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

Here's a quick demo..

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

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.

Built on a breadboard for testing:

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.

• The 555 Timer Applications Sourcebook
• Engineer's Mini-Notebook 555 Timer IC Circuits
• LM555 Datasheet
• CD4011 datasheet
• 2N3906 datasheet