Recently, I am working on ECU of CNG Auto Rickshaw having 4 Stroke Engine.

Here, Image shows the Engine of Auto Rickshaw.Engine of Auto Rickshaw.

I have already taken reading of MAP, TPS and RPM. And now I am working on Injection Timing. For that, I am taking signals from RPM socket(As I don't know which sensor it is).

Here, In Image there is RPM Socket from which I am taking Signals by connecting probe of Oscilloscope to Yellow Wire.RPM Socket

And I am getting Signal like given in this Image,RPM Signals

Now, I want signals as showed in below Image,Desired Output Signals

I have tried getting this signal using LOW PASS FILTER followed by SCHMITT TRIGGER and getting this kind of result,Using LOW PASS FILTER and SCHMITT TRIGGER

So, I need help in getting proper square wave signal.....

Circuit that I have implemented....

1) Schmitt Trigger

Schmitt Trigger

2)Low Pass Filter Low Pass Filter

  • \$\begingroup\$ so, are you looking for waveform in white color to be produced using the yellowish waveform? can you share the low pas filter and schmitt trigger section you have implemented? \$\endgroup\$
    – User323693
    Dec 30, 2016 at 8:15
  • \$\begingroup\$ A comparator and a reference? \$\endgroup\$
    – winny
    Dec 30, 2016 at 8:47
  • \$\begingroup\$ Yes...Attaching in Question...@Umar \$\endgroup\$
    – karanrp
    Dec 30, 2016 at 9:44
  • \$\begingroup\$ I don't get you...@winny \$\endgroup\$
    – karanrp
    Dec 30, 2016 at 9:44
  • \$\begingroup\$ Wrong ref for V- to Op Amp. It should be 2~3V. \$\endgroup\$ Dec 30, 2016 at 10:24

1 Answer 1


It takes me 20x as long to explain this as it would for me to design it.

The optimal design should minimize latency with adequate noise resonance filtering , i.e. matched filter with maximum signal to noise ratio.

  • My timing analysis indicates your pulse has a resonance in the 3.75kHz region and the pulse interval is 32ms (31Hz) or 1875 RPM ( if 1/rev, 938 RPM if 2/rev).

  • Tolerance to latency of 1 deg at 6000 RPM is equivalent to 28 us which needs to be accounted for in filter ignition timing vs RPM. A 28us = T for low pass filter, LPF (maximum. )

    • if 1 pulse/rev then 100Hz = 6000 RPM = 10ms interval
  • Alternative to a LPF filter is a non-retriggerable one shot with much like a scope trigger with a dwell of 6 cycles @3.75kHz unless there are conditions that exceed this. Thus 6/3.75kHz= 1.6ms
  • The signal is the negative edge from +22V for the leading edge of the Hall sensor while there is resonant noise on the trailing edge of the sensor after a period of reaching +22V.
    • There is some DC dwell to be ignored after the sensor's leading active low pulse and the trailing active high sense positive pulse.
  • The ideal threshold appears to be in the 2 to 4V range for hysteresis thresholds.

    • it is worthy to note that the positive feedback ratio defines the threshold and hysteresis as a function of the output swing with respect to the Reference level for a a differential amp. So ground is a poor choice of reference. It should be ~ V/2 or if 2,4V thresholds desired then V- =(2+4)/2 = 3V ( not ground )

Design Recommendation

Confirm signal, noise range and sensor if 1 or 2 pulse per rev and max RPM.

  • Stage 1: a 28us LPF with Rseries 100k with shunt of C= 28us/100k = 280 pF max.
  • Stage 2: 5V 74HC14 Schmitt trigger using Vcc=5V with hystereis thresholds of Vcc/3, 2Vcc/3
  • Stage 3 rising edge 1 shot of 1.6ms with feedback to make non-retriggerable to disable input using. 74HC123


simulate this circuit – Schematic created using CircuitLab

100K current limits to 0.22mA satisfies the max 5mA internal ESD clamp diode specs. Latency of 28us needs to checked for max RPM. One shot syncs to leading negative edge and filters out trailing edge glitch at all RPM. ( T=1.6ms needs to be increased to satisfy minimum RPM)


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