melody that will get attention of any one standing at the back of your vehicle.

The circuit is totally contact less and there is no installation required. To get it working simply attach it at the reverse light of your vehicle. Fit the circuit in a small box and make a small hole for the LDR, for best results leave the LDR inside the box infront of the small hole so it will only receive the vehicle reverse light, if it will receive lights other than reverse light for example day light or other vehicles light then it will start producing melody.
 
For speaker use a another box if you are using big 8ohms speaker like 0.5watt or 1watt and hide it some where in the back of the car. For this purpose a good quality 1 meter audio wire should be used from the circuit to the speaker. If you want to use a small speaker try to fit it inside the circuit box. The operating voltage of the circuit is 6V DC and you can use to 3V lithium coin cell as supply. For more loud sound use BD139 transistor in the place of 2N4401. The sensitivity of the light detection of the circuit can be adjusted with the 50K variable resistor.

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Here is a very interesting project of a simple contact less car reverse or backup alarm circuit. It cannot only use with car but you can use it with any vehicle. The circuit doesn't require any installation and gets automatically activated when your car is in reverse gear. When it is activated it will generate a good
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Contact Less Car Reverse Or Backup Alarm Circuit Diagram