Warehouse anti-theft alarm circuit

Some warehouses with valuable items have their doors closed all year round, which can easily attract the attention of thieves. Once given the opportunity, thieves will break in. The circuit in Figure 13-58
is a gate monitor. As long as a thief breaks into the door, it will sound a siren to notify the on-duty personnel to pay attention. The alarm circuit is shown in the figure.
    The circuit in the picture consists of a three-tone (referring to three alarm sounds) music chip ICl (UM3561), an infrared light diode LED1, a phototransistor VTl
(2N5777) and related transistors. The entire circuit is mounted on a printed board (schematic). Then place the assembled printed circuit
board on the door (indoor part).
    Then install a small lens on the door frame and install it in line with the printed board.
    When the door is closed, the infrared light emitted by LED1 on the printed board falls on the phototransistor VT1 of the printed
board through specular reflection, causing the equivalent resistance of VT1 to decrease. The voltage at point A decreases and VT1 is cut off. The ⑤ pin of IC1 (the power terminal of IC1) is powered off, the ③ pin has no alarm signal
, the speaker does not sound, and the entire circuit is in a waiting state.
    Once a thief opens the door, the phototransistor VT1 will not get the infrared light signal. Its equivalent internal resistance increases greatly, the voltage at point A increases, the transistor
VT2 is turned on, the ⑤ pin of Icl obtains the working voltage through R5, and its ③ pin outputs an alarm signal, which is amplified by the Darlington pair tubes of VT3 and VT4
to drive the speaker. Sound an alarm.
    The speaker can also be sent directly to the duty room via a transmission line to remind the duty personnel to pay attention.
    The three-tone musical ICI in the picture can emit firetruck sirens, police sirens and ambulance sirens.
    S2 in the circuit is a single-pole three-position switch. The pole of the switch is connected to the D pin of IC1, and its position is connected to the F, P, and A pins in the figure. When S2 is placed in
different positions, the music piece will emit one of the three siren sounds mentioned above, allowing the user to choose the siren sound he needs.
    The power supply in the picture is powered by a +12y battery, which is then stabilized by IC2 (7805) for circuit operation.