Multi-Utility Flashing Light

The circuit described below can be used in many applications like a fake light to create an impression that alarm has been fitted or in vehicles so that when the car breaks down to catch the attention of the vehicle passing nearby. This flashing light circuit can also be used for decorative purposes. In this circuit the intensity of fleshing LED can also be varied according to the requirement.

To understand the working you should know about the truth table of NAND gate which is as follows-

INPUT	INPUT	OUTPUT
A	B	Y
0	0	1
0	1	1
1	0	1
1	1	0

The truth table of NAND gate shows that the output will remain HIGH until any of the signals is LOW. If both the signals go HIGH then in that case the output will be LOW.

In this circuit the pin 3 always oscillates back and forth from high to low. During one cycle, capacitor charge in one direction giving output and again charges in opposite direction to remain off. Consider the case when circuit is switched on for the first time. At this time, voltage at pin 3 is zero and voltage at pin 1 and 2 is high which is fed to pin 4 which is also high. Pin 5 and 6 are initially connected to pin 3 through capacitor and resistor, which are low. After some time capacitor connected between the pin 3 and 5 starts charging with the help of C1 and R1. Voltage starts building at pin 3 which is internally connected to pin 12 and 13 .As a result, the output pin 11 becomes low and this low voltage is supplied to pin 8, 9 making pin 10 high and LEDs connected to it will start conducting thus giving the output.

After preset interval of time depending on the value of capacitor and variable resistor, the polarity of the charge across C1 and R1 will reverse. Pin 4 goes low and pin P3 gets high. C1 again starts charging once more. But this time the charging is in the opposite direction. Since pin 3 is now high and pin 4 is low, the junction between C1 and R1 represents the negative side of the capacitor. As the capacitor charges, the voltage on the junction of C1 and R1 will fall. This means that the voltage on pins 5 & 6 will fall. When it falls to just below half the supply voltage, the inputs change from high to low. And the output of gate 2 will go high. Pin P4 will take pins 1 & 2 high. So pin 3 will go low. And again the LED will start flashing. In other words the first cycle is complete Repetition of this cycle will continuously produce the output .

If the time period has to be increased only the values of R1 and C1 have to be changed.