Monostable Multivibrator

Why a one-shot?

Many circuits need to convert a random trigger event into a precisely-timed pulse. A pushbutton press is messy — contact bounce, finger time, switch chatter. A sensor edge might be nanoseconds wide or microseconds wide depending on conditions. Downstream logic often needs a predictable, fixed-duration HIGH signal: a relay must close for at least 50 ms, a triac gate must be driven for 10 μs, a camera flash must fire for exactly 1 ms. The monostable multivibrator solves this by emitting one calibrated pulse for each trigger.

It is the analog cousin of a microcontroller's "delay" function — except it costs ten cents in parts, runs without firmware, and has nanosecond-class trigger latency. The 555 timer, introduced in 1971, has been the dominant monostable implementation for half a century.

555 monostable topology

           V_CC
             │
             ├─── R ──┬─── 7 (DIS, discharge transistor)
             │        │
             │        ├─── 6 (THRESH)
             │        │
             │        ├─── 2 (TRIG) ←── trigger pulse (briefly < ⅓ V_CC)
             │        │
             │        ─── C ─── GND
             │
        ────┤NE555├──── 3 (OUT) → HIGH for T = 1.1·R·C after trigger
             │
            GND

   Internal: two comparators tied to ⅔ V_CC and ⅓ V_CC,
   one SR flip-flop, one discharge transistor.

The trigger arrives at pin 2. Internal comparator 2 compares it to ⅓ V_CC; if the trigger dips below, the SR flip-flop sets. The output (pin 3) goes HIGH and the discharge transistor (pin 7) turns OFF — releasing the timing capacitor C, which begins charging through external R from V_CC. When C reaches ⅔ V_CC, internal comparator 1 fires, resetting the flip-flop. Output goes LOW and the discharge transistor turns back on, dumping C to ground in microseconds, ready for the next trigger.

Worked example: 555 one-shot for relay control

Design: pushbutton triggers a relay to close for exactly 5 seconds. Mains-rated relay coil draws 30 mA at 12 V; 555 output sources up to 200 mA — plenty.

Pulse width. T = 5 s. Need R·C = T / 1.1 = 4.55 s.

Pick C first. Long timings want low-leakage capacitors. 22 μF tantalum or low-ESR electrolytic. R = 4.55 s / 22 μF = 207 kΩ. Nearest E12: 220 kΩ. Final T = 1.1 × 220 kΩ × 22 μF = 5.32 s — close enough; trim with a 200 kΩ pot if exact 5 s is needed.

Trigger. Pushbutton from V_CC to TRIG pin through a 100 nF coupling cap, plus a 10 kΩ pull-up keeping TRIG normally at V_CC. The press injects a brief drop below ⅓ V_CC.

Output driver. The 555's OUT directly drives the relay coil through a 1N4148 flyback diode across the coil. No transistor needed at this current level.

Total bill of materials: 555, R = 220 kΩ, C = 22 μF, trigger cap 100 nF, pull-up 10 kΩ, pushbutton, flyback diode, relay. Eight parts, under $2.

Monostable family comparison

Part	Topology	T formula	Trigger	Retriggerable	Notes
NE555	Bipolar timer IC	1.1·R·C	TRIG < ⅓ V_CC	No	50-year workhorse, 5–18 V supply
CMOS 7555 / TLC555	CMOS timer	1.1·R·C	TRIG < ⅓ V_CC	No	Low quiescent (60 μA), higher R range
74LS123	TTL dual one-shot	0.45·R·C (typ)	Edge (A or B)	Yes	Independent A/B trigger logic
74HC4538	CMOS dual one-shot	R·C × ln(2) ≈ 0.69·R·C	Edge + level	Yes	Crystal-controlled variants available
RC + Schmitt + diode	Discrete	R·C (approx)	Edge through cap	No	Tinkerer's cheap one-shot, ~10% accuracy
MCU GPIO timer	Firmware	Programmable	Interrupt	Yes	Microsecond-class resolution, software-defined

Non-retriggerable vs retriggerable

This is the key behavioural distinction. Imagine triggers arriving at 100 Hz (every 10 ms) into a one-shot programmed for T = 50 ms.

Non-retriggerable (555). First trigger starts a 50 ms pulse. Triggers 2–5 (arriving at 10, 20, 30, 40 ms after trigger 1) are ignored — the comparator is already SET. At 50 ms the pulse ends. The next trigger after that (whenever it arrives) starts a new pulse. Effective output: 50 ms pulse, 0 ms gap (depending on phasing), 50 ms pulse, ... — duty cycle depends on trigger timing.

Retriggerable (74LS123 with R/T tied high). First trigger starts a 50 ms timer. Each subsequent trigger resets the timer back to 50 ms. As long as triggers arrive at intervals shorter than 50 ms, the output stays HIGH forever. If triggers stop, the output finally goes LOW after one more 50 ms timeout. This is the classic "activity detector" or "wake-on-motion" pattern.

