Inpatient pager circuit

(1) Circuit composition The circuit is composed of power circuit, call control and sound and light circuit. As shown.
    The power circuit is composed of power switch S, fuse FU, power transformer T, rectifier bridge stack UR, filter capacitor C and three-terminal voltage stabilizing integrated circuit
IC.
    The call control circuit is composed of control buttons SB1-SB3, capacitors C, ~C3, thyristors VTHl--VTH3 and light-emitting diodes LED 1-
LED3.
    The sound and light call circuit is composed of resistors Ri~R3, reset buttons SB4~SB3, light-emitting diodes LED4~LED6, diodes VD1~VD3,
transistors VT and buzzer HA.
    (2) Working principle Turn on the power switch S. After the AC 220V voltage is stepped down by T, rectified by UR, filtered by c1 and stabilized by lC, it
provides __-12V working voltage for the whole machine.
    When SB1~SB3 is pressed, VTH1~VTH3 and VT are all in cut-off state, and LED1~LED3 and LED4~LEt)6 are not
bright. HA is silent.
    When a button among SB1 ~ SB3 is pressed, the thyristor connected to the button will be triggered to conduct, and the LED of that path will light up to indicate the
bed number. At the same time, the VT will be saturated and conduct, and the HA will make a buzzing sound; After the medical staff at the nurse station receives the ward call signal, they press
the reset button of the channel to cut off the thyristors and transistors, the two light-emitting diodes go out, and the HA stops sounding.
    If, when SB2 is pressed, VTH2 is triggered and turned on, LED2 and LⅡ)5 light up, VT turns on, and HA buzzes. When SB2 is pressed
again, VTH2, vr and VD2 are cut off. LED2 and LED 5 go out, and HA stops sounding.