Logic Gate Board Instructions
Logic Gate Board Instructions
Table of Contents
1. Introduction
1.1. What are logic circuits?
Before we can understand what logic circuits are, we first need to understand the difference between digital and analogue circuits.
Analogue circuits are those whose voltages can be any continuous value, such as 5V, 3V, and 1.5V, and such circuits can be found in various MitchElectronics kits, including the Simple Function Generator, the 555 astable and monostable kits, and the discrete op-amp kit. Digital circuits, however, are those that use two specific voltage levels, such as 5V and 0V, with one representing true (a binary 1) and the other representing false (a binary 0).
Circuits that process digital signals to compute logical functions are known as logic circuits. For example, the logic function AND produces a logical 1 output when all inputs to the AND gate are a logical 1 (more on this later), otherwise the output will be a logical 0.
The overwhelming majority of logical circuits use 0V as the false value (or binary 0), but the voltage for a true signal depends on the power supply of that circuit. Common values for the logical value of true (or binary 1) include 5V, 3.3V, and 1.2V, and in MitchElectronics kits, a true value is defined as 5V.
Key Takeaways
- Logic circuits are made using digital elements
- Logic works with truth/false (binary 1 and 0)
- Binary values can be any two voltages, but most commonly 5V and 0V
1.2. Glue logic and logic families
Logic gates are the fundamental building block of any logic circuit, and are found in almost all electronic devices. While modern devices such as CPUs integrate millions of logic gates onto a single chip, the first logic devices had to rely on sticking individual logic gates together using wires, which was (and still is) known as glue logic.
Creating logic gates can be done using discrete transistors, but such a design gets extremely large very fast (early computers in the 50s made using this technology would take up entire rooms). Instead, we can use discrete logic chips that integrate a few gates, helping to reduce the size of designs.
The two most common (and famous) family of logic chips are the 7400 series (introduced 1966) and the 4000 series (introduced 1968), which include numerous logic functions such as AND gates, NAND gates, flip-flops, shift registers, and even full adders. The 7400 series of chips came out before the 4000 series and originally utilise TTL logic, whereas the 4000 series utilised CMOS logic.
Early designs preferred the 7400 series due to its faster speeds, but as CMOS technologies improved (offering both speed and low energy consumption), it didn’t take long for the 7400 series to introduce CMOS versions. In fact, the benefits that the 7400 series and 4000 series provide still justifies their manufacture even till this day.
Key Takeaways
- Logic ICs help to reduce the size of circuits
- The two most common logic families are the 7400 series and 4000 series
- CMOS is preferred over TTL due to its lower power consumption
1.3. What is the Logic Gate Board?
Both the 7400 and 4000 series costs of hundreds of chips, with each one being useful in its own right. However, trying to learn what each chip does is completely useless as most will rarely be used in modern designs. Instead, it is better to learn how the most crucial chips work, so that you can readily use them in projects.
The Logic Range of kits, including the Gate Board, is designed to help you understand and learn how to use these chips by providing the various inputs and outputs needed to drive them. In the case of the Gate Board, this kit will allow you to test some of the most crucial logic gate chips; the 4001, 4070, 4071, 4081, and 4093.
Key Takeaways
- Logic Gate Board helps test different 4000 series logic gate chips
- Allows for testing the AND, NAND, OR, NOR, and XOR gates
2. Schematic
3. Simulation
4. How does the Logic Gate Board work?
4.1. The chips
The Gate Board is designed to allow you to test AND, NAND, NOR, OR, and XOR gates utilising the 4001, 4070, 4071, 4081, and 4093 ICs. The board itself only has one DIP socket, meaning that only one chip can be tested at a time, and because each of these chips has the same pinout, no changes to the board need to be made.
Each of these chips is a 14-DIP IC that house 4 identical logic gates, and each of these logic gates has the same pin locations for each gates inputs and outputs, allowing the chips to be swapped with ease (of course, the logic function is different). The reason why this kit doesn’t allow for testing a NOT gate chip is because there are no NOT gate ICs in a 14 DIP package following the same pinout, but a NOT gate can be realised with the NAND and NOR ICs via the second sub-circuit (see The Board for more details).
