Understanding the Concept:
A monostable multivibrator, commonly referred to as a one-shot multivibrator, possesses a single stable rest state and one quasi-stable state. When an external narrow triggering pulse is applied, the circuit transitions from its stable state into its temporary quasi-stable state. It remains in this quasi-stable state for a predetermined duration $T$ governed by an internal RC timing network:
\[
T = 1.1 \cdot R \cdot C
\]
After time $T$ elapses, the circuit automatically drops back to its stable resting state.
Step 1: Understanding Missing Pulse Detection Operation
In biomedical missing-heartbeat detector systems, the monostable circuit is configured to act as a missing pulse detector:
• Regular heartbeats are conditioned into a stream of periodic negative-going trigger pulses.
• The time period of the monostable quasi-stable state ($T$) is adjusted to be slightly *longer* than the expected normal interval ($T_n$) between consecutive heartbeats ($T > T_n$).
Step 2: Tracking Circuit Dynamics under Normal Signals
When heartbeats trigger the circuit at a normal rhythm:
• The first heartbeat arrives, switching the monostable output to HIGH (quasi-stable state).
• A discharge transistor wired across the timing capacitor resets the capacitor's voltage to zero every time a new trigger arrives.
• Because the next normal heartbeat arrives *before* the monostable timing period $T$ runs out, the capacitor is continuously discharged before it can finish its timing cycle.
• Consequently, the monostable output stays consistently HIGH as long as pulses arrive regularly.
Step 3: Behavior during a Missing Beat
If a heartbeat is skipped or dropped, the triggering pulse does not arrive on schedule. This gives the internal timing capacitor enough time to charge all the way up to its threshold value ($\frac{2}{3}V_{CC}$ in a 555 timer).
As a result:
• The timing cycle finishes, and the monostable immediately drops back to its stable LOW state.
• This sudden transition to a LOW output voltage flags the system that a heartbeat is missing, triggering a medical alarm.
Step 4: Evaluating alternatives
• Astable Multivibrator: Free-running oscillator with no stable state; used for clock generation, not event timing checks.
• Schmitt Trigger: A voltage comparator with hysteresis used for wave-shaping; it cannot track elapsed time on its own.
• Filter: Removes noise frequencies but lacks the state-changing timing logic needed to flag missing events.