Using FrequencyMeasure with external pulse generator


I am new to the Arduino and want to try to measure the frequency of a pulsed signal from a 555 circuit see link. The output frequency is approx 100Hz.

I want to use the FrequencyMeasure.h library and example program to display the frequency either serial or LCD. see library page. FreqMeasure Library, for Measuring Frequencies in the 0.1 to 1000 Hz range, or RPM Tachometer Applications

input is to digital pin 8 on the Uno. Since my input circuit above is an external one where do I ground the signal?
As shown in the circuit I have limited the voltage to < 5v and I have assumed the signal is clean and needs no filtering.

So far I cannot get the Arduino/program to display the frequency, either serial monitor or lcd. Whether I connect the output of the circuit to pin 8 (or connect a dead input wire) I just get a value of approx 50Hz .

Where should I ground the signal? What am i missing here? any help would be appreciated .