Output LED
The LED is an electronic device that converts current into visible light. Similar to regular diodes, LEDs pass current in only one direction. Also, there is a voltage drop across the LED, but the value is not 0.7V as with regular diodes.
Obtain a red LED and identify cathode and anode leads. The cathode is usually marked by the flat spot on the plastic case. If it is a brand new LED, the longer pin is the anode.
Tip: The flat spot on the case corresponds to the flat line of the diode symbol.
We start with the simple circuit shown at the right. Build the circuit on your breadboard. Measure voltage Vout when the switch is open and when it is closed. Calculate the current Ilo.
Important: Any time you calculate something, provide 3 things: formula, individual values of every item in the formula, final result (number and unit).
Switch open:
Vcc=, V:
Vout=, V
LED is ON or OFF (circle correct answer)
Switch closed:
Vcc=, V
Vout=, V
Ib=
LED is ON or OFF (circle correct answer)
Let us see now how we can use an LED to visualize logic level at a gate output. There are two ways to connect an LED to the output of a gate. Both will work, but one is the preferred way.
Build the circuit shown below. This configuration is called "current sourcing" because the output of the gate delivers current for the LED, i.e. the gate works as a source of current. A 74LSxx gate can only "source" about 0.4mA without losing output voltage.
