74LS21 Dual 4-input AND gate
The 74LS21 holds two independent 4-input AND gates. Each output is high only when all four of its inputs are high. Pins 3 and 11 are not connected.
74LS21 pinout
Top view of the 14-pin DIP package. Pin 1 is at the bottom left, beside the notch, and the numbers run anticlockwise. Power is on pin 14 (VCC, +5 V) and pin 7 (GND).
| Pin | Name | Type | What it is |
|---|---|---|---|
| 1 | 1A | Input | Input of gate 1 |
| 2 | 1B | Input | Input of gate 1 |
| 3 | NC | — | Not connected |
| 4 | 1C | Input | Input of gate 1 |
| 5 | 1D | Input | Input of gate 1 |
| 6 | 1Y | Output | Totem-pole output |
| 7 | GND | Ground | 0 V |
| 8 | 2Y | Output | Totem-pole output |
| 9 | 2A | Input | Input of gate 2 |
| 10 | 2B | Input | Input of gate 2 |
| 11 | NC | — | Not connected |
| 12 | 2C | Input | Input of gate 2 |
| 13 | 2D | Input | Input of gate 2 |
| 14 | VCC | Power | +5 V supply |
74LS21 truth table
| A | B | C | D | Y |
|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 0 | 1 | 0 |
| 0 | 0 | 1 | 0 | 0 |
| 0 | 0 | 1 | 1 | 0 |
| 0 | 1 | 0 | 0 | 0 |
| 0 | 1 | 0 | 1 | 0 |
| 0 | 1 | 1 | 0 | 0 |
| 0 | 1 | 1 | 1 | 0 |
| 1 | 0 | 0 | 0 | 0 |
| 1 | 0 | 0 | 1 | 0 |
| 1 | 0 | 1 | 0 | 0 |
| 1 | 0 | 1 | 1 | 0 |
| 1 | 1 | 0 | 0 | 0 |
| 1 | 1 | 0 | 1 | 0 |
| 1 | 1 | 1 | 0 | 0 |
| 1 | 1 | 1 | 1 | 1 |
What the 74LS21 is used for
- Four-literal product terms, such as the minterms of a four-variable function
- Detecting one 4-bit value
Electrical summary
| Package | 14-pin DIP |
| Supply voltage | 5 V (4.75 V to 5.25 V) |
| Supply current, typical | 1.7 mA |
| Output type | Totem-pole — drives both high and low |
| Output current, low | sinks up to 8 mA |
| Output current, high | sources up to 0.4 mA |
| Input current, low | 0.4 mA out of each input held low |
| Fan-out | one output can drive about 20 LS inputs |
Typical figures for quick reference when planning a board. Check the manufacturer's datasheet for limits and timing before committing a design. The 74HC21 and 74HCT21 use the same pin numbers, with different electrical values.
Try the 74LS21 on a breadboard
The Breadboard Simulator has the 74LS21 in its parts library: seat it on a virtual breadboard, wire it, power the board and probe the pins before you build the real thing.