Any idea on how to build a TTL random number circuit that slows as the trigger is released, as to simulate a lucky wheel spin? I wanted to use a 74LS90 and 74LS48 (or 248 for tailed 6 and 9), along with a few MPSA42 transistors going to a Beckman SP-101 display for the number output.

The problem is that I can't figure out how to get an oscillator circuit going to run a clock pulse of 15 Hz once it sees 5V in, and gradually slow to a stop once the input goes low.

Any ideas on how to do this the hard way, without using a Maxim or PIC microcontroller?