The Simplest Frequency Counter for the Arduino
I needed a frequency counter for settng up 555 timer chips, so I came up with the simplest, fairly accurate method of acheiving this.
This has just one variable and two lines of control code.
You can connect pin 3 of the 555 timer or any other positive output, as long as you limit the potential difference to between 0.5 - 5 volts, to the Arduino interrupt pin D2. And gnd to gnd, of course.
I have tested this up to about 150 kHz, using a sample time of 1000 milliseconds, but if you want a more accurate results at lower frequencies, then you will have to increase the sample time by factors of 10, for mathermatical simplicity.
At about 160 kHz, the loop code becomes corrupted and so I have kept the loop code to the bare minimum. If you add more code to the loop, the corruption threshold will lower.
Have fun!