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Faizan
An LED ("Light Emitting Diode") is a semiconductor that glows when current passes through it. As a diode, it only allows current to pass in one direction: from the "anode" to the "cathode." An LED always needs to be connected through a resistor to limit the voltage and current. You can calculate the value of the resistor using Ohm's law. Subtract the LED's forward voltage from the supply voltage, and divide by the LED current, which is usually 0.02 amps. Round the result up to the nearest common resistor value. You can find an LED's voltage and current ratings in its datasheet. These simple circuits use a small number of components to create different types of LED flashers.
Basic LED Blinking LED Circuit
You can use a common, inexpensive chip called a 555 timer to create a blinking LED circuit. A 555 timer is an integrated circuit ("IC") that outputs a square wave that oscillates between zero volts and the supply voltage. You control the frequency and duty cycle of the timer by adjusting the values of the resistors and capacitors connected to the timer. The duty cycle of a square wave is the ratio of high and low signals in a single cycle. For example, an oscillator that goes high for two milliseconds and then low for two milliseconds has a duty cycle of 50 percent. Connect the power supply to pins four and eight of the timer IC. Connect pin seven to the power supply through a resistor, and connect pin seven to both pins six and two via a second resistor. Connect pin two to ground through a capacitor, and tie pin one directly to ground. The output is available at pin three. Connect the anode of your LED to pin three through a resistor, and connect the cathode to ground. The LED will blink. The frequency is controlled by both the total resistance and capacitance, where higher values make the light blink more slowly. The ratio of the two resistors controls the duty cycle. For a 50 percent duty cycle, use the same value for both.
Alternating Flashing LEDs
Using a pair of NPN-type bipolar transistors you can make a simple circuit that will alternately flash a pair of LEDs. A transistor is a semiconductor that can function as an electronic "switch." Transistors have three leads, called the base, collector and emitter. The names can be confusing because they come from the internal structure of the transistor and aren't related to its practical use. Connect the base of each transistor to a nine-volt power supply through a 100k ohm resistor. Connect the anode of each LED to the power supply, and connect the cathode of each LED to the collector pin of a transistor through a resistor. Choose the value of the LED resistor based on Ohm's law, as described in the introduction. Connect the collector of each transistor through the base of the other through a 10 uF capacitor, with the positive end of the capacitor connected to the collector of the transistor. Connect the emitters of the transistors to ground. The LEDs will blink alternately. Control the frequency by changing the values of the resistors connected to the base of each transistor.
Blinking Night Light
With the addition of a phototransistor and a comparator, you can adjust your basic LED flasher circuit so that it only lights up at night. A phototransistor is similar to a normal bipolar transistor, except instead of a base, it has an element that detects light. Run the power supply into the collector of a phototransistor through a 470k ohm resistor. Higher resistor values will make the transistor less sensitive to light. Also connect the phototransistor to the inverting input of the comparator IC, and connect the emitter to ground. Connect the non-inverting input of the comparator to the power supply through a resistor, and connect it to ground through an identical resistor. The values of the two resistors connected to the non-inverting input are irrelevant as long as they are the same. Wire your flashing LED circuit as normal, except instead of connecting the cathode of the LED to ground, connect it to the output of the comparator. The comparator will cause the LED to light only when the voltage across the phototransistor, which increases in the dark, is greater than half the supply voltage. Attach this circuit to something you need to be able to find in the dark, such as the remote control.
Basic LED Blinking LED Circuit
You can use a common, inexpensive chip called a 555 timer to create a blinking LED circuit. A 555 timer is an integrated circuit ("IC") that outputs a square wave that oscillates between zero volts and the supply voltage. You control the frequency and duty cycle of the timer by adjusting the values of the resistors and capacitors connected to the timer. The duty cycle of a square wave is the ratio of high and low signals in a single cycle. For example, an oscillator that goes high for two milliseconds and then low for two milliseconds has a duty cycle of 50 percent. Connect the power supply to pins four and eight of the timer IC. Connect pin seven to the power supply through a resistor, and connect pin seven to both pins six and two via a second resistor. Connect pin two to ground through a capacitor, and tie pin one directly to ground. The output is available at pin three. Connect the anode of your LED to pin three through a resistor, and connect the cathode to ground. The LED will blink. The frequency is controlled by both the total resistance and capacitance, where higher values make the light blink more slowly. The ratio of the two resistors controls the duty cycle. For a 50 percent duty cycle, use the same value for both.
Alternating Flashing LEDs
Using a pair of NPN-type bipolar transistors you can make a simple circuit that will alternately flash a pair of LEDs. A transistor is a semiconductor that can function as an electronic "switch." Transistors have three leads, called the base, collector and emitter. The names can be confusing because they come from the internal structure of the transistor and aren't related to its practical use. Connect the base of each transistor to a nine-volt power supply through a 100k ohm resistor. Connect the anode of each LED to the power supply, and connect the cathode of each LED to the collector pin of a transistor through a resistor. Choose the value of the LED resistor based on Ohm's law, as described in the introduction. Connect the collector of each transistor through the base of the other through a 10 uF capacitor, with the positive end of the capacitor connected to the collector of the transistor. Connect the emitters of the transistors to ground. The LEDs will blink alternately. Control the frequency by changing the values of the resistors connected to the base of each transistor.
Blinking Night Light
With the addition of a phototransistor and a comparator, you can adjust your basic LED flasher circuit so that it only lights up at night. A phototransistor is similar to a normal bipolar transistor, except instead of a base, it has an element that detects light. Run the power supply into the collector of a phototransistor through a 470k ohm resistor. Higher resistor values will make the transistor less sensitive to light. Also connect the phototransistor to the inverting input of the comparator IC, and connect the emitter to ground. Connect the non-inverting input of the comparator to the power supply through a resistor, and connect it to ground through an identical resistor. The values of the two resistors connected to the non-inverting input are irrelevant as long as they are the same. Wire your flashing LED circuit as normal, except instead of connecting the cathode of the LED to ground, connect it to the output of the comparator. The comparator will cause the LED to light only when the voltage across the phototransistor, which increases in the dark, is greater than half the supply voltage. Attach this circuit to something you need to be able to find in the dark, such as the remote control.
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