Magnetic door and window anti-theft alarm

This article introduces a magnetically controlled door and window burglar alarm with low cost, simple design, stable performance, easy production, simple debugging, easy installation and small size.

This anti-theft alarm is a very suitable "door and window electronic dog" for daily work and life (especially in living rooms, warehouses, secret rooms, etc.). When the closed doors and windows are accidentally opened, the anti-theft alarm will immediately issue a continuous, sharp and loud "beep..." sound to warn until the doors and windows are closed again.

1. Working Principle

The circuit principle of the magnetically controlled door and window anti-theft alarm is shown in Figure 1. S1 is a normally open reed switch that needs to work with a magnet. Q1 is an NPN transistor and Q2 is a PNP transistor. The emitter junction of Q2 is the load of Q1, and Q1 provides the base operating current for Q2. Bu is a high-loudness active buzzer (containing an oscillation circuit) and is the alarm sounding device of the entire circuit. R1 is a bias resistor that provides bias current for the normal operation of Q1. C1 is an AC bypass capacitor. It is mainly used to reduce the AC internal resistance of the battery BT, making the buzzer sound louder and having a certain effect of extending the battery life. S2 is a small ordinary single-pole single-throw toggle switch that controls the operation of the entire circuit. BT is a 9V battery that provides working power for the entire circuit.

When the magnet leaves the normally open reed switch s1 for a certain distance, s1 is disconnected. The base potential of the NPN transistor Q1 rises, and when Q1 is in a positive bias, Q1 enters the on state. At this time, the PNP transistor Q2 is also in the on state due to the base operating current provided by Q1. Finally, the high-loud active buzzer loses power and starts to sound an alarm.

When the magnet approaches the normally open reed switch s1, s1 is closed. The base potential of the NPN transistor Q1 drops, vbe is almost zero, and Q1 is in the cut-off state. At this time, the PNP transistor Q2 is in the cut-off state due to the loss of the base operating current provided by Q1. Finally, the high-loud active buzzer Bu loses power and stops buzzing.

The magnet of the magnetically controlled door and window anti-theft alarm is generally installed on the indoor side of the door and window, on the edge of the top near the lock or bolt side; and its circuit board is also installed on the indoor side of the door and window, on the corresponding frame near the top of the lock or bolt side.

When enabled, just turn switch s2 to the open position. When the door and window are closed normally, reed switch s1 is closed, transistors Q1 and Q2 are in the cut-off state, the buzzer does not sound, and the static current of the whole machine is about 100μA (when the resistance value of R1 is increased, the static current of the whole machine will become smaller); when the door and window are accidentally opened (such as pried open, bumped open, opened by a key, etc.), reed switch s1 is disconnected, transistors Q1 and Q2 are in the on state, and the buzzer immediately emits a loud "beep..." sound.

In addition, if you want to increase the alarm sound of the magnetically controlled door and window anti-theft alarm and improve its alarm reliability, you can also add a PNP transistor and a high-loud active buzzer; if you want to increase its working status indication, you can also connect a current-limiting resistor and a light-emitting diode in series and connect them to the circuit. The circuit principle of the improved magnetically controlled door and window anti-theft alarm is shown in Figure 2.

2. Component Selection

The components used in this magnetically controlled door and window anti-theft alarm are all ordinary

Components, their appearance is shown in Figure 3 (omitted).

Figure 3 Components used in magnetically controlled door and window anti-theft alarms

Switch S1 uses a normally open reed switch. It consists of a pair of elastic reeds made of magnetic material and is sealed in a glass tube. The end faces of the reeds overlap with a gap, and the contacts are plated with a layer of precious metal. The glass tube is also filled with inert gas. This makes the reed switch have both stable performance and a long service life. When a magnetic field generated by a permanent magnet or coil approaches the switch, once the magnetic field attraction generated is greater than the resistance generated by the elasticity of the reed, the two reeds come into contact, causing the switch to close; when the magnetic field generated by the permanent magnet or coil moves away from the switch, once the magnetic field attraction generated is less than the resistance generated by the elasticity of the reed, the two reeds separate again, causing the switch to open. Switch S2 can use a small ordinary single-pole single-throw toggle switch.

Q1 can use genuine S9014 (maximum allowable collector current IcM=0.15A, maximum allowable power consumption PCM=450mw) or S9013, 3DGG8 and other NPN silicon low-power transistors (current amplification factor β value is 50~200); Q2 or Q3 can use genuine S9012 (ICM=0.5A, PCM=625mw) or 3CX200, 3CG23 and other PNP silicon medium-power transistors, and the β value can be selected between 30 and 100.

