Blocking monitoring circuit

The blockage monitoring circuit is shown in the figure. When the light path is blocked by materials, the phototransistor VT1 is cut off, the transistor VT2 is cut off, and
the injector-coupled bistable flip-flop composed of VT3 and VT4 flips to the state where VT3 is turned on and VT4 is cut off. The diode VD2 cannot be turned on, and the diode VD2 is turned off by the double base. The constant current source composed of the pole
tube VT5 and the potentiometer RP charges CzJ. After the voltage on C2 rises to a certain value, the composite tubes VT6 and Vr7 are turned on. J1 is closed to
control external circuit operation or alarm. If the voltage on C2 has not increased enough to turn on VT6. When the optical path is connected, the laser beam will flip to
the state where VT3 is cut off, VT4 is on, and VD2 is on, clamping the terminal of the capacitor Cz to a low potential, and Jl cannot pull in. It is said that adjusting RP can change
the maximum blockage time allowed by the circuit (that is, the circuit will not alarm for a short time of blockage). VT8 forms an automatic light source replacement alarm circuit .
    Component selection and debugging are as follows.
    Choose 3DU5 for VT1, 3DG6 for VT2~VT4, and VT6, transistors with Lu value between 50-80. Choose 3DG12 or 3DK4 for VI'7 and VT8
  . The reed value is between 40-50.
    Jl and JZ use JQX-4F type 1ZV relays or other sensitive relays. During
    debugging , when light shines on VT1, VT3 should be cut off and Jl should be released. If not, VT2 can be replaced with a transistor
with a larger J9 value (eg 100   times) . If the light path is blocked for a long time and Jl does not close, you can replace Ru with one with a smaller resistance.