#729 Missing Pulse Detector

Presenting a missing pulse detector circuit based on a 555 timer monostable, and tested using a 555/4011 interruptible pulse generator.

Build

Here’s a quick demo..

Notes

The missing pulse detector is made using a 555 time in monostable mode. Incoming pulses continually reset the timing cycle. A missing pulse allows the timing cycle to complete, changing the output state.

The circuit is described in many places, including:

The 555 Timer Applications Sourcebook - experiment 8
Engineer’s Mini-Notebook 555 Timer IC Circuits p12

Circuit Design

The circuit includes three main sub-systems:

a test pulse generator (555 U1)
a test pulse interrupter (pushbutton and CD4011 U3)
the actual missing pulse detector (555 U2)

The test pulse generator:

simply a 555 timer (555 U1) configured in astable mode running at 3.396 Hz, 88.9% duty cycle.
An indicator LED1 is attached to show the output PULSE_GEN in action

The test pulse interrupter:

uses one NAND gate from CD4011 U3(1) to invert the PULSE_GEN output, so pulses are HIGH spikes at ~11.1% duty cycle
combines with the normally-HIGH push-button into a second NAND gate from CD4011 U3(2), the output labelled SIGNAL
when the push-button is not pressed:
- CD4011 U3(2) output SIGNAL reproduces the PULSE_GEN signal
when the push-button is pressed:
- CD4011 U3(2) output SIGNAL is pulled HIGH

The missing pulse detector:

is based on a 555 timer (555 U2) configured in monostable mode with Time High = 1706 ms
while the SIGNAL is pulled LOW (every pulse)
- it continuously triggers the timing cycle, keeping the output HIGH
when the SIGNAL LOW pulse is missed for longer than the monostable timing, then the output goes LOW
the indicator LED2 is configured on the high side of the output, so that is lights when pulses are missed.

The monostable timing is set by R5/C2. These need to be adjusted so that the timing is longer than the expected period of incoming pulses. A practical missing pulse detector would switch R5 with a small fixed resistor (>=1kΩ) and a large potentiometer, and possible make C2 switchable between different “ranges” so that the sensitivity can be adjusted for the frequency of the signal being monitored.

schematic

Built on a breadboard for testing:

bb_build

Test

Here is the scope trace of a sample run. The signals

CH1 (Yellow) - traces the output of the missing pulse detector
CH2 (Blue) - traces the continuous PULSE_GEN pulse train
CH3 (Red) - traces the SIGNAL, normally PULSE_GEN except when the push-button is pressed

The button was pressed 4 times during the sweep, but on ly the first and last were long enough to cause the missing pulse detector to indicate.

scope_test1

Project Notes

#729 Missing Pulse Detector

Notes

Circuit Design

Test

Credits and References