Buzzer Bu1 or Bu2 uses a high-volume active buzzer (including an oscillation circuit) with a withstand voltage of more than 9V.

Resistor R1 or R2 can be RTXE, 1/8W carbon film resistor. R1 is 100KΩ, R2 is 10KΩ. In addition, increasing the resistance of R1 appropriately can reduce the static power consumption of the entire circuit, but it will also reduce the sound of the active buzzer. In addition, the light-emitting diode D1 will increase the power consumption of the entire circuit, so it is best not to use it after debugging.

Capacitor c1 is a cD11-16v, 1000uF electrolytic capacitor.

The light emitting diode D1 uses a φ3mm ordinary light emitting diode (red, green, yellow, blue and other luminous colors are all acceptable).

The battery BT uses a 6F22 9V laminated battery. To connect this type of battery, a special battery buckle is usually required to facilitate replacement.

3. Production and Use

Since the design of the magnetically controlled door and window anti-theft alarm is very simple, a universal circuit board can be selected for production. Figure 4 (omitted) is a physical picture of the improved magnetically controlled door and window anti-theft alarm made of a universal circuit board.

Figure 4 Actual picture of magnetically controlled door and window anti-theft alarm

Below, taking the improved magnetically controlled door and window burglar alarm as an example, we will introduce the relevant production and usage techniques.

1. Welding of components

Since the pins of the selected components are relatively small, a soldering iron with a power of about 25W can be used for soldering. When soldering components, please pay attention to the following points:

(1) Pay special attention when bending the pins of the reed switch: the joint between the glass tube body and the metal pin will be subjected to external force, which may easily cause the glass tube to break, causing the inert gas filled inside to leak out, affecting the normal performance and service life of the reed switch. Therefore, when bending, it is advisable to first use the left hand to hold the needle-nosed pliers to clamp the metal pin close to the glass tube body, and then use the right hand to hold the tweezers to gently bend the corresponding position of the metal pin according to the installation position. In addition, in order to maximize the sensitivity of the reed switch when in use, the bending direction of the metal pin should be in the same plane as the tongue reed. In this way, when the metal pin of the reed switch is vertically soldered to the circuit board, the magnet placed perpendicular to the circuit board will have the greatest attraction effect on the tongue reed, thereby ensuring that the reed switch can exert the greatest sensitivity when in use.

(2) When connecting the active buzzer to the circuit, you need to pay attention to its positive and negative poles. Among the pins of the active buzzer, the longer one is usually the positive pole (the "O+" symbol is usually printed at the approximate position corresponding to its top or outer wall), and the shorter one is the negative pole. In addition, after soldering the active buzzer, you must also tear off the round seal that was pasted on its top before leaving the factory to facilitate its high-loud sound.

(3) When fixing the 6F22 type 9V stacked battery to the circuit board, you can cut two sections of thin copper core cables with a length of about 60mm, bend them into an "n" shape, weld them on the circuit board at an appropriate distance, and then insert the 9V stacked battery into the two "n" shaped strapping strips.

2. Testing

The magnetically controlled door and window burglar alarm can work normally as long as the components are of guaranteed quality and the welding is correct. During the test, the normal working characteristics of the magnetically controlled door and window burglar alarm are: when the power switch S2 is closed, the light-emitting diode D1 should light up and indicate, and the buzzers Bu1 and Bu2 should also immediately make a "beep..." sound. The working current of the whole machine is about 190mA; when the magnet is close to the reed switch s1, the light-emitting diode D1 should still light up and indicate, but the alarm sound should stop immediately, and the static current of the whole machine is about 1.9mA.

3. Installation

During installation, according to the need of anti-theft, the magnet is usually installed on the indoor side of the door and window, on the edge of the top of the lock or bolt side, and the circuit board is installed on the frame of the top of the door and window, corresponding to the lock or bolt side. Then, according to the magnetic field attraction of the magnet, the maximum sensing distance can be measured by the debugging method of slowly approaching the reed switch with the magnet, and then 1/2 to 2/3 of the maximum sensing distance is taken as the actual installation distance (this can not only ensure the sensitivity and service life of the reed switch, but also facilitate the installation of the circuit board and magnet). Finally, the gap of the door or window is used as the middle position, and the circuit board and magnet of the magnetically controlled door and window anti-theft alarm are fixed to the appropriate position respectively.

When defense is needed, just close the corresponding doors and windows and turn the power switch S2 to the closed position. When the doors and windows that are intentionally closed are accidentally opened, the anti-theft alarm will immediately issue a continuous, sharp and loud "beep..." sound to warn until the doors and windows are closed again.

Since the anti-theft device is easy to install and small in size, it can also be used as an anti-theft guard for cabinets and drawers. The installation and use methods are similar to the above.