When the vehicle`s backup lights kick on, or when the manual switch (SI) is closed, a small current is fed to the base of Ql.
Transistor Ql allows current to flow through it and, if the enable switch (S2) is closed, it sends 12 V to Ul, a 555 timer. Timer Ul sends high pulses that last 0.97713 s and low signals that last 0.488 565 s to the base of Q2. When Ul switches Q2 on, it sends 12 V to BZ1, a piezoelectric buzzer. For best results, the buzzer should be mounted under the vehicleâ€”somewhere where people around the car can hear the warning beeps.