The OP wanted a trigger when the pulse train from the tacho ceased. The circuit link supplied is for a missing pulse detector which will do the same sort of thing. If the pulse train was fed through a diode into an RC combination we get a charge pump, and the resulting charge fed into the base of a transistor to keep it biased on. When the pulse train ceases the transistor turns off. Charge pump / missing pulse TRAIN...
We could of course use CMOS gates, or a PIC micro or a myriad of other methods, all of which work. The OP already has a relay and manual switch which he is using, read the post.