Missing-pulse-detector

This circuit detects a missing pulse or an abnormally long interval between consecutive pulses in a pulse train. The timer is configured in monostable mode, with the time delay set slightly longer than the duration of the input pulses. The timing interval of the monostable circuit is continuously retriggered by the input pulse train. If the pulse spacing is less than the timing interval, it prevents the voltage at the timing capacitor (Vc) from rising sufficiently to terminate the timing cycle. A longer pulse spacing, a missing pulse, or a terminated pulse train allows the timing interval to complete. The circuit generates an output pulse (V0) as illustrated in a referenced figure. The output remains high on pin 3 until a missing pulse is detected, at which point the output decreases. The NE555 monostable circuit should operate at a frequency slightly lower than that of the pulse being analyzed. Additionally, the input frequency cannot exceed twice the free-running frequency, as this would cause retriggering before the timeout, resulting in a continuous low state output. The circuit functions in monostable mode at approximately 8 kHz, enabling the observation of pulse trains ranging from 8 to 16 kHz.

The described circuit utilizes the NE555 timer in a monostable configuration to effectively monitor pulse trains for integrity. In this setup, the NE555's output (pin 3) is critical for indicating the presence or absence of pulses. When the circuit is powered on, the timing capacitor begins to charge upon receiving a pulse. If the incoming pulse train maintains intervals shorter than the configured timing interval, the capacitor voltage (Vc) remains below the threshold required to reset the timer, thus keeping the output high.

The design requires careful selection of the timing resistor and capacitor values to ensure that the timing interval exceeds the duration of the input pulses. This setup allows for reliable detection of any abnormalities in the pulse train, such as missing pulses or extended gaps between pulses. The output pulse width can be adjusted by varying the resistor and capacitor values, providing flexibility in the detection threshold.

For practical implementation, it is important to ensure that the input frequency does not exceed twice the monostable circuit's free-running frequency. This limitation is crucial to prevent premature retriggering of the timer, which would keep the output low and defeat the purpose of monitoring the pulse train. The circuit's effective operating range of 8 kHz allows it to analyze pulse trains between 8 kHz and 16 kHz, making it suitable for various applications in digital signal processing and timing analysis.

In summary, this NE555 monostable circuit serves as a reliable pulse train monitor, capable of detecting missing pulses and irregularities in pulse spacing, with output characteristics that provide clear indications of the pulse train's integrity.This circuit will detect a missing pulse or abnormally long spacing between consecutive pulses in a train of pulses. The timer is connected in the monostable mode. The time delay should be set slightly longer than the timing of the input pulses. The timing interval of the monostable circuit is continuously retriggered by the input pulse train, 1-J.

The pulse spacing is less than the timing interval, which prevents Vc from rising high enough to end the timing cycle. A longer pulse spacing, a missing pulse, or a terminated pulse train will permit the timing interval to be completed.

This will generate an output pulse, V0 as illustrated in Fig. 25-3b. The output remains high on pin 3 until a missing pulse is detected at which time the output decreases. The NE555 monostable circuit should be running slightly slower, lower in frequency, than the frequency to be analyzed.

Also, the input cannot be more than twice this free-running frequency or it would retrigger before the timeout and the output would remain in the low state continuously. The circuit operates in the monostable mode at about 8kHz, so pulse trains of 8 to 16 kHz can be observed.

🔗 External reference