CSC270  Lab #2

EXPERIMENT 1: The Diode. Finding the passing direction

Using the breadboard part of the kit, take a 1N914 diode and put it in a circuit with a 1K-Ohm resistor. Power the circuit with a 5V supply and find its passing direction, that is the direction in which the current flows through it. What is the relationship between the symbol for a diode and its physical markings?

EXPERIMENT 2: A diode and resistor AND-gate.

Figure 2 shows an AND gate implemented with two diodes and a resistor. The figure shows only the gate, and not the input and output circuits necessary to test it.



Connect the inputs to Data Switches and the output to an Indicator, and test the AND gate. Use the voltmeter to Measure the voltages at the inputs and at the output when you activate the switches and set them to all four possible combinations (00, 01, 10, 11). Record your observation in a truth table.

Look especially at the way the current flows through the output wire when the output voltage is low and when it is high. What is interesting (puzzling maybe) about the flow of information and the flow of current.

What happens when the two inputs are disconnected from the Data Switches? What signals does the circuit "see" on its inputs in that case? In other words, is a disconnected input treated as a high signal, or as a low signal?

EXPERIMENT 3: The 7408 Integrated Circuit

Get a 7408 (or 74LS08, or 74S08) Integrated Circuit (IC) and put it on the breadboard part of the kit. Use the information shown in Figure 3 to wire the circuit up.

Use the data sheet distributed in class (or get a new copy by clicking here) to see the actual contents of the 7408, and to understand what your experiment actually tests. Observe that the 7408 contains 4 AND-gates, and you are testing one of them, the one with inputs connected to Pins 1 and 2, and the output connected to Pin 3.

Always make sure that the circuit is oriented with its notch to your left, the inscriptions on it facing you, so that the top left pin is Pin 14, and the bottom right Pin 7.

GENERAL RULE: the top left pin is always connected to +5V and the bottom right to Ground (GND).



Figure 3: Wiring of an IC on the breadboard. Always identify Pin 1 of the IC which and orient the IC so that Pin 1 is at the lower left corner of the chip when looking at it from your seat.

Connect Pin 1 of the IC to Switch 1 (input A), Pin 2 to Switch 2 (input B), and Pin 3 to a logic indicator. Connect Pin 1 and Pin 2 to two other logic indicators.

Activate the switches in all four possible combinations, and create the truth table for the gate.

A	B	Output
0	0
0	1
1	0
1	1

Now, repeat the same experiment, but record the voltages on the inputs and on the output for all four combinations of the inputs. Since you have only 1 volt-meter, you will have to make several measurements. Record your observations in the table below.

A	B	Output

What voltages qualify as "low" and "high" voltages? Are you surprised at all by the measurments you made?

EXPERIMENT 4: The Transistor

The transistor is a three-pole semiconductor. The poles are polarized and have well defined functions. They are called the collector, the base, and the emitter.

The transistor you will be using today is the 2N5772, and the manufacturer (fortunately) marked the poles on the package (E, B, and C).

Implement the circuit shown in Figure 4. Connect the input of the circuit to a switch, and its output to an Indicator. Observe how the output changes as a function of the input. Generate a truth table for this circuit. What logic function does it implement?

EXPERIMENT 5: An OR-gate

Using all the information you know, the circuits you have seen so far, implement an OR-gate (you cannot use the 74LS32 circuit that you used last week!). Demonstrate that it works.