Both are useful, but for different problems. A debouncer wants non-retriggerable: ignore the bounces, then emit one clean edge. A motion sensor wants retriggerable: hold the light on while motion continues, switch off after 30 s of inactivity.

Frequency-to-voltage conversion

Feed an unknown-frequency square wave into a monostable's trigger, set T to a fixed value, and the output is a stream of identical pulses occurring at the input frequency. Low-pass that output and you get a DC voltage proportional to frequency:

V_avg = V_CC × T × f_in

For T = 1 μs and V_CC = 5 V:
   f_in =  10 kHz  →  V_avg = 50 mV
   f_in = 100 kHz  →  V_avg = 500 mV
   f_in = 1 MHz    →  V_avg = 5 V (saturates)

The linearity is excellent up to f_in ≈ 1/T (where pulses start running into each other). Used in tachometers, automotive speedometers, FM detection, and analog phase-locked loops as the F-to-V stage. Far simpler than digital counting in the days before microcontrollers — and still useful when you want microsecond response with no firmware.

Precision and pitfalls

• Capacitor leakage dominates long timings. Aluminium electrolytic leakage can be tens of microamps per microfarad — comparable to charging current at large R. Use film or tantalum for T > 1 s.
• Temperature drift. 555 internal references drift about 75 ppm/°C, plus 100–1000 ppm/°C from the external R and C. Aim for ±1% accuracy over 0–70 °C with care.
• Trigger edge sharpness. Slow trigger edges can cause the 555 to mistrigger or oscillate. AC-couple the trigger or use a Schmitt buffer in front.
• Power-supply ripple. Thresholds are V_CC-relative, so noisy V_CC translates to pulse-width jitter. 100 nF bypass at the IC and 10 μF bulk are mandatory.
• R range. Bipolar 555: 1 kΩ < R < 10 MΩ. CMOS 7555: extends up to 100 MΩ. Outside, comparator input bias or saturation effects degrade timing.
• Avoiding double-triggers. Long trigger pulses extended beyond T cause the 555 to immediately retrigger when T expires. Add a coupling capacitor on TRIG (10–100 nF) to differentiate the leading edge.

Logic-family one-shots

For digital systems, the 74LS123 and 74HC4538 are dual monostables in 16-pin DIP packages. Each has:

• Two trigger inputs (A, B) with edge-detect logic — you can require a rising A AND a HIGH B, falling A OR a LOW B, etc.
• A CLEAR input that forces the output LOW asynchronously.
• Retriggerable timing — each new trigger restarts the timer.
• True and complementary outputs (Q and Q-bar).
• R and C connect between dedicated pins; recommended R range 5 kΩ to 50 MΩ.

The 74LS123 was the dominant TTL one-shot through the 1980s; the 74HC4538 succeeded it as CMOS supplies dropped to 3.3 V. Both are still in production and standard parts in any digital-electronics lab.

Where one-shots show up

• Switch debouncers. First bounce triggers a one-shot of 20–50 ms; subsequent bounces during the pulse are ignored. Output is a single clean edge.
• Watchdog timers. Firmware regularly retriggers a retriggerable monostable. If the firmware hangs and stops retriggering, the monostable times out and pulls the microcontroller's RESET line. STM32 and AVR microcontrollers all integrate this on-chip as the "independent watchdog".
• Pulse stretching for slow logic. A 100 ns sensor pulse triggers a 10 μs one-shot to drive a CMOS counter that's too slow for the original edge.
• Camera flash control. Trigger arrives from the shutter; one-shot fires the IGBT driving the xenon tube for exactly the calibrated 100 μs to 5 ms flash duration.
• Strobe synchronisation. One-shots in series form a programmable delay generator — useful for high-speed photography and instrumentation triggering.
• Touch sensors. Skin contact changes capacitance by a few pF; a 7555 with R = 10 MΩ and C ≈ trace + finger = 20 pF responds with detectable pulse-width changes.
• Engine ignition. Cam-position sensor triggers a one-shot whose calibrated pulse drives the spark coil for fixed dwell — older car ignition modules use this exact circuit.

A 1971 chip still ruling the world

The NE555 was designed by Hans Camenzind at Signetics in 1971 — a 23-transistor bipolar IC sold for less than a dollar. It was originally conceived as a more flexible replacement for a custom timing chip on PLL projects, but its analog flexibility (monostable, astable, voltage-controlled oscillator, Schmitt trigger, missing-pulse detector — all from one part with different external networks) made it explosively popular. Estimates put cumulative 555 shipments at over a billion per year for decades. The original Signetics mask was so successful that pin-compatible copies are still produced today by NXP, Texas Instruments, ON Semiconductor, and Chinese fabs — fifty-five years later, and the part is unchanged.

The CMOS variants (7555, TLC555, ICM7555) followed in the 1980s, trading current capacity for lower quiescent and wider R range. Modern MCUs have largely replaced one-shots in new designs, but for analog-pure applications — where firmware would be overkill — the 555 monostable remains unbeatable in parts count, latency, and price.