• Let's assume that we have 1.8v across the LED.

• Then there's 3.2 volts across the resistor.

• Then, 3.2/R amps flows through the resistor.

• That current is what causes the LED to light.

Using Light Emitting Diodes

        LEDs have many uses.

• Specialized LEDs can be used in various kinds of displays.

• You can use LEDs as indicators with logic circuits.

A typical circuit for logic signal indication is shown below.  In this circuit, you want the LED to emit light when the output of the AND gate is a logical one.

        If you have a logic signal, then the following will occur.

• If Vin is 5v (logical 1) and R is properly chosen and the LED is in the circuit correctly, the LED will light.

• If Vin is zero, the LED will not light.

• These statements will be true even in the real situation where a logical 1 is not exactly 5v, and where a logical zero may be closer to 1v.  It helps that the LED takes over 1.5v to turn on and start emitting light because a logical one can be a voltage up to 1.5 v.

        To use this circuit there is at least one other caution.

• The LED can be inserted into the circuit two ways.  One way works, the other doesn't.

• The LED package has a small "flat" on the flange.  The flat corresponds to the end of the diode.  In the circuit model for the diode, the flat corresponds to the bottom end in the picture below.

What If?

        What if you put 3.2W

in the circuit inadvertently?  That would mean you might have had one ampere flow through the LED.  In that case, the LED will probably never again emit any light.

• The LED has gone through an LED to DED conversion.

• DED stands for Darkness Emitting Diode.

• By virtue of this act you are now permitted to go on the Gradeful DED Tour.

• That's it for this lesson, and

• It's about time!