Lab 1 - LED Circuit

For the first part of the lab, we were asked to design a circuit using a transistor and a logic gate of our choice to light up an LED when both switches are closed. The circuit schematic, as well as the circuit breadboard diagram are shown below.

The resistor values for this circuit were justified as follows:

Pull-Up Resistor:
The value of the pull-up resistor depended on the high level input current and the high level output voltage the logic gate was rated for. Reading from the data sheet for this particular 2-input NOR Gate, V_in = 2.7 V and i1 = 20e-6 A. Using Kirchhoff's current laws, the equation we used for this particular part of the circuit is:









We could not find a resistor that was exactly 115kOhms, so we opted for a 112kOhm resistor instead. The value for R2 is identical to that of R1, since both resistors serve the same purpose and are there to limit the input current into the logic gate.

Current Limiting Resistor (LED):
The current limiting resistor, true to its name, was limited by the maximum current rating of the LED we were using in our circuit. The maximum current the LED could handle was approximately 0.02A, which means that the current from the collector to the emitter could not exceed this value. The voltage drop due to the LED, or V_d, was approximately 1.8V. We also knew from the spec sheet of the transistor that the voltage drop from the current to the emitter was approximately equal to 0.3V. Thus, the equation we used to calculate R4 is as follows:




We found a resistor close to this value, 150 Ohms, and used it in our circuit instead.

Base resistor:
The resistor between the output of the logic gate and the base of the transistor, R3, was determined by the specifications of the transistor. The transistor had a voltage drop of 0.6V from the base to the emitter and a DC Current gain of about 10. This means that the current i, from the logic gate to the base of the transistor is approximately 10 times the current from the collector to the base of the transistor. Since the current from the base of the transistor is limited by the maximum current rating of the LED to be 0.02A (as determined above), we could simply scale that by adding the beta value into the equation as follows:




Once again, we could not find a resistor that was exactly the value we have calculated to be ideal and we used a 1.5kOhm resistor instead.