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Missing Pulse Detector (MEC187E)

This project is from my book Robotics, Mechatronics and Artificial Intelligence (sold out) published in the United States. It shows how to mount a tachometric missing pulse detector that can be used in mechatronics or robotics projects with microcontrollers or as a building block for simple projects.

An important function for projects in robotics and mechatronics is the missing pulse detector. This circuit keeps its output in the high logic level, presenting the power supply voltage while a constant pulse sequence is applied to the input as shown in Figure 1.



Figure 1 - The missing pulse detector action
Figure 1 - The missing pulse detector action



If one or more pulses are missing in this sequence, the output of the circuit passes for a brief time interval to the low level, generating a “flag” signal. This signal can be used to trigger an alarm or another circuit.

The circuit is shown in Figure 2 and is a monostable based in the 555 IC. R and C are calculated to provide the circuit with a time constant that is longer than the distance between two incoming pulses but shorter than the distance between two pulses. This means that the circuit will be retriggered by the next pulse before the end of the timing process, keeping the output high. If a pulse is missing, the time constant ends before the next pulse has arrived, and the output goes to the low level in an instant, producing the flag output signal.


Figure 2  Missing pulse detector.
Figure 2 Missing pulse detector.



The time constant is given by t = 1.1 × R × C, and R can be kept in the range between 1 kΩ and 1 MΩ.

The bipolar 555 can drain about 200 mA, but we also can build a CMOS version (TLC7555) that can drain and supply 200 mA. If the load needs more current, an output stage must be used such as the ones described in other articles in this section.