NE555 Timer, wired as an astable clock
The NE555 is the timer every logic lab uses as its clock. Wired as an astable — R1 from VCC to DIS, R2 from DIS to THR, THR joined to TRIG, and a capacitor C from there to ground — its output is a square wave of f = 1.44 / ((R1 + 2 R2) C), high for 0.693 (R1 + R2) C and low for 0.693 R2 C. It is not a 74-series part, but it sits on the same breadboard and drives TTL inputs directly.
NE555 pinout
Top view of the 8-pin DIP package. Pin 1 is at the bottom left, beside the notch, and the numbers run anticlockwise. Power is on pin 8 (VCC, +5 V) and pin 1 (GND).
| Pin | Name | Type | What it is |
|---|---|---|---|
| 1 | GND | Ground | 0 V |
| 2 | TRIG | Input | Input |
| 3 | OUT | Output | Totem-pole output |
| 4 | RESET′ | Input | Input, active-low |
| 5 | CTRL | Input | Input |
| 6 | THR | Input | Input |
| 7 | DIS | Input | Input |
| 8 | VCC | Power | +5 V supply |
NE555 function table
| RESET′ | OUT |
|---|---|
| 0 | 0 — held low |
| 1 | a square wave of f = 1.44 / ((R1 + 2 R2) C) |
In the Breadboard Simulator the frequency is set on the chip (0.5 to 4 Hz), and its inspector gives R1, R2 and C for it; Run and Step move time on.
What the NE555 is used for
- A slow clock for counters, shift registers and state machines you can watch
- Blinking an LED
- Debouncing a push button, wired as a monostable
- Tone for a buzzer at audio frequencies
Not 74-series: the timer every lab uses as a clock. Wired as an astable — R1 from VCC to DIS, R2 from DIS to THR, THR joined to TRIG, and C from there to ground — OUT is a square wave of f = 1.44 / ((R1 + 2 R2) C). Here the frequency is set on the chip, and its inspector gives R1, R2 and C for it. RESET′ low holds OUT low.
Electrical summary
| Package | 8-pin DIP |
| Supply voltage | 5 V (4.75 V to 5.25 V) |
| Supply current, typical | 3 mA |
| Output type | Totem-pole — drives both high and low |
| Output current, low | sinks up to 100 mA |
| Output current, high | sources up to 100 mA |
| Input current, low | 0.01 mA out of each input held low |
| Fan-out | one output can drive about 250 LS inputs |
Typical figures for quick reference when planning a board. Check the manufacturer's datasheet for limits and timing before committing a design.
Try the NE555 on a breadboard
The Breadboard Simulator has the NE555 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.