Monostable Multivibrator Symbol

The Monostable Multivibrator schematic symbol. Standard-specific details are listed below where available.

Definition: The Monostable Multivibrator symbol represents a single-output pulse-generating circuit block — also called a one-shot timer — that produces one fixed-width output pulse for each valid trigger event, as standardised in IEC 60617 and implemented by devices such as the 74121 and 555 timer in monostable mode.

Also known as: one-shot, one-shot timer, 74121, monostable timer, single-shot multivibrator, pulse stretcher.

What the Monostable Multivibrator symbol means

The Monostable Multivibrator symbol denotes a digital or analog timing block that has exactly one stable state. When a trigger signal is applied to the TRIG pin, the output switches to its unstable state for a defined period set by the RC timing network, then automatically returns to the idle state.

In schematic diagrams, the monostable multivibrator symbol communicates that the block converts an edge or pulse of arbitrary width into a single, predictable output pulse whose duration is determined by an external resistor-capacitor (RC) network connected at the RC pin. It is widely used in debouncing, pulse shaping, and time-delay applications.

How to identify the Monostable Multivibrator symbol

The Monostable Multivibrator symbol is drawn as a rectangular block labelled '1' or 'MONO' (or sometimes with a stylised rising-edge followed by a fixed-width pulse waveform). Three terminals are shown: TRIG on the left (trigger input), OUT on the right (pulse output), and RC at the bottom (timing network connection). Some symbols show the IEC trapezoid shape with a small square waveform icon inside.

Function and usage boundary

A monostable multivibrator generates one precisely timed output pulse each time a trigger event is detected on the TRIG pin. The pulse duration T is set by the external RC timing network (T ≈ 0.7 × R × C for a 555-based design, or per the specific IC datasheet), after which the output automatically resets. The circuit is used to create clean, repeatable pulses from noisy or variable-width input signals.

Standards: IEC vs ANSI

IEC 60617	IEC 60617-12 (binary logic elements) defines the monostable multivibrator as a rectangular logic block with the qualifying symbol '1' inside denoting one stable state. Trigger and output pins are labelled per IEC 60617 logic-symbol conventions.
ANSI/IEEE 315	ANSI/IEEE 315-1975 (and IEEE 91-1984 for logic symbols) represents the monostable as a rectangular distinctive-shape block; the input is labelled with an edge-trigger indicator and the output with a pulse-width symbol. IEEE 91 uses the dependency notation consistent with ANSI Y32.2.
Key difference	IEC 60617 uses the qualifying symbol '1' inside a rectangle to indicate monostability, while ANSI/IEEE 91-1984 may use the label 'MONO' or show a small waveform icon; both use identical rectangular block outlines and are functionally equivalent.

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Logical terminals

Logical ID	Editor terminal name
trig	TRIG
out	OUT
rc	RC

Where the Monostable Multivibrator symbol is used

Push-button debouncing: converts a noisy mechanical contact closure into one clean output pulse
Pulse-width generation: produces a fixed-duration enable or clock pulse in digital systems
Time-delay circuits: delays a downstream event by a precise RC-set interval after a trigger
Watchdog reset logic: generates a CPU reset pulse of defined width after a timeout event
Missing-pulse detection: combined with a second stage to detect absence of expected trigger pulses
One-shot strobe generation: drives camera flash or LED strobe with a fixed exposure time

Key facts

The Monostable Multivibrator symbol represents a one-shot timing circuit with exactly one stable state, returning automatically to idle after each triggered output pulse.
The three standard terminals are TRIG (trigger input), OUT (pulse output), and RC (external resistor-capacitor timing network).
Pulse duration formula for 555-based monostable: T ≈ 0.7 × R × C, where R is in ohms and C is in farads.
The 74121 is a classic TTL monostable multivibrator IC; CMOS equivalents include the 74HC123 and 74HC221.
IEC 60617-12 identifies the monostable with the qualifying symbol '1' inside a rectangular logic block, indicating one stable state.
The monostable multivibrator is non-retriggerable by default (output ignores further triggers during the pulse); retriggerable variants (e.g. 74123) restart timing on each new trigger.
Common applications include debouncing, pulse stretching, time delays, and one-shot strobe generation.

Frequently asked questions

What does the monostable multivibrator symbol mean in a schematic?

The monostable multivibrator symbol means the block generates a single fixed-width output pulse each time a valid trigger signal is applied. It has one stable state and one unstable state. The pulse width is set by an external RC network connected to the RC pin.

What does the monostable symbol look like?

The monostable multivibrator symbol is a rectangle with the label '1' or 'MONO' inside. It has a trigger input (TRIG) on the left, a pulse output (OUT) on the right, and an RC pin at the bottom for the external timing resistor and capacitor.

What is the difference between a monostable and a bistable multivibrator symbol?

A monostable multivibrator symbol ('1' inside block) shows a circuit with one stable state that returns to idle after each pulse; a bistable multivibrator symbol (flip-flop, '2' or labelled SR/JK/D) shows two stable states that hold indefinitely. The monostable produces timed pulses; the bistable stores a bit.

How is monostable pulse width calculated?

For a 555 timer in monostable mode, pulse width T ≈ 0.7 × R × C, where R is the timing resistor in ohms and C is the capacitor in farads. For a 74121, the formula is T ≈ 0.7 × REXT × CEXT per the datasheet.

What IEC standard covers the monostable multivibrator symbol?

The monostable multivibrator symbol is defined in IEC 60617-12, which covers binary logic elements. The qualifying symbol '1' inside the rectangular block indicates one stable state.

What is the designator letter for a monostable multivibrator?

Monostable multivibrator ICs are typically designated U (or IC) in schematics, following the general integrated circuit reference designator convention (e.g. U1 = 74121). The block symbol itself does not carry a single-letter standard designator.

What are common ICs that use the monostable symbol?

Common ICs represented by the monostable symbol include the 74121, 74123 (retriggerable dual), 74HC221, and the NE555/LM555 timer configured in monostable mode. All produce a single output pulse per trigger event.

Related symbols

Physical breadboard, PCB, and panel mapping

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