The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation, and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-flop element.
The 555 has three operating modes:
Monostable mode: In this mode, the 555 functions as a "one-shot" pulse generator.
Astable (free-running) mode: The 555 can operate as an oscillator.
Bistable mode or Schmitt trigger: The 555 can operate as a flip-flop.
| Pin | Name | Purpose |
|---|---|---|
| 1 | GND | Ground reference voltage, low level (0 V) |
| 2 | TRIG | The OUT pin goes high and a timing interval starts when this input falls below 1/2 of CTRL voltage (which is typically 1/3 of VCC, when CTRL is open). |
| 3 | OUT | This output is driven to approximately 1.7V below +VCC or GND. |
| 4 | RESET | A timing interval may be reset by driving this input to GND, but the timing does not begin again until RESET rises above approximately 0.7 volts. Overrides TRIG which overrides THR. |
| 5 | CTRL | Provides "control" access to the internal voltage divider (by default, 2/3 VCC). |
| 6 | THR | The timing (OUT high) interval ends when the voltage at THR is greater than that at CTRL. |
| 7 | DIS | Open collector output which may discharge a capacitor between intervals. In phase with output. |
| 8 | VCC | Positive supply voltage, which is usually between 3 and 15 V depending on the variation. |
Modes
The 555 has three operating modes:
Monostable mode: In this mode, the 555 functions as a "one-shot" pulse generator.
Astable (free-running) mode: The 555 can operate as an oscillator.
Bistable mode or Schmitt trigger: The 555 can operate as a flip-flop.