Positive flank triggered retriggerable timer?

Hi! I did a lot of TTL logic back in the nineties but now I am lost.
Need a circuit that can trigger a relay when movement or stop is detected on a axle.
My approach is that the relay should stay active as long as the axle is turning. So, I decided to have the axle generate a pulse train.
Next I need a timer that triggers on positive flank and then turns off the output after maybe half a second. If there is a new positive flank during this time the timer will restart. The reason I want it this way is that the axle might stop on a positive flank and not indicating a stop. As long as the axle is spinning the relay will stay active.

Any ideas?

Also: Once upon a time there was a TTL logics datablade book. Is such documentation still available?

By positive flank you mean on the rising edge of the pulse or the pulse top? What is the nature of the pulse train? Voltage and pulse rates.

"Need a circuit that can trigger a relay when movement or stop is detected on a axle." How does that relate to the pulse from the timer being on only 0.5 secs? Does that mean that if the pulse train is off for more than 0.5 seconds (axes stopped turning) the  timer drops out resetting the relay?

A retriggerable timer would stay on while it is  constantly being  reset else if not reset after 0.5 secs its output would go low.

.

For TTL logics datablade books see ebay.com

Or even better  Forrest Mims's   book "Engineer's Note Book !!  - A Handbook of Integrated Circuits   Applications" is available online in pdf  from:       IC Book      

 Dated but still the best book ever on ICs. The book has an NE555 circuit for missing pulse detection; which maybe what you need.