555 Timer IC Symbol
Definition: The 555 Timer IC symbol represents a versatile monolithic timing integrated circuit — originally designated NE555 by Signetics in 1972 — shown in circuit diagrams as a rectangular block with eight labelled pins (GND, TRIG, OUT, RESET, THRESH, DISCH, VCC, CTRL) that can be configured as a monostable (one-shot), astable (free-running oscillator), or bistable (flip-flop) circuit, following IEEE 315-1975 / ANSI Y32.2 rectangular IC symbol conventions.
Also known as: NE555, LM555, NA555, 555 IC, timer IC, 555 oscillator, 555 monostable, 555 astable, SE555 (military grade).
What the 555 Timer IC symbol means
The 555 Timer IC symbol identifies a general-purpose timer chip in a schematic, indicating that the circuit at that node controls timing intervals, oscillation frequencies, or pulse-width generation. The symbol's eight pins provide voltage supply, trigger input, threshold detection, discharge control, reset, output, and a control voltage reference, enabling a wide variety of timing functions using only a few external resistors and capacitors.
In circuit diagrams the 555 Timer IC symbol signals to the reader that the surrounding RC network defines the timing behaviour: in monostable mode a single RC pair sets a one-shot pulse width; in astable mode two resistors and one capacitor set the oscillation frequency and duty cycle; in bistable mode trigger and threshold pins are used as set/reset inputs. The 555 timer is one of the most widely used ICs in electronics history.
How to identify the 555 Timer IC symbol
The 555 Timer IC symbol is drawn as a rectangle with eight pins labelled on the perimeter: GND (pin 1) and TRIG (pin 2) on the left, OUT (pin 3) on the left or bottom, RESET (pin 4) on the top or left, CTRL (pin 5) on the right, THRESH (pin 6) on the right, DISCH (pin 7) on the right, and VCC (pin 8) on the top. The IC reference designator 'U' (e.g., U1) and part number '555' or 'NE555' are printed beside or inside the rectangle. Some schematics show the internal voltage divider and comparator structure inside the symbol, but most use the simplified 8-pin box.
Function in a circuit
The 555 Timer IC uses an internal voltage divider (three 5 kΩ resistors) to set two reference thresholds at 1/3 VCC and 2/3 VCC, feeding two comparators that control an SR flip-flop which drives the output and an open-collector discharge transistor. In monostable mode, a negative trigger pulse at TRIG starts a timed output pulse whose width is T = 1.1 × R × C. In astable mode, the output oscillates at f = 1.44 / ((R1 + 2×R2) × C) with adjustable duty cycle. The RESET pin (active-low) immediately forces the output low regardless of timer state. The CTRL pin can modulate the threshold voltage for frequency modulation or pulse-width modulation applications.
Standards: IEC vs ANSI
| IEC 60617 | The 555 Timer IC symbol in IEC-style schematics follows IEC 60617-12 rectangular logic/IC symbol conventions with a reference block labelled by function or part number. The internal structure may be shown using IEC qualifying symbols for comparators and flip-flops. |
|---|---|
| ANSI/IEEE 315 | IEEE 315-1975 / ANSI Y32.2 defines the general rectangular IC symbol used for the 555 timer; pin numbers and labels follow the manufacturer's datasheet. The designator letter is 'U' for integrated circuit per IEEE 315. |
| Key difference | IEC 60617-12 and IEEE 315-1975 both use a rectangular block for the 555 IC. IEC schematics may show functional internal blocks; ANSI/IEEE schematics typically show only the perimeter pins with numeric labels. Functional representation is identical. |
Terminals / pins
| Pin | Name |
|---|---|
| gnd | GND |
| trigger | TRIG |
| output | OUT |
| reset | RESET |
| threshold | THRESH |
| discharge | DISCH |
| vcc | VCC |
| ctrl | CTRL |
Typical values
Supply voltage: 4.5V–16V (NE555), 2.7V–16V (CMOS TLC555). Output current: sink/source up to 200mA. Timing range: microseconds to hours. Astable frequency range: <1Hz to approximately 500kHz. Comparator input threshold: 1/3 VCC (trigger) and 2/3 VCC (threshold). Timing accuracy: ±1% typical with stable RC components.
