mx270a - 3/19/2016 16:49

I build electronics. I'll try to explain what your schematic is doing.

Cap 1 is to smooth the input to the voltage regulator. Anything from 100uF to 470uF would be fine. Size doesn't matter much. Voltage does. Figure the maximum voltage you'll ever see on that side and then add 50%. Your cap needs to be rated for that amount or higher. I'd go with 35 or 50v caps on the input to be safe.

Cap 2 is to smooth the output from the voltage regulator. 100uF is a good value. Since its on 5v, get a cap rated for at least 10v.

Resistor 3 is sized to regulate the current going through the power LED. Lets say you use an LED that uses 2.2v at 20mA. The remaining 2.8v will go to the resistor, so to get 20ma at 2.8v requires 150 ohms. For resistor sizing on LEDs, I use a LED calculator like this one:
http://led.linear1.org/1led.wiz

The comparitor simply compares two inputs. If input A has a higher voltage that input B, the output will be high. If A is less than B, the output is low. You can't size the resistors without knowing what voltage the ultrasonic sensor outputs for the target distance you want to measure.

Capacitor 5 is just there to smooth the input to the comparitor. I'd guess 10uF would be good. Maybe 1uF. A larger cap here will mean a slower response with the ultrasonic sensor output changes.

The MOSFET #9 is a N-channel. You need one rated to carry more current than the buzzer will consume. My guess is your current will be less than half an amp (500mA). At that low of a current, you have a lot of options. As the load increases, you would need to pay attention to RdsON resistance, which will allow you to determine the power consumption and thus heat dissipation required for the FET.

Resistor 8 is there to regulate current through the buzzer. Again, size depends on what the buzzer specs are.