WEEK

We are going to explore electricity and electronics.

GOALS (This week will be a continuation of week 4):

· Build an RS flip flop using two NAND gates.

· To be able to understand the RS truth table based on measurements.

· To be able to understand the truth table based on the truth table for the NAND.

· To use the RS flip-flop in conjunction with a switch to make an electronic signal that can be counted.

Last week some students already began to work on combining two NAND gates together to form an RS flip-flop. This is an example of how one can take the basic logic gates and build a device with new characteristics. In principle an entire computer could be built using just these gates. We can think of them as the basic building blocks.

The RS flip-flop begins to address the transitory nature of information. If you provide two true signal to the inputs of an AND gate the resulting output is true. This result is lost however when the input information changes. In many applications we would like the result of some logic operation to set a particular response for a time and for a time period that is not just equal to the time the information is available. The RS flip-flop can be used to do just that function. The S or set line can set the Q output while the R or reset line can reset the Q output. After you build the device you can explore how it works. You will discover that once the set line has set the flip-flop ots value is irrelevant as long as the R line is held at 5V. Also once the flip-flop is reset the state of the reset line doesn't change the output as long as the S line remains at 5V. We then proceed to add some resistors and a switch to make a circuit that generates one and only one pulse every time we change the switch position. Again the understanding of the circuit is greatly facilitated by building it. For now let us consider some important aspects of the circuit components. First the logic gates that we are using act like very big resistors. When you attach 5 Volts to the input through a 2.2kW resistor, the voltage drop across the 2. 2kW resistor is very small because the current drawn through the gate is very small. This is a bit complicated but something we can begin to understand if we think about it. The voltage at the input will be about 5 Volts even with the resistor in the circuit because all the "push" is used across the gate itself and not much is used to push the tiny current through the resistor.. (A student could experiment and see what happens if a 100 MW were used instead.) The fact that these gates are high impedance (look like large resistors) is an important characteristic of the gates.

The switch is the used to generate signals to count. We run into a problem however. We cannot guarantee that the switch will generate just one contact. The switch might bounce thereby opening and closing several times. While these short make-break connections would be undetectable to us, a counting circuit would see several rather than one signal. The RS flip-flop alleviates the problem. It will be easy to see this by exploring this in the lab.

Once we have something to count we can use a special IC to do the counting. We need to power up the IC and set some inputs to specific values and provide a connection to the signal we wish to count. A handout will show these connections. The current value of the counter will be displayed as 1's an 0's or on/off using LEDs. This will introduce the binary numbers. This number system is important because it uses only two symbols 1,0 to write a number. The decimal system requires ten (0,1,2,3,4,5,6,7,8,9) symbols. Hopefully we know how to count so we can try and relate the number displayed in binary form to the counter value. Again this is easy to explore in the lab.

QUIZ *******

QUIZ *******

In order to receive a grade of **A** you must successfully complete the
lab portion of the course, read prelab material, and pass the lab quiz. In
order to pass you must get 75% of the questions correct.

In order to receive a grade of **B** you must successfully complete the
lab portion of the course. The quiz is optional. For fun feel free to try the
quiz.

You are not allowed to consult with other students while taking the quiz. You will be given a limited time period to complete the quiz. You may refer to any written material.

You may take the quiz at any time during the week but only once.

The quiz will be available starting on the day you cover the material and ending one week later.

You are not allowed to consult with other students while taking the quiz. The quiz will be available at least from lab until following lab. You will be given 30 minutes to complete the quiz and you may refer to any written material.