Digital Circuit Design Tutorial

Digital circuits make decisions.

Digital circuit design, the use of "logic gates" to design electronic circuits, allows you to create electronic circuits that can make decisions. A logic gate will produce one of two states at its outputs: true or false. This output state is dependent on what combination of true or false states are applied to the inputs of the logic gate.

Two basic types of logic gates include the "NOT" gate and the "OR" gate. Understanding basic digital circuit design requires that you know what output state each gate will produce for a given input state.

Things You'll Need

• Logic simulator
• Logic gates
• Breadboard
• Power supply
• Voltmeter

Instructions

1. Understand that the true and false state correspond to two distinct voltage levels. Realize that these two states include the battery supply voltage level, often 5 volts, known as the true state, and the battery ground voltage level, often 0 volts, known as the false state.

2. Analyze the NOT gate's logic operation. Use a logic simulator or breadboard--an actual NOT gate. Place 5 volts on the input of a NOT gate. Record the voltage at the output of the NOT gate. Conclude that when 5 volts is applied to the input of a NOT gate, the NOT gate produces a voltage of 0 volts at its output.

   Now apply a voltage of 0 volts on the input of the NOT gate. Record the voltage at the output of the NOT gate. Conclude from your measurement that when 0 volts is applied to the input of a NOT gate, the NOT gate produces 5 volts at its output.

3. Analyze the operation of a two-input OR gate. Use a logic simulator or breadboard--an actual OR gate. Apply the 0 volts to both inputs of the OR gate. Record the resultant output voltage of the OR gate. Conclude that when both the inputs of the OR gate are at 0 volts, the output of the OR gate is also at 0 volts.

4. Apply 0 volts to one input of the OR gate and 5 volts to the other input of the OR gate. Record the output voltage of the OR gate. Interchange the voltages on the input pins of the OR gate. Conclude that when one input of the OR gate is at 0 volts, and the other input is at 5 volts, the output of the OR gate will be at 5 volts.

5. Apply 5 volts to both inputs of the OR gate. Record the output voltage of the OR gate. Conclude that when both inputs of an OR gate are at 5 volts, the output of the OR gate will be 5 volts.

   Conclude that for a two-input OR gate, the output voltage will only be at 0 volts when all the inputs of the OR gate are also at 0 volts. Also conclude that if one or more inputs of the OR gate is equal to 5 volts, the output of the OR gate will also be equal to 5 volts.

Tips & Warnings

• A NOT gate is designed to convert a 5-volt signal to a 0-volt signal, and a 0-volt signal to a 5-volt signal. Also, the NOT gate will only convert 0 and 5 volt signals; it will not reliably convert voltage levels between 0 and 5 volts to a specific voltage level. Don't apply any other voltage levels to the input of the NOT gate. Remember: Many digital logic circuits are designed to operate at 3-volt and 0-volt logic levels as opposed to 5-volt and 0-volt logic levels.

• A voltage of 5 volts or 3 volts is often referred to as the high, logic one, true or binary one state. A voltage of 0 volts is often referred to as the low, the logic zero, the false or the binary zero state.

References

• Play Hookey: Digital Logic
• All About Circuits: Volume IV Digital
• Innovatia Technologies: Logic Gates
• Digital Logic Systems: Digital Circuit Introduction for Beginners and Bystanders