555 Timer
The 555 timer is one of the most recognizable general-purpose ICs in electronics. It can create a single timed pulse, oscillate continuously, stretch a pulse, blink an LED, create a clock signal, or act as part of many simple control circuits.
Why it belongs here
Inside the 555 are analog comparators, a resistor divider, switching circuitry, and a latch. In modern language it is reasonable to call it a mixed analog/digital timing IC, although it has traditionally been grouped with linear or analog ICs.
| Pin name | Basic job |
|---|---|
| TRIG | Starts a timing action when the voltage falls below the lower internal threshold. |
| THRES | Ends a timing action when the timing voltage rises above the upper internal threshold. |
| DISCH | Provides a path to discharge the external timing capacitor. |
| RESET | Overrides the timing cycle and forces reset when asserted. |
| CONT | Allows the internal threshold levels to be shifted. |
| OUT | The timer output. |
| VCC / GND | Power connections. |
Two thresholds make the timing work
In the classic bipolar 555, the internal reference divider establishes thresholds near 1/3 VCC and 2/3 VCC. The external capacitor charging and discharging through those thresholds creates the timing action.
Two famous modes
- Monostable: one trigger produces one timed output pulse.
- Astable: the circuit repeatedly charges and discharges the timing capacitor and becomes an oscillator.
Why it survived
The 555 is not always the best modern solution, but it remains an excellent teaching part because the timing capacitor, thresholds, latch, and output action can all be followed directly.
Manufacturer reference: TI NE555.