R15_____________3K3 1/4W Resistor R16___________330K 1/4W Resistor R17___________470K 1/4W Resistor R18___________150K 1/4W Resistor C6______________1µF 63V Electrolytic or Polyester Capacitor D8,D9,D10____1N4148 75V 150mA Diodes IC4_____________555 Timer IC BZ1___________Piezo sounder (incorporating 3KHz oscillator)
This modification was designed on request: some people prefer an audible alert instead of looking at the LED display, making easier the parking operation.
The original Park-aid circuit was retained, but please note that the input pins of IC2B, IC2C and IC2D are reversed. LEDs D5, D6 and D7, as also resistors R12, R13 and R14 are omitted. IC2B, IC2C and IC2D outputs drive resistors R15, R16 and R17 through D8, D9 and D10 respectively, in order to change the time constant of a low frequency oscillator based on the 555 timer IC4.
This allows the Piezo sounder to start beeping at about 2 times per second when bumper-wall distance is about 20 cm., then to increase the beeps to about 3 per second when bumper-wall distance is about 10 cm. and finally to increase further the beeps frequency to more than 4 beeps per second when the distance is about 6 cm. or less.