Tacho (RPM) filter schematic

Using Arduino General Electronics
1

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?

2

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

3

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.

4

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.

5

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

6

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

7

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:

8

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

9