74LS42 BCD to decimal, 1-of-10, active-low
The 74LS42 is a BCD to decimal decoder. It takes a 4-bit BCD digit on D C B A and pulls exactly one of its ten active-low outputs low. For the six invalid codes, 10 to 15, every output stays high.
74LS42 pinout
Top view of the 16-pin DIP package. Pin 1 is at the bottom left, beside the notch, and the numbers run anticlockwise. Power is on pin 16 (VCC, +5 V) and pin 8 (GND).
| Pin | Name | Type | What it is |
|---|---|---|---|
| 1 | 0′ | Output | Totem-pole output, active-low |
| 2 | 1′ | Output | Totem-pole output, active-low |
| 3 | 2′ | Output | Totem-pole output, active-low |
| 4 | 3′ | Output | Totem-pole output, active-low |
| 5 | 4′ | Output | Totem-pole output, active-low |
| 6 | 5′ | Output | Totem-pole output, active-low |
| 7 | 6′ | Output | Totem-pole output, active-low |
| 8 | GND | Ground | 0 V |
| 9 | 7′ | Output | Totem-pole output, active-low |
| 10 | 8′ | Output | Totem-pole output, active-low |
| 11 | 9′ | Output | Totem-pole output, active-low |
| 12 | D | Input | Input |
| 13 | C | Input | Input |
| 14 | B | Input | Input |
| 15 | A | Input | Input |
| 16 | VCC | Power | +5 V supply |
74LS42 truth table
| D | C | B | A | 0′ | 1′ | 2′ | 3′ | 4′ | 5′ | 6′ | 7′ | 8′ | 9′ |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 0 | 0 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 0 | 1 | 0 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 0 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 1 | 0 | 0 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 |
| 0 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 |
| 0 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 |
| 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 |
| 1 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 |
| 1 | 0 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
What the 74LS42 is used for
- Selecting one of ten devices or digits
- Decoding a decade counter
- A 3-to-8 decoder, by using D as an active-low enable
Electrical summary
| Package | 16-pin DIP |
| Supply voltage | 5 V (4.75 V to 5.25 V) |
| Supply current, typical | 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 74HC42 and 74HCT42 use the same pin numbers, with different electrical values.
Try the 74LS42 on a breadboard
The Breadboard Simulator has the 74LS42 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.