LED CHASER CIRCUIT (555 TIMER & 4017)

The LED chaser circuit is a circuit that lights up a line of LEDs in a repeating sequence which gives the illusion of one LED "chasing" down the line. You've probably seen this when a marquee is used in a theatre, when you get a scanner on a Christmas tree or when Knight Rider scanner hissed KITT. It is easy to use and inexpensive, and provides movements without any programming.

It has two principal chips: the 555 timer (configured as an astable multivibrator) that produces a steady pulsing clock signal; and the CD4017 decade counter that lights one output (and one LED) at a time, advancing with each pulse and looping back when it reaches the end. A potentiometer allows you to vary the speed of the chase by varying the timing of the 555.

In the above breadboard diagram, the 555 timer and potentiometer on the left serve as the clock pulse generator, the center CD4017 is simply used to distribute the clock pulse to six LEDs, and the Arduino Uno is only used as a stable 5V power supply — all the chasing logic is done in hardware, not in code.

Here are the materials required to build this circuit!

The majority of these parts (resistors, capacitors, LEDs, the 555 chip and 4017 chip) are generally available in bulk packs, which are typically less expensive per piece than being sold individually — It is suggested that if your electronics store sells a basic component package, you purchase that rather than buying the individual parts.

Links to purchase each part:

https://www.amazon.com/s?k=10uf+electrolytic+capacitor

https://www.sparkfun.com/categories/portenta

https://www.amazon.com/s?k=250k+ohm+potentiometer

https://www.amazon.com/s?k=560+ohm+resistor

https://www.digikey.com/product-detail/en/texas-instruments/CD4017BE/296-2037-5-ND/67253

https://www.amazon.com/Eagles-NE555-Single-Astable-Monostable/dp/B07H6F22RH

https://www.amazon.com/s?k=1k+ohm+resistor

https://www.amazon.com/s?k=5mm+red+led

https://www.amazon.com/s?k=5mm+blue+led

https://www.sparkfun.com/arduino-uno-r3.html

https://www.amazon.com/s?k=breadboard

https://www.amazon.com/s?k=jumper+wires

All seven RED LEDs were first mounted in the middle of the main breadboard, equally spaced so that each would be aligned with its corresponding output on the CD4017. LEDs have two leads; the short lead is the cathode (negative lead, sometimes with a flat edge on the plastic casing) and the longer one is the anode (positive lead). This orientation is crucial as LED will simply not light up if installed back to front.

I then placed a 560Ω resistor in series with the cathode (negative lead) of each LED, and connected them to the circuit. These resistors are essential; LEDs have no internal resistance to limit the flow of current through them, and if they were connected directly to a power supply without a resistor then too much current could flow through them, causing them to become hot and burst into flames. From trial and error, 560Ω was found to be suitable for reducing the current to a safe level for the red and blue LEDs whilst maintaining the brightness level such that the chase effect could still be seen easily.

At this stage, each LED-resistor pair was simply set up as an independent output channel — not yet connected to the CD4017 or power. This allowed for easier testing of each LED individually in the next steps and helped to keep the wiring neat and tidy before introducing more complex logic and timing connections in the later steps.

All seven LED-resistor combinations from Step 1 were connected to the respective output pins of the CD4017 decade counter, which was configured as a divider by 10. The next step was to connect each LED-resistor combination to a respective output pin of the CD4017 decade counter that has been set up as a divider by 10. The CD4017 has 10 output pins (Q0 to Q9), each of which turns on one at a time, in sequence, with each clock pulse — that is how the chasing happens down the line of LEDs.

As seen in the photo, one yellow and green wire leads from each of the output pins (at the bottom of the chip) up to the resistor leg that feeds into one of the seven red LEDs above. Most of the connections were done in yellow, a few of the longer ones in green, just to make it easier to see them on the schematic — color isn't important here, it's merely for organization. In this build, only seven of the CD4017's 10 outputs are used, so the remaining unused output pins were simply left disconnected, the chip will cycle through all 10 internally, but only the 7 with output connections perform a visible action.

