a bit - just one microcircuit and a couple of radio components that can be soldered from old equipment.

The SC1006 series IC is one of many specialized ICs that have been designed to operate in high powered alarms only. If you look at the data sheet about the microcircuit, it becomes clear that the input voltage rating of the microcircuit allows it to be used for car alarms.

Do-it-yourself car alarm

It is on these microcircuits that many alarms and signaling devices that we use in the car are implemented. The microcircuit generates pulses, the frequency of which changes at each moment of time, as a result, a siren howl is obtained, which is amplified and fed to the head.

The power amplifier is assembled according to a fairly simple scheme. For this, two transistors were used, the last of which is a power transistor, to which the head is connected.

At the power input there is a 1 Amp semiconductor diode, which plays the role of protection against power reversal, can be removed or replaced by another.

The 330kΩ resistor regulates the sound of the siren, you can replace it with a variable resistor and adjust the sound you need.
The power of the siren mainly depends on the transistors used to amplify the signal. Instead of both transistors, you can put the KT829 transistor, which is a composite transistor and has a very high gain.

This article provides diagrams of the simplest electronic alarms, which can be done by anyone who is at least minimally familiar with electronics or simply knows how to hold a soldering iron in his hand. Such alarms come in handy in many cases. They can be put on the windows if there is a small child in the house who can open them. On the doors of an apartment or garage of a guarded parking lot. And when triggered, the watchman will call the police. You can put such an alarm in the apartment if you are friends with your neighbors. Even if you go camping, it’s not a sin to spread a security train around the camp at night in case of wild animals or strangers.

First scheme electronic signaling is simple to the extreme, there is nowhere easier. This is just one transistor, a resistor and an executive relay. If an audible alarm is expected, then instead of a relay, an audible siren or howler is turned on.

Principle of operation: A security loop is a thin wire, or a closed contact. When the wire is intact (or the contact is closed), the base of the transistor is grounded and the transistor is closed. No current flows between collector and emitter.

If you break the security wire, or open the contact, the base will be connected to the power source through the resistor R1, the transistor will open and the relay (or siren) will work. You can turn it off only either by turning off the power, or by restoring the security loop.
Such an alarm can be used to protect your belongings, for example. A reed switch is used as a security contact, the alarm is hidden in the side pocket of a bag or backpack, and a magnet is placed nearby. If the magnet is removed from the alarm itself (move the thing), the siren will squeal in all voices.

Second scheme with more advanced user features

As in the first case, a security loop, a normally closed (in armed mode) contact or a reed switch closed by a magnetic field serves as a sensor. If the loop is violated, an alarm is triggered and its operation continues until the power is turned off. Restoring the loop does not turn off the alarm, it will still continue to work for some time. The alarm has a button for temporary blocking, which is necessary for the owner to leave the protected area. The alarm also has a response delay, which is necessary for the owner to turn it off when he enters the protected area.

Let's analyze how the circuit works. Before arming the alarm, it is necessary to turn off (open) switch S1. It must be installed in a secret place near the entrance. You can use, for example, a hidden reed switch, which closes - opens by rearranging some object with a magnet built into it, etc. This switch blocks the operation of the system and it stops responding to a broken loop. When leaving, the switch S1 opens and the capacitor C2 begins to charge through the resistor R2. Until the capacitor is charged to a certain value, the system is "blind". And you have time to leave the object, restoring the security loop or closing the contacts. By selecting the values ​​​​of resistor R2 and capacitor C2, achieve an acceptable output delay for yourself.

If the security loop is broken, then the capacitor C1 will start charging through the resistor R1. This pair creates a slight delay in the alarm, and the owner has time to neutralize it by turning on switch S1. It is necessary to choose the values ​​​​of the resistor and capacitor for a comfortable response delay time.
If the loop is broken by an intruder who does not know how to turn off the alarm, then some time after the loop is broken, the alarm will work (at both inputs of element D1.1 they will be logical "1", respectively, at the output "0". After passing through the inverter D1 .2 it will again become "1" and open the transistor VT1. The transistor will discharge the capacitor C3 and open the transistor VT2 through the inverter, which will make the executive relay work or turn on the siren.

Even if the attacker quickly restores the loop, the siren will continue to work, since the capacitor C3 will charge through the resistor R3 for a sufficient time. It is the ratings of this pair that determine the operating time of the alarm after the loop is restored. If the loop is not restored, the alarm will work constantly.
Microcircuit - K561LA7, transistors - any n-p-n (KT315, KT815, etc.) Power supply - any with a voltage of +5 - +15 Volts. The executive relay or siren can be connected to a more powerful power source than the circuit itself. In standby mode, the circuit consumes practically no current (at the level of self-discharge of batteries).