door opener circuit

The circuit of the door opener is as shown in the figure. As long as someone opens the door, the bell will ring. It is useless to close the door immediately, the bell
will still ring. Until the circuit switch S is turned off. The ringing will stop.
    (1) Circuit principle: Install a reed switch on the door frame, and place a small bar
magnet above the reed switch so that the reed switch can be reliably attracted. Place another bar magnet on the door with the polarity
opposite to the magnet on the door frame. When the door is closed, the magnetic field of the magnet on the door weakens the original magnetic
field and the reed switch opens. K is an electromagnetic relay, which has two normally open contacts Kl and K2. Close
the door and then close the switch K. Since the thousand reed tube is disconnected, if no current passes through the relay, it is in the release
state, the normally open contacts Kl and K2 are disconnected, and the bell does not ring. When the door is opened, the reed switch is closed
, and the relay K is closed due to current passing through it. The normally open contacts K1 and K2 are connected, and the bell rings
. At this time, the door is closed. Although the reed switch is disconnected, because Kl acts as a self-locking function, current
still flows through the relay , and the bell rings continuously. The ringing will stop only when the switch SB is turned off, the power is cut off,
the relay is released, and Kl and K2 are disconnected.