74HC gate and buffer drawer
13 chips on breadboards, powered, every input tied low and ready to rewire. There is no microcontroller in it: the simulator solves the 13 parts as a circuit, so it runs the moment you press Run, and nothing needs compiling.
Teaching? Assign it to your class in one click.
Beginner About 20 minutes to build and run. Runs in your browser. Free, and no account needed.
The parts retail for about $21. On Mokxi they're free.
What you'll learn
- Gates with three and four inputs
- What a tri-state buffer does with its output enable
- How a bus buffer passes eight lines at once
- Comparing two 8-bit values with a comparator
- Why unused inputs must be tied to a known level
- Difficulty: Beginner
- Time: About 20 minutes
- Parts: 19
How it works
This is a drawer of 13 chips on three breadboards: multi-input NAND, AND and NOR gates, an XNOR, a Schmitt NAND, tri-state buffers, bus buffers and an 8-bit comparator. Each has 5 V on VCC and ground on GND.
Every input is wired to ground, so nothing floats. Nothing else is connected, and no code runs, because there is no microcontroller.
You pick a chip and move an input's wire to the plus rail. Then you put an LED and a resistor on an output and watch it. Each chip's help page has its pins and truth table.
Parts list
19 parts, plus the jumper wires. They retail for about $21; on Mokxi they're free.
- 3 × Full-size breadboard
- 3 × Power, 5 V
- 1 × 74HC10 triple 3-input NAND
- 1 × 74HC11 triple 3-input AND
- 1 × 74HC20 dual 4-input NAND
- 1 × 74HC21 dual 4-input AND
- 1 × 74HC27 triple 3-input NOR
- 1 × 74HC30 8-input NAND
- 1 × 74HC7266 quad XNOR
- 1 × 74HC132 quad Schmitt NAND
- 1 × 74HC125 quad tri-state buffer
- 1 × 74HC126 quad tri-state buffer
- 1 × 74HC244 octal buffer
- 1 × 74HC245 two-way bus buffer
- 1 × 74HC688 8-bit comparator
Wiring table
Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.
| Part and pin | Connects to |
|---|---|
| 74HC10 triple 3-input NAND pin VCC | 5 V |
| 74HC11 triple 3-input AND pin VCC | 5 V |
| 74HC20 dual 4-input NAND pin VCC | 5 V |
| 74HC21 dual 4-input AND pin VCC | 5 V |
| 74HC27 triple 3-input NOR pin VCC | 5 V |
| 74HC30 8-input NAND pin VCC | 5 V |
| 74HC10 triple 3-input NAND pin 1A | Ground |
| 74HC10 triple 3-input NAND pin 1B | Ground |
| 74HC10 triple 3-input NAND pin 2A | Ground |
| 74HC10 triple 3-input NAND pin 2B | Ground |
| 74HC10 triple 3-input NAND pin 2C | Ground |
| 74HC10 triple 3-input NAND pin GND | Ground |
| 74HC10 triple 3-input NAND pin 3A | Ground |
| 74HC10 triple 3-input NAND pin 3B | Ground |
| 74HC10 triple 3-input NAND pin 3C | Ground |
| 74HC10 triple 3-input NAND pin 1C | Ground |
| 74HC11 triple 3-input AND pin 1A | Ground |
| 74HC11 triple 3-input AND pin 1B | Ground |
| 74HC11 triple 3-input AND pin 2A | Ground |
| 74HC11 triple 3-input AND pin 2B | Ground |
| 74HC11 triple 3-input AND pin 2C | Ground |
| 74HC11 triple 3-input AND pin GND | Ground |
| 74HC11 triple 3-input AND pin 3A | Ground |
| 74HC11 triple 3-input AND pin 3B | Ground |
| 74HC11 triple 3-input AND pin 3C | Ground |
| 74HC11 triple 3-input AND pin 1C | Ground |
| 74HC20 dual 4-input NAND pin 1A | Ground |
| 74HC20 dual 4-input NAND pin 1B | Ground |
| 74HC20 dual 4-input NAND pin 1C | Ground |
| 74HC20 dual 4-input NAND pin 1D | Ground |
| 74HC20 dual 4-input NAND pin GND | Ground |
| 74HC20 dual 4-input NAND pin 2A | Ground |
| 74HC20 dual 4-input NAND pin 2B | Ground |
| 74HC20 dual 4-input NAND pin 2C | Ground |
| 74HC20 dual 4-input NAND pin 2D | Ground |
| 74HC21 dual 4-input AND pin 1A | Ground |
