All the components in an automobile are powered by internal combustion engines and thereby reducing its mechanical efficiency. Battery in an automobile is charged with an internal combustion engine with the conversion of mechanical to electrical energy by an alternator also known as generator. Most of the people often forget to turn off their light in the morning which reduces mechanical efficiency of an engine. We have designed an electrical circuit which will automatically cut-out the headlight circuit during the daytime. This circuit will increase the mechanical efficiency of the engine by reducing power loss due to the headlights and also save the important fuel.

Working:
It is clear from the attached Illustration-1 that -- When light falls on the phototransistor, it begins to conduct up to about 1.5 mA, which pulls down the voltage at the lower side of the resistor by 1.5 V, turning off the transistor, which turns off the lamp. When it’s dark, the transistor is able to conduct about 15 mA through the lamp. So the lamp will start.

List of symbols:
W = watt
A = ampere
hr = hour

Assumed data:
Battery = 12 V, 65 A-hr
Lamp = 50W * 2 = 100W
Alternator rating = 150 amp

Current requirement by lamp:
Power = Voltage * Current
100 = 12 * I1
Hence, I1 = 8.33 Amp

Time of complete discharge of battery:
Time = (65A-hr)/ (8.33A)
= 7.80 hrs

Resistance offered by alternator:
Voltage = Alternator current * Resistance
V = I2 * R
12/150 = R
R = 0.08 ohm

Power generated by engine to drive the alternator:
P= (I^2) * R
P = (65 * 65) * 0.08
P = 338 W

Inference:
The ampere rating of the battery is 65 that is it can deliver 65 A of current continuously for an hour or for a complete discharge. If the headlights are
continuously running for 7.8 hours than the engine has to deliver 338 watts for
running it for 7.8 hours. In other words, if the light remain on for 7.8 hours during day time than 338 watts from an engine are wasted.

Conclusion:
There are ample of vehicles including 2- wheelers and 4- wheelers having their lights turned on in day time which results in loss of engine’s mechanical power. The circuit designed can help to reduce this loss up to some extent which in turn helps to save fuel.