NE 555 Timer based Automatic Headlight

Automatic headlamp is very useful for miners as it eliminate the need to manually switch the headlamp ON and OFF while entering into the mines where light is not sufficient. The automatic headlight circuit described below works as a human eye to outside light levels and independently turns on and off the lights when needed .It hence offers both safety and convenience. This circuit is very helpful when you are entering into a tunnel at twilight or sunlight even in foggy and rainy condition. For example when you enter into a dark tunnel you will not have to search for headlight switch it will automatically turn on after sensing the poor quality of light and when you come out from tunnel it will turn off automatically.The headlamp circuit is based on NE555, which is widely known to generate accurate timing pulses. Being a multivibrator IC, NE555 works in monostable and astable mode. In this circuit, it is working in monostable state. Let's see what lies inside a standard NE555 IC:

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Fig. 1: Circuit Diagram Of a Standard NE555 IC

The circuit is built around commonly known NE555 timer IC, light dependent resistor (LDR) and few more discrete components. LDR is a resistor, whose resistance decreases with increase in the intensity of the incident light. This means that LDR exhibits very high resistance in darkness and low resistance in presence of bright light. A variable resistor is used to set the light sensitivity of LDR. LDR is connected to the trigger input pin 2 of IC. The output of IC is connected to base of transistor through a resistor. LDR and variable resistor will provide a voltage divider arrangement at pin 2.

When there is sufficient light, resistance of LDR is low , and sovoltage at pin 2 is greater than two-third of 6V. As a result output at pin 3 of IC remain low and the light connected to headlight will remain off.

When you enter into the darkness, ambient light decreases and resistance of LDR becomes high. As a result voltage at pin 2 of IC decreases to less than one third of 6V and output pin 3 goes high which turn on the LED’s connected in the output.