OK, thanks audioguru, I think you have answered most of my questions. I admit that I didn't see these notes when reading the datasheet.
My circuit uses a 7555 chip as a monostable pulse generator that is triggered by a light sensor monitoring the speed of a perforated rotating wheel. The timer is used to clean up the signal and provide a clean square wave output. The signal passes to the LM2917N speed switch that is set up to give a high output on pin 5 when the input frequency passes the reference level. The output signal from pin 5 then goes on to trigger the analog switch and the LED.
It works, but the response time seems slow and inaccurate. It could be because I am using the 8 pin chip and should be using the 14 pin version. I'm not sure. If there are any other faults in the schematic please let me know. Thanks!