The triple presence of the 555 does the following...
The first one provides a frequency to count for a certain periods of time.
The second 555 switches the count on or off and is switched off to NO COUNT by the third 555.
So you can securely measure any frequency, just add mor displays ...
In this example it counts 11 within 10 seconds...
That'll mean 66 rpm or Hz....
Perfect for several purposes.
