Roulette Kit

On this page the circuit of the roulette kit is explained.
[Purchase the kit here.]

An American roulette wheel has 38 positions, this accounts to 38 LEDs. Driving 38 LEDs directly from a single counter is not possible, because a typical counter does not have enough output lines to control such a large number of LEDs individually. To solve this, the circuit makes use of a matrix arrangement.

A matrix is a method of organizing LEDs in rows and columns. Instead of giving each LED its own dedicated control line, LEDs are wired at the intersections of these row and column lines. This reduces the total number of required outputs significantly. For example, in this design, the LEDs are arranged in 4 rows and 10 columns, giving the capacity to control up to 40 LEDs with only 14 signals (10 for the columns and 4 for the rows).

1. Starting the sequence:

• When the button is pressed, capacitor C2 charges.
• This powers the NE555 timer in astable mode, which starts oscillating at around 56 Hz.

2. Driving the first counter:

• The NE555 output acts as the clock input to U2.
• U2 is a decade counter that has 10 outputs (Q0–Q9).
• On each clock pulse from the 555, one output goes high in sequence, effectively scanning through 10 columns of LEDs.
• This allows one LED in the current row to be selected at a time.

3. Row selection with carry-out:

• After U2 finishes one full cycle (Q0 → Q9), it sends a carry-out pulse.
• This pulse is used as the clock input to U3, another counter.
• U3 switches between 4 outputs, each one enabling a different row of LEDs via transistors.
• This way, after every 10 LEDs, the counter moves to the next row.

4. Matrix operation:

• The combination of 10 column outputs (from U2) and 4 row outputs (from U3) creates a 4×10 matrix, capable of driving 40 LEDs with only 14 control signals.
• In your circuit, only 38 LEDs are used; the last two positions are unused.
• To solve the mismatch, the unused outputs are tied into the reset, ensuring that the sequence loops cleanly after 38 LEDs instead of trying to light non-existent ones.

5. Frequency ramp-down:

• As C2 discharges, the charging rate of the 555 changes.
• This gradually slows down the oscillation frequency, making the LEDs appear to move more slowly over time.
• Eventually, when C2 is fully discharged, oscillation stops completely, and the sequence halts.

6. Transistor drivers:

• Since the row outputs of U3 cannot directly drive 10 LEDs at once, each row output controls a transistor switch.
• The transistors provide enough current to activate an the LEDs when selected.

Summary

• The NE555 provides a clock signal that slowly decreases in speed as C2 discharges.
• This clock drives the U2 counter, which scans across 10 columns of LEDs.
• Every time 10 LEDs have been scanned, the carry-out signal moves the second counter (U3) to the next row.
• Together, the two counters form a matrix that can light all 38 LEDs one by one.
  
• Over time, the LEDs slow down and eventually stop as the capacitor empties.