Where the 555 Timer IC symbol is used
- Monostable pulse generators for debouncing, delay circuits, and one-shot timing in digital systems
- Astable oscillators generating square waves for LED flashers, buzzers, and clock signals
- PWM (pulse-width modulation) generation for motor speed control and LED dimming
- Missing-pulse detectors that trigger an alarm when expected pulses stop arriving
- Touch/capacitance sensors exploiting the TRIG pin's sensitivity to capacitive coupling
- Frequency-to-voltage converters and tone generators in audio circuits
- Educational electronics kits — the 555 timer is the canonical first IC circuit for beginners
Example
In a simple LED flasher circuit, the 555 Timer IC symbol appears with a 10kΩ resistor from VCC to DISCH, another 10kΩ from DISCH to THRESH/TRIG, and a 10µF capacitor from THRESH to GND; this astable configuration produces a ~1.4Hz square wave at OUT, causing an LED (with series resistor) to flash once per second — the 555 timer symbol on the schematic immediately identifies the oscillator core.
Key facts
- The 555 Timer IC symbol represents a monolithic timing IC that can be configured for monostable (one-shot), astable (oscillator), or bistable (SR latch) operation using external resistors and capacitors.
- The IC has eight pins: GND (1), TRIG (2), OUT (3), RESET (4), CTRL (5), THRESH (6), DISCH (7), VCC (8) — the pin numbers are standardised across all NE555-compatible devices.
- In monostable mode the output pulse width is T = 1.1 × R × C seconds; in astable mode the frequency is approximately f = 1.44 / ((R1 + 2×R2) × C) Hz.
- The internal voltage divider sets comparator thresholds at 1/3 VCC (trigger) and 2/3 VCC (threshold/reset), which is the origin of the '555' name (three 5kΩ resistors).
- The RESET pin (pin 4, active-low) immediately forces OUT low when pulled to GND, overriding all other timing states.
- The CTRL pin (pin 5) allows the internal 2/3 VCC threshold to be varied externally, enabling frequency modulation or PWM applications.
- The 555 timer is defined in IEEE 315-1975 / ANSI Y32.2 as a standard rectangular IC symbol with reference designator 'U', and is one of the best-selling ICs of all time.
Diagrams that use this symbol
Frequently asked questions
What does the 555 timer symbol look like in a schematic?
The 555 timer symbol is a rectangle with eight perimeter pins labelled by function: GND, TRIG, OUT, RESET on one side and CTRL, THRESH, DISCH, VCC on the other. The reference designator U (e.g., U1) and the part number NE555 or LM555 are written beside the block. Active-low pins (RESET) are sometimes marked with an overbar.
What does the 555 timer IC symbol mean in a circuit diagram?
The 555 timer IC symbol means a precision timing integrated circuit is used at that point. The surrounding RC components (resistors and capacitors connected to TRIG, THRESH, DISCH pins) determine whether the circuit operates as a one-shot timer (monostable), a free-running oscillator (astable), or a flip-flop (bistable). The OUT pin delivers the timed or oscillating signal.
How many pins does the 555 timer IC symbol have?
The 555 timer IC symbol has eight pins: pin 1 GND, pin 2 TRIG (trigger), pin 3 OUT (output), pin 4 RESET (active-low), pin 5 CTRL (control voltage), pin 6 THRESH (threshold), pin 7 DISCH (discharge), and pin 8 VCC (supply voltage). All pins are standardised across NE555, LM555, and CMOS TLC555 variants.
What is the difference between monostable and astable modes of the 555 timer?
In monostable mode, a single trigger pulse on TRIG starts one output pulse of width T = 1.1 × R × C, then the output returns low until retriggered. In astable mode, the output continuously oscillates between high and low with no trigger needed; frequency is set by f ≈ 1.44 / ((R1 + 2×R2) × C). Monostable produces a single timed pulse; astable produces a continuous square wave.
What standard defines the 555 timer IC symbol?
The 555 timer IC symbol follows IEEE 315-1975 / ANSI Y32.2 for the rectangular IC block format used in North American schematics. IEC-style schematics use IEC 60617-12 conventions. Both standards use a rectangular symbol with labelled pins; the designator letter 'U' is assigned to integrated circuits per IEEE 315.
What is the CTRL pin on the 555 timer symbol used for?
The CTRL pin (pin 5) of the 555 timer provides access to the internal 2/3 VCC voltage divider threshold. By applying an external voltage to CTRL, the oscillation frequency or pulse width can be varied without changing the RC components, enabling voltage-controlled oscillator (VCO) and pulse-width modulation (PWM) applications. In circuits where CTRL is unused, a 10nF bypass capacitor to GND is connected to reduce noise pickup.
What are common 555 timer IC variants?
Common 555 timer variants include the NE555 (commercial grade, bipolar, Signetics original), LM555 (National Semiconductor/TI equivalent), SA555 / SE555 (industrial/military grade), and the CMOS TLC555 / ICM7555 (lower power, wider voltage range, 2.7V–16V, suitable for battery circuits). The 556 is a dual 555 in a single 14-pin package.
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