As shown, in the circuit, 555 and R2, C1, and VT1 frames a monostable trigger circuit . The general difference between the trigger is VT1,  R1, Rz, R3: forms  a constant current , ie., a constant-current charge to C1. C1 charges with a voltage linearity, and good precision delay . The Delay time is calculated by ˜ 1.1R2C1. Required trigger pulse cycle T> td. The output waveform shown in Figure (b).