The circuit design of lock-controlled anti-theft alarm composed of TWH8751

As shown in the figure, this is a lock-controlled anti-theft alarm circuit. The circuit consists of an RS trigger, a gated oscillator, an alarm circuit, etc. The RS trigger consists of NAND gates F2 and F3, and the gated oscillator consists of the integrated circuit TWH8751 and R and C.

When SKK is in the "lock door" position, the 2 input terminals of the NAND gate F1 are at a high level, and the state of DBK1-n is directly reflected by F1. If there is an open circuit in DBK1-n, F1 outputs a low level, setting the RS trigger to "1" (Q is high level, Q is not low level), and the corresponding TWH8751's select terminal ② is at a low level, triggering the oscillator to start oscillating, and the oscillation signal causes the speaker to emit an alarm sound.

When SKK is in the "open door" position (i.e., the door lock is opened normally with the key), the input terminal ② of F1 is at a low level, so the input terminal of F1 is blocked, and the RS trigger is reset (Q is at a low level, Q is not at a high level). The corresponding TWH8751 does not oscillate because the strobe terminal ② is at a high level, and the alarm does not work. At this time, the owner can freely use all the items monitored by DBK1-n. When the owner locks the door and leaves, the alarm will automatically resume normal operation. To eliminate the alarm, first use the key to open the door lock, and then press AN.