Tacho (RPM) filter schematic

Greetings guys, wanted to ask if anybody could help me out with hooking up a motorcycle rpm/tacho signal to an arduino uno. I want to use the FreqMeasure lib to calculate the current RPM, however the schematics I've found online don't exactly work as expected (they either work at low-RPM, don't work at high-RPM or just plain don't work at all as in 0hz).

Hornet-RPM graph

Thats how the signal looks when viewed from an oscilloscope, any suggestions what schema to use in order to filter it out so its usable by the arduino?

An RC filter and a schmitt trigger. An Op Amp can be wired up to have hysteresis.

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Does the voltage really peak at 5 Volts? You could process the raw signal with an Op-Amp Comparator with Schmidt Triggering.

Use this link: Comparator to design the basic op-amp circuit ...

Vcc = 5 R1 = 10k R2 = 7k R3 = 30k

Turn ON = 3 Volts Turn OFF = 2 Volts

Circuit will output a clean "Square Wave" to feed into the Arduino. Of course, there will always be a few extra components for reliability.

Survey says..."Schmidt triggered op amp"

I recommend the LT1215. I ordered mine directly from Linear online. They are specifically designed for use at 5V.

The AD8656 will work 2.7V to 5.5V but had full rail-to-rail inputs as well as outputs.

The AD8656 is not available in DIP package like the LT1215.

Thank you guys for the suggestions, I will look up the op-amp links/ics you wrote about and do some testing this weekend, thank you again :slight_smile:

If the signal is that clean then maybe a software only solution using the built in analogue comparator will do the trick.