Here is a little circuit to measure low frequency ...
You got two switch :
- the left one to reset counters,
- the right one to start counting during one second. So you can measure the frequency.
You may change the frequency with the potentiometer of second 555.
I change the second oscillator to get a better duty cycle.
It works from 15Hz to 67Hz changing the potentiometer from 10% to 90%