Each output should drive exactly one LED and no wires should connect two outputs together, otherwise it may cause unpredictable behavior or even damage the chip if two outputs were to simultaneously drive the same point at the same time. It was also good to route the wires so they did not cross over the top of the breadboard, close to the LEDs, as this helped to prevent shorts and kept the board easy to read in the troubleshooting process.

With each LED channel wired back to a different output of the CD4017, the chip was now set up to receive the clock signal from the 555 timer (which is described in the next step) (Power connected to Power, Ground to Ground, Clock Enable to Ground, Reset to Ground, Clock to LED and 555).

Potentiometer: The potentiometer's wiper (the center terminal, which moves as the knob turns) connects to the 555's TRIGGER pin (pin 2). The second terminal is connected to both RESET pin (pin 4) and DISCHARGE pin (pin 7). When the knob is turned, the resistance between these points varies, and this resistance can be used to increase or decrease the rate at which the timing capacitor charges and discharges — hence the LED chase can be speeded up or slowed down.

Capacitor: The capacitor is connected between the THRESHOLD pin (pin 6) and the TRIGGER pin (pin 2). It charges and discharges through the resistance of the potentiometer and periodically compares the voltage on this capacitor to its internal reference voltages and controls the output high or low as appropriate; this charging and discharging process determines the oscillation frequency.

Output to CD4017 clock: The output pin (pin 3) of the 555 is directly connected to the clock pin (pin 14) of the CD4017. Each time this output pulses high, the next output on the CD4017 is activated and the lit LED is moved one step further down the chain.

Blue LED on the clock line: The blue LED is wired in parallel to the output/clock line, through an LED current limiting resistor, from this same output/clock line (between pin 3 of the 555 and the CD4017's CLK pin). It flashes at the same rate as the 555 is oscillating, so that there is a visual indication that the clock signal is on and that the chase speed is regardless of what red LED is lit. It does not have any logic to the circuit, it's just a visual representation of the clock pulse.

This network of potentiometer, capacitor, and the trigger, threshold, discharge and reset pins is the timing circuit, when combined with the 555, that causes it to self-oscillate (create a multivibrator) and this oscillation powers the entire chase sequence through the CD4017.

Video showing how the circuit is supposed to work.

The purpose of the 555 Timer is to create the clock signal. The 555 Timer is used to generate the clock signal.

The 555 timer used in this circuit is configured as an astable multivibrator, which is a circuit that will repeat a high to low output without the need of an external trigger. It does that on the basis of the potentiometer, the 1kΩ resistor and the capacitor: The capacitor charges up via the resistor and potentiometer until it reaches the internal threshold voltage of the 555, after which the chip switches its output to discharge the capacitor through the same path until it falls below the trigger voltage, after which the output switches back. This is a continuous square-wave output (pin 3), which is essentially a clock signal, repeated with every repetition of the charge/discharge cycle. The potentiometer adjusts the duration of the chase cycle by adjusting the time for each charge/discharge cycle.

This involves sequential logic and counting circuits.This includes the sequential logic and counting circuits, CD4017.

The CD4017 is a decade counter, which works like a series of flip-flops (a simple memory/logic unit that holds one bit of state). It counts up every time a rising-edge pulse appears on its clock (CLK) input and outputs the new count value on the next output pin, with the previous output pin low. It is a clean and useful example of sequential logic (as opposed to combinational logic, in which outputs only depend upon the current inputs), since the output of a CD4017 depends upon its current state as well as its past history of clock pulses. That's the basic concept behind latches, counters and registers, and it's something that this chip does in an elegant and visual way, as you can literally watch the "1" bit move from output to output as the chase goes.

Putting It Together

The 555 is used as the clock generator and the CD4017 is used as the sequential counter that is clocked by the 555. The output of each of the CD4017s illuminate one LED (with a current limiting resistor in between), and the "active output" of the chip is displayed as a "moving point of light". This is a hands-on, real-world example of how a simple oscillator can be used with a counter chip to create digital sequences with timed, repeated chases (these are found in many applications such as traffic light controllers, digital clock timers, and event counters — but in a much more limited form with only one visible chase pattern).