LED Roulette Circuit (555 + 4017) – B2P13

Another interesting circuit that can be built using the binary counter is a roulette like LED wheel. It’s pretty much based on the same principle as before so let’s take a look at the schematic.

LED roulette

The 4017 is used like before but we add a total of 10 LEDs which share a single current limiting resistor. A 555 timer is used to generate the required clock pulses but the RC network is slightly more complicated like before.

Q1 is a PNP transistor which means that it’s base has to be negative for it to switch ON. When switch S1 is pressed, it pulls the base to the ground and this switches Q1 ON. R1 and R2 form a voltage divider network whose output voltage charges C1. The 555 timer IC works in the astable mode to generate the required clock pulses. When the switch is released, the charge held by C2 keeps the transistor ON and the capacitor discharges through the voltage divider network. As the capacitor discharges, the transistor moves from the saturation to the linear region and the bias voltage keeps reducing as it proceeds to enter the cut-off region. The voltage across R1 & R2 decreases as the transistor switches regions and this also reduces the voltage across the capacitor, C1. The end result is that we see a trail like effect once the switch is released. The LEDs continue to illuminate at a lower frequency until the clock pulses come to a halt and this occurs when the capacitor C2 is fully discharged.

LED roulette

Here we have the assembled PCB and we can see the LEDs being lit as we press the switch. The LED trail effect is not that easily seen with the standard 1uF value of C2 so we can increase this to lengthen the delay.

LED roulette

Here’s what it looks like when C2 is changed to 10uF and you can tweak the circuit values to suit your application. Let’s now learn about a new type of counter in the next project.