| 74HC21 dual 4-input AND pin 1B | Ground |
| 74HC21 dual 4-input AND pin 1C | Ground |
| 74HC21 dual 4-input AND pin 1D | Ground |
| 74HC21 dual 4-input AND pin GND | Ground |
| 74HC21 dual 4-input AND pin 2A | Ground |
| 74HC21 dual 4-input AND pin 2B | Ground |
| 74HC21 dual 4-input AND pin 2C | Ground |
| 74HC21 dual 4-input AND pin 2D | Ground |
| 74HC27 triple 3-input NOR pin 1A | Ground |
| 74HC27 triple 3-input NOR pin 1B | Ground |
| 74HC27 triple 3-input NOR pin 2A | Ground |
| 74HC27 triple 3-input NOR pin 2B | Ground |
| 74HC27 triple 3-input NOR pin 2C | Ground |
| 74HC27 triple 3-input NOR pin GND | Ground |
| 74HC27 triple 3-input NOR pin 3A | Ground |
| 74HC27 triple 3-input NOR pin 3B | Ground |
| 74HC27 triple 3-input NOR pin 3C | Ground |
| 74HC27 triple 3-input NOR pin 1C | Ground |
| 74HC30 8-input NAND pin A | Ground |
| 74HC30 8-input NAND pin B | Ground |
| 74HC30 8-input NAND pin C | Ground |
| 74HC30 8-input NAND pin D | Ground |
| 74HC30 8-input NAND pin E | Ground |
| 74HC30 8-input NAND pin F | Ground |
78 more connections: open it in the simulator to see them all.
The notes that come with it
There is no microcontroller in this one, so nothing is compiled: the simulator solves the circuit itself. The file in the editor is the notes that come with it.
// 74HC gate and buffer drawer.
//
// Every chip is on a breadboard and powered, and every one of its inputs is
// tied low by a wire to a ground rail, so nothing floats. Nothing else is
// wired: pick a chip, move an input's wire to the + rail, put an LED with a
// resistor on an output, and see what it does. Each chip's help page has its
// pins and its truth table.
Troubleshooting
The usual mistakes with this circuit, and the fix for each.
- A buffer output does not follow its input. This project
Check the output enable pin. Tri-state chips can disconnect their outputs, and the help page says which level turns them on. - A multi-input gate does not change when you move one wire. This project
A 3-input or 4-input AND or NAND needs every input high to change. A NOR needs every input low. Check the truth table. - A gate output is stuck or the chip behaves randomly. 74HC10 triple 3-input NAND
Never leave an input floating. Tie unused inputs to VCC for a NAND, or tie them with a used input. All three inputs must be high for the output to go low. - The chip is unpredictable or noisy. 74HC10 triple 3-input NAND
Connect VCC (pin 14) and GND (pin 7). Add a 0.1 uF decoupling capacitor close to them. 74HC runs on 2 V to 6 V, and its outputs follow the supply. - The output is stuck or random. 74HC11 triple 3-input AND
Tie unused inputs of a 74HC gate to VCC for AND, or to GND for OR. A floating CMOS input picks up noise and may draw extra current. If one input of a gate is unused, tie it HIGH. - The chip misbehaves or gets hot. 74HC11 triple 3-input AND
Power pin 14 with VCC (2 V to 6 V) and pin 7 with GND. Add a 0.1 uF capacitor close to the chip. Leaving the supply pins out gives odd results. - The output never goes low. 74HC20 dual 4-input NAND
A NAND output goes low only when all four inputs are high. A floating input is not a safe high. Tie unused inputs to VCC, or join them to a used input. - The chip behaves oddly or is hot. 74HC20 dual 4-input NAND
Power pin 14 with VCC (2 to 6 V) and pin 7 with ground. Add a 100 nF decoupling cap near the chip. The pinout has two unused pins, so do not wire to them.
Change it and keep it
Open it in the editor, change the circuit or the code, and keep your version in a free account.