Parking a car in congested parking lots and tiny spaces is a tedious job and the important thing is that you must be very careful while reversing so that you do not damage the car (your car or the adjacent one).
Almost all modern cars are equipped with reverse parking sensors that are activated when the car is put in reverse gear and beep at variable rate depending on the distance between the car and the closest obstacle.
The Reverse Parking Sensors that are equipped in the car are basically Ultrasonic Proximity Sensors i.e. they use Ultrasonic Sensors to measure the distance between the car and the object and warn the driver if the car is too close.
In this project, I have built a simple prototype of a Car Reverse Parking Sensor with the help of ATmega8 and the very famous HC-SR04 Ultrasonic Sensor.