The operation of the circuit is beginning when the IC1 reduces the car battery voltage to 5V. The diodes D1 & D2 in combination produces an additional drop of 1.4 V to give a 3.6 V supply for the UM66. The supply voltage of UM 66 should not be more than 4V. When ever the car is in reverse gear, the reverse gear switch of the car gets activated and the circuit gets connected to the car battery. The UM66 starts playing the music tone. The transistor T1 amplifies the output of UM66 to drive the loudspeaker.
Before attempting the circuit, have a good idea about the electrical wiring of your car. A wrong connection may damage your car’s electrical system. The transistor Q1 is not very specific. Any medium power NPN audio transistor will do the job. You could easily find one from your electronics junk box. The switch S1 is the reverse gear switch of the car. Don’t forget to place in waterproof dashboard in the car.