I built a Tach for my BVM Sabre. I painted two stripes on the spinner of the fan and I had a LED and a photo transistor embedded in the inlet duct so they would "look" at the spinner. The output of the photo transistor was fed to another LED that was placed near the tach pickup of my 9ZAP radio.
The electronics to condition the pulses was made by a friend of mine and I would have to ask him for the cct. It worked very well but I only used it for a while and then I seemed to know how the engine was running by the sound.
If i remember correctly it showed about 23000 rpm on the ground.
I have just sold the BVM fan stuff. If you are really interested email me and I will put you in touch with the guy that has it now.

Regards

Tom Watson
Sydney Australia