Inverted Square waveform

Here in figure, we can see two waveforms.

Below one is waveform of raw RPM signal. I want to find square waveform which based on that LOW INTERVAL of RPM signal and length must vary according our requirement. So for that purpose, I just made inverter circuit using 74HC14 and Low pass filter(for removing noise).

As a result of that circuit, I got Upper one waveform means square wave signal. Now, you can see it has also one spike beside square wave(I don't want that). And I can't vary length because inverter circuit gives only perfect inverting signal.

Is it possible to make square wave like this and also vary the length of that square wave with help of Arduino?

Here, I want to use Arduino because I have to vary the length of square wave with help of programming. For example, if I want 10ms square wave then just enter value 10 in programming then generate 10ms square waveform. Rising edge must be same. And right now, I have no clue or idea how to do this.

If possible then please give me some suggestion how to do this.

  • You should probably try to filter out the spike before it gets into the Schmitt triggers. – Edgar Bonet Jan 9 '17 at 9:26
  • @EdgarBonet I already tried out Low pass filter and then output of that going into 74HC14. – Hasan Jan 10 '17 at 9:37
  • needs by bypass caps. tiny ones; less than 100 puff. – dandavis Feb 19 '17 at 7:40

The most steightforward decision is to use this spike to trigger interrupt in Arduino. The interrupt procedure starts one of the counters and sets 1 at one of the pins. After N milliseconds by counter or by inner timer the Arduino outputs 0 to the same pin, so you will have needed length of the positive pulse. This will work while pulse length will be less then time between successive spikes. Do not forget to make standard spike amplitude enough to start interrupt, but not too big not to damage Arduino. Of course you can work without interrupts, just testing the input in main cycle, but it will be less presise. You will need a decision, how to conveniently control the pulse length by some tool. It can be infrared pult or some other tool and you must see the set time value, so you will need to use some indicator. May be LCD 1602 keypad shield will be enough for this.

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