In this electronic roulette wheel (see Fig. 1.) the timer 555 works as a generator of pulses with fading frequency. The counter 4017 switches LED's one by one in the selected string, and the divider by two based on D-type flip flop (4013 IC) selects the LED's strings. Actually the base of this circuit diagram is the counter DD2 and the string of LEDs D1...D10 (if we connect the resistor R6 to the ground and remove the D-trigger and LEDS DD11..DD20 from the circuit then we get simple electronic roulette wheel with 10 light-emitting diodes), so by adding a D-trigger to the circuit we can add more LED's.
When the button SB1 ("Spin") is first pressed and then released, the generator produces pulses with frequency that getting lower, and at the end the generator stops. The decade counter DD2 counts this pulses, and the state of its outputs changes with every pulse. The light emitting diodes is connected in tho strings D1...D10 or D11...D20 lights up one by one (it looks like it "spins"). At the end an only one LED will illuminate. It is imitate a roulette spin. The DD3 divider by two changes its state every 10 pulses, and this divider selects the first or the second string of LEDs. Resistors R6, R7 prevents overload of outputs of DD3 flip flop trigger. All inputs of the second flip flop of IC DD3 should be grounded.
The 555 integrated circuit produces pulses. When the power switch SA1 is on and the button SB1 is pressed then the frequency of the timer is constant. When the button SB1 is released then the frequency of the timer will slowly fading down. The capacitor C2 was charged while the button SB1 was pressed, so now C2 discharges through resistors R2, R3, capacitor C3 and the output "DIS" ("discharge") of the 555 IC. Because the current that charges the capacitor C3 getting weaker, it takes more time to charge the capacitor C3 to the voltage of activating the trigger of the timer. So the frequency of the 555 timer will fading down. When this voltage at the capacitor C2 gets reduced below the trigger level then the timer will stopped.
The second circuit diagram (see Fig. 2.) reminds the first one. It uses four strings of 9 light emitting diodes in each one string, and instead on D-type flip flop trigger it uses the second decade counter. The outputs of the counter are connected to the inverters DD4.1...DD4.4.
Click to enlarge the circuit diagram
This circuit uses four LEDs string, each string has 9 light emitting diodes. The cycle of the counter DD2 is shortened to 9 bits, and the cycle of the counter DD3 is shortened to 4 bits, so we got 9*4=36 LEDs.
Both devices don't need any adjustments, but we can try to match capacitors C2 and C3 to get more realistic effect.