Alarm That Uses A Secret Lock To Switch On: Traditional Chinese Circui

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Alarm that uses a secret lock to switch on and offThe main reason why users are reluctant to accept anti-theft alarms for homes is that the control switches are too cumbersome and unsafe. Commonly used switches are password switches, hidden switches or touch switches. Their shortcomings are either easy to destroy or cumbersome to operate, making them unsuitable for the elderly and children.Alarms that use hidden locks for switching have the following characteristics: first, they use common unlocking methods, which are easy to use for the elderly and children, and do not require additional memory; second, they are easy to install, work reliably, and have good confidentiality; third, there is no need to modify the existing mechanical hidden locks, and only a metal piece is added to function as a password switch.1． How the circuit worksThe alarm circuit that uses a secret lock to switch is shown in Figure 41-1. It is composed of a password switch circuit and a disconnected alarm circuit. Figure 41-1In the figure, resistor R1, capacitor C1, unidirectional thyristor SCR and relay J form a password switch circuit. It is mainly used to cut off the power of the alarm circuit when the owner of the room opens the door to prevent the alarm from occurring when the owner of the room opens the door. Resistor R1 and capacitor C1 are delay circuits, and K2 is the hidden lock key. When the owner uses the key to unlock, he turns the key to the predetermined position (that is, the position where the lock tongue is pulled open) and stays there for 3 to 5 seconds (this time is set as needed). Capacitor C1 is charged by resistor R1, and the charging voltage rises to the trigger voltage of the thyristor SCR control pole. SCR is turned on, relay J operates, and the normally closed contact cuts off the power of the alarm circuit. When the door is opened, the alarm will not sound. When a thief picks the lock or uses a master key to open the lock, he firstly does not know where to stop the key or how long it will stay; secondly, the thief is guilty of thievery but wants to open the door lock quickly. Generally, the key is inserted into the hidden lock hole and turned to the end. Since the charging voltage on C1 has not reached the trigger voltage of the SCR, the thyristor SCR is still off, the relay J does not operate, the alarm circuit power is on, and the lock tongue is pulled out, that is, K3 is disconnected, and an alarm sounds immediately.Resistor R2, capacitor C2, transistors BG1, BG2 and speaker Y form a delayed disconnection alarm circuit. R2 and C2 form a delay circuit. Its function is to prevent an alarm when the power switch K1 is turned on before the door is closed, because the door lock is not locked before the door is closed (that is, K3 is in an off state). If there is no delay circuit, the alarm circuit will alarm immediately. With the delay circuit composed of R2 and C2, since the charging voltage on C2 has not reached the operating voltage of transistor BG1, the circuit will not alarm even if the power switch K1 is turned on before the door is locked. When the door is locked (that is, K3 is closed), although the charging voltage of capacitor C2 reaches the base operating voltage of the BG1 tube, K3 is turned on at this time, causing the base and emitter of BG1 and BG2 to be short-circuited, causing the alarm circuit to not work. When a thief opens the door lock (K3 is off), since the power supply of the alarm circuit is on, the charging voltage on capacitor C2 immediately turns on BG1 and BG2, forming an oscillation, and the alarm makes a sound similar to that of a police car, ambulance or air defense alarm.2. Component selection and debuggingJ is a DC relay. In order to ensure that the alarm can work normally when the power supply voltage decreases, according to experience, there is no need to select a 6V DC relay that matches the power supply voltage. Instead, a 3V DC relay is selected. This is both reliable and ensures the service life of the 3V relay, because no current flows through the relay coil in the alarm state, and the alarm time is very short. …

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