ELEC 101, Spring 2005
Prof. Rich Kozick

## Laboratory 13 Sequential Logic Circuit Design: Game Show Detector

Please bring the Bobrow textbook and your lecture notes to lab to help you with sequential logic circuit design.

Problem: Design and implement a two-button game show detector circuit. A summary of the circuit operation is as follows.

Inputs: Boolean X1, X2 for the contestants' buttons, and CLEAR
Outputs: Boolean Q1, Q2 to indicate which contestant pressed the button first
Initially, Q1 = Q2 = 0
If X1 = X2 = 0 and no button was pressed, then remain at Q1 = Q2 = 0
If X1 = 1 first, then Q1 = 1 and Q2 = 0
If X2 = 1 first, then Q1 = 0 and Q2 = 1
CLEAR input resets the state to Q1 = Q2 = 0

Question: Can you design this circuit using combinational logic only? Please explain.

Suggested Procedure:

1. Develop a state diagram and a transition table.
2. Decide whether to use D flip-flops (7474 IC) or JK flip-flops (7476 IC). You may want to design the circuit both ways: once with D flip-flops and again with JK flip-flops. Then you can evaluate the designs and decide which to implement.
3. ICs containing 2-input NAND gates (7400) and 3-input NAND gates (7410) will be available in lab.
4. Record in your lab notebook all of the steps used to design your system.
5. Use push-buttons for the inputs X1 and X2 (wiring instructions are given below), LEDs for the outputs Q1 and Q2, and for the clock use either the 555 timer or a square wave from the function generator.

** Each person should be sure that they understand the design procedure. **

Group Implementation: Since time in the lab is limited, please work together to implement two or three working systems. You may want to divide the wiring into subsystems, with different lab groups wiring the combinational logic, the flip-flops, and the push button inputs & LED outputs.

Wiring the push-buttons: The push-button switches have four pins and have the following characteristics:

• There are two pairs of internally connected pins, the left pair and the right pair.
• The left pair of pins are connected internally to each other.
• The right pair of pins are connected internally to each other.
• The left pair of pins are not connected to the right pair of pins when the button is not depressed.
• The left pair of pins are connected to the right pair of pins when the button is depressed.

Use your ohm meter to determine which pins form each pair of internally connected pins. Also, use your ohm meter to confirm that both pairs of pins are not connected when the switch is not depressed and are connected when the switch is depressed.

Follow the circuit diagram below and construct the switches such that when the switch is not depressed, Vout is 0V, and when the switch is depressed, Vout is ~5V. Instead of the 1 Mohm resistor in the circuit diagram, you may use resistors in the kohm range.

Thank you and have fun.