Resistors R1, R2 form a voltage divider, to the negative input of operational amplifier A1 to provide bias voltages U1, as a comparison voltage reference. Static, the capacitor C1 is fully charged, the positive input of the op amp A1 voltage U2 equal to the supply voltage V +, so the A1 output high. When the input voltage Ui the regulator goes low, diode D1 turns on, the capacitor C1 discharges rapidly through D1, so that U2 suddenly falls to the ground level, at this time because U1> U2, so the operational amplifier A1 outputs a low level. When the input voltage becomes high, the diode D1 is turned off, the power supply voltage of the capacitor C1 is charged R3, when the charging voltage on C1 is greater than U1, both U2> U1, A1 output goes high again, ending a one-shot trigger.