This is the circuit of a simple LED chaser. The LEDs lights one by one for a period
of 1second and the cycle repeats giving the running light appearance. The circuit
uses two ICs (one is 555) to drive the LEDs. IC1 (NE555) is the popular timer IC
wired in the Astable Multivibrator mode. Resistors R1, VR1 and capacitor C1 act
as the timing components and the output pulses are available from the output pin 3
of IC1. These pulses are given to the input pin 14 of the Johnson decade counter IC
CD4017. Out of the 10 outputs of IC2, eight outputs are used to drive the LEDs.
The ninth output pin 9 is connected to the reset pin 15 to stop the counting. So that
the cycle repeats. With the value of C1, each LED remains on for 1second. When
one LED turns off, the second on turns on. This cycle repeats giving the running
light appearance. Resistor R3 keeps the input pin 14 of IC2 low after each
pulse.VR1 adjusts the speed of LED chasing.
LED Chaser Circuit
The popular Timer IC 555 is extensively used in short duration timing applications. IC 555 is a
highly stable integrated circuit functioning as an accurate time delay generator and free running
multivibrator. But one of the serious problem in 555 timer design is the false triggering of the circuit
at power on or when voltage changes. The article describes how IC555 is designed perfectly to avoid
555 IC pin functions
Pin1 Ground Pin2 Trigger Pin3 Output
Pin 4 Reset Pin 5 Control voltage Pin 6 Threshold Pin 7 Discharge Pin 8 Vcc
Functional aspects of pins
Trigger Pin 2
Usually pin2 of the IC is held high by a pull up resistor connected to Vcc. When a negative going
pulse is applied to pin 2, the potential at pin 2 falls below 1/3 Vcc and the flip-flop switches on. This
starts the timing cycle using the resistor and capacitor connected to pins 6 and 7.
Reset pin 4
Reset pin 4 can be controlled to reset the timing cycle. If pin 4 is grounded, IC will not be triggered.
When pin4 becomes positive, IC becomes ready to start the timing cycle. Reset voltage is typically
0.7 volts and reset current 0.1 mA. In timer applications, reset pin should be connected to Vcc to get
more than 0.7 volts.
Control Voltage pin 5
Pin5 can be used to control the working of IC by providing a DC voltage at pin5. This permits the
control of the timing cycle manually or electronically. In monostable operation, the control pin5 is
connected to ground through a 0.01 uF capacitor. This prevents the timing interval from being
affected by AC or RF interference. In the Astable mode, by applying a variable DC voltage at pin 5
can change the output pulses to FM or PWM.
Threshold pin 6 and Discharge pin 7
These two inputs are used to connect the timing components- Resistor and Capacitor. The threshold
comparator inside the IC is referenced at 2/3 Vcc and the trigger comparator is referenced at 1/3 Vcc.
These two comparators control the internal Flip-Flop of the circuit to give High or Low output at pin
3.When a negative going pulse is applied to pin 2, the potential at pin2 drops below 1/3 Vcc and the
trigger comparator switches on the Flip-Flop. This turns the output high. The timing comparator then
charges through the timing resistor and the voltage in the timing capacitor increases to 2/3 Vcc.( The
time delay depends on the value of the resistor and capacitor. That is, higher values, higher
time).When the voltage level in the capacitor increases above 2/3 Vcc, the threshold comparator
resets the Flip-Flop and the output turns low. Capacitor then discharges through pin 7.Once triggered,
the IC will not responds to further triggering until the timing cycle is completed.
The time delay period is calculated using the formula T= 1.1 Ct Rt. Where Ct is the value of
Capacitor in PF and Rt is the value of Resistor in Ohms. Time is in Seconds.
How to eliminate false triggering?
The circuit diagram shown below is the simple monostable using IC 555. To
eliminate the false triggering resistor R1 and Capacitor C1 are connected to the
reset pin 4 of the IC. So the reset pin is always high even if the supply voltage
changes. Moreover capacitor C3 connected close to the Vcc pin 8 acts as a buffer
to maintain stable supply voltage to pin 8. Using this design, it is easy to avoid
false triggering to a certain extent.
555 Monostable circuit
A ready recknor to select timing resistor and capacitor
Theoretically long interval is possible with IC 555,but in practical conditions, it is difficult to get
more than 3 minutes. If low leakage Tantalum capacitor is used, this can be increased to 5 minutes or
more. If the value of the timing capacitor is too high above 470 uF, charging time will be prolonged
which will upset the timing cycle and the output remains high even afterthe desired time is over.
The internet is overfilling with LED chasers, Knight Riders, and different ways to blink a series of
LEDs in a certain pattern. In this article, I'm going to make yet another LED chaser, or a Knight
Rider, but this one is with a little twist: we'll make an LCD with a menu that will allow you to choose
different patterns and speeds.
This project has five PCBs: four PCBs with LEDs and one with a microcontroller. On the LED's
PCB, there are two ICs: the HCF4017BE Johnson counter. The counter has one clock input pin and
10 output pins. For each high pulse received on the clock pin, the next output pin is set high, starting
with output pin 0. Read more about the IC in the link.
The last PCB has a PIC16F877A microcontroller. To this microcontroller I've connected a few
switches, a potentiometer, and an LCD. The microcontroller clock on each counter IC is in a different
order. The microcontroller can also reset the counter ICs. In the software, there's a menu that allows
you to choose between some predefined patterns. You navigate with one of the switches and enter the
menu choice with another. The third switch is the reset switch. One of the patterns lets you use the
potentiometer to change the speed for the "running" LEDs. This is done with the microcontroller's
built-in ADC module. ADC will not be discussed in this article.
When we power the circuit, LEDs start glowing one by one for a defined time period. Means first
LED Q1 glows and then Q2 glows and Q1 turned OFF and then Q3 glows and Q2 turned OFF and so
on. When we change resistance of variable resistor then speed of LED’s increase. Because frequency
of 555 timer increases and this increases frequency signal is directly connected to counter’s trigger
pin. So that counter changes its state faster.
555 timer frequency formula:
The charge time (output HIGH) is given by :
T1 = 0.693 (R1 +VR) C1
The discharge time (output LOW) by :
T2 = 0.693 (R2) C1
Thus the total period T is given by :
T = T1 + T2 = 0.693 (R1 + 2VR) C1
The frequency of oscillation is:
The main part of this LED chaser circuit diagram is 555 timer IC which generates some variable
frequency. 555 timer IC is a general purpose IC which can be configured in some different modes
like Astable, Monostable and Bistable. Here in this project we configured 555 timer as an Astable
multivibrator in which both the stages of signal are unstable. Some
time we call frequency generator also. Here we use output signal of this Astable multivibrator to
trigger IC CD 4017 counter to change its state to perform desired task.
Here we have connected 555 timer IC in Astable mode for generating a trigger pulse of some time
period. A variable resistor is connected for changing the cycle frequency of 555 timer’s output. A
CD4017 counter IC is also connected with this circuit for lighting LEDs. 10 red LED’s are connected
to Q0-Q9 pin (pin 3) through 150 ohm resistor. MR pin (pin 15), enable or clock inhbit pin (pin 13)
is directly connected with ground and Clock pin of counter directly connected with output pin of 555
In this article I've made yet another LED chaser for the internet, but the little twist here is the LCD
with a menu. A total of 32 LEDs are running up and down, or in different patterns. The
microcontroller has some unused pins, which means that you can add even more PCBs with LEDs.