Key Takeaways
- Multiple 4000 series logic chips share common layout
- NOT gates do not come in the same packages
4.2. The board
The Gate Board integrates three input buttons, three input LEDs, and three output LEDs, which allow for the testing of various logic gates, and the entire board is split into three main logic sub-circuits. The first sub-circuit, indicated by the output LED A, allows the logic gate to be tested independently with the two switches A and B being connected into its inputs.
The second sub-circuit, indicated by the output LED B, allows the logic gate to be tested with a single switch, A, where both inputs are connected together. The final sub-circuit, indicated by the output LED C, allows for a combination of two gates to be tested, with all three switches providing inputs to this circuit.
On the sides of the board are further expansion connectors that you can use with other Logic Range kits. However, instead of providing connections to each logic circuit, only the inputs and outputs are provided on these pins.
Key Takeaways
- First sub-circuit tests gate on its own
- Second sub-circuit tests all inputs connected together
- Third sub-circuit tests simple combinational circuit
5. Component List
| Component | PCB Reference | Quantity | Looks Like |
|---|---|---|---|
| 14 DIP Socket | U1 | 1 |  |
| 100nF Capacitor | C1 | 1 |  |
| 1K Resistor | R1 - R9 | 9 |  |
| 3mm Red LED | D1 - D6 | 6 |  |
| Tactile Switch | SW1, SW2, SW3 | 3 |  |
| 4001 | U1 | 1 |  |
| 4070 | U1 | 1 |  |
| 4071 | U1 | 1 |  |
| 4081 | U1 | 1 |  |
| 4093 | U1 | 1 |  |
| PP3 Connector | BT1 | 1 |  |
6. PCB
7. Practical Exercises
7.1. Develop truth tables
In digital logic, truth tables are tables that show what the state of outputs will be depending on the state of inputs. Truth tables are absolutely vital when it comes to digital theory, as they create well defined states, allowing for predictable and repeatable behaviour.
For example, in computers, digital circuits that add numbers must always come to the same answer, otherwise it would be virtually impossible to create reliable program code. Imagine the uproar that would be caused if two different machines provided different answers to 1 + 1.
Of course, truth tables are generally made before designing a circuit, as engineers need to know what output is expected of their design. However, it can sometimes be require to work in reverse, and develop a truth table from an existing design, such as in reverse engineering or fault finding.
So, using this kit, insert each chip one by one, and develop its truth table to identify the true function of each sub-circuit and the real logic function provided by each chip!
7.2. Combinational logic
The Gate Board comes with multiple chips and a single PCB to help reduce the cost of testing various chips. However, if multiple Gate Boards are used, it is possible to connect them together to create combinational logic circuits, allowing for more advanced functions.
Furthermore, it is also possible to create feedback circuits whereby the output of a combinational logic circuit can be connected to an input (something which is important in latch circuits). Thus, using multiple Gate Boards, see if you can create the following combinational logic circuits.
- 3 Input AND Gate
- NOR Latch
- Flip-Flop
- Half Adder & Full Adder
- Rising Edge Pulse Detector (also known as a clock edge)
8. Construction Tips
8.1. Electronics Construction Guide
To learn more about how to solder electronic components, download the Electronics Construction Manual free using the button below
8.2. Component Order
Solder the components in this order to keep things simple
- Resistors
- IC Socket
- Capacitors
- LED
- Switches
Watch out for the PP3 connector!
- Make sure the red wire is connected to VCC (sometimes V+)
- Make sure the black wire is connected to 0V (sometimes GND)
Double-check your components BEFORE soldering!
- MitchElectronics kits use double-sided PCBs with plated through-holes
- This makes the PCBs extremely strong
- It also makes de-soldering very hard, so be sure components are inserted correctly
8.3. Final Thoughts
Try stress-testing the chips!
- Use a signal generator to test how fast the logic circuits can respond to signals