“WELCOME” LED Display Circuit

The post explains how to make a chasing "WELCOME" display circuit sign board, which illuminates each alphabet sequentially until all the 7 alphabets are lit and then the whole display shuts off, the cycle continues permanently as long as the circuit is powered.

Overview

I have already discussed a similar concept explaining a bar graph display LED circuit for car turn signal, the same idea is implemented for the present welcome chasing light display circuit.

The figure below shows the details of the circuit:

Circuit Diagram

Parts List

All Resistors are 1/4 watt 5%

• SCR gate resistors are all 1k.
• T1 base resistor is 1k
• 33k = 1no
• potentiometer 100k = 1no

• Capacitor 4.7uF/25V/Electrolytic = 1no
• Capacitor 0.1uF/Disc = 1no
• Capacitor 0.01uF/Disc = 1no

• T1 2N2907 = 1no
• IC 4017 = 1no
• IC 555 = 1no
• SCR BT169 = 7nos

How it Works

Referring to the circuit above, the entire design is configured around a standard IC 4017, and IC 555 chaser circuit, wherein the IC 555 transmits the required sequences clocks at pin#14 of the IC 4017 and enables a sequential chasing of the high logic across the selected output pins of the IC 4017.

Here the pinouts from pin#3 and pin#5 are rigged for illuminating the "welcome" display while pin#6 is used for resetting the sequence after each complete cycle.

Meaning once the whole "welcome" sign is lit, subsequently pin#6 triggers the 2N2907 to switch OFF the SCRs and reset the sequence from the beginning at pin#3.

The 4017 IC outputs sequence with a "jumping" high logic which switch ON only momentarily while shifting from one pin to the next, this implies that if the LEDs were connected directly with the pinouts would cause each alphabet to illuminate only for a moment until the next alphabet was lit, enabling only one alphabet to be lit at a given instant. This would make the display unreadable and the "welcome" sign would be unrecognizable.

In order to ensure that all the alphabets are lit and stay latched during the sequencing, SCRs are introduced with the pinouts for illuminating the LEDs.

The SCRs trigger and latch during the sequence until all the the alphabets are illuminated and then finally shut off to begin a new sequence.

The IC 555 generates the clocks for the sequencing, and the speed of the sequencing can be adjusted through the associated 100K pot.

How to Wire the LEDs for Creating the WELCOME alphabets

Now let's learn how all the alphabets involved in the display may be wired using many LEDs in series and parallel connections.
Since the supply for the circuit is around 12V to 15V, and assuming the LEDs are 5mm/20mA type red LEDs, wiring groups of 4 LEDs in series seems to suit the best.
The following example figure clearly illustrates how the letter "W" may be wired using groups of 4 series LEDs, and connecting these 4 LED strings in parallel, such that the final outcome resembles the letter "W".

In the same way other alphabets could be easily configured and wired together for developing the required "welcome" chasing LED display circuit.
The series resistors across each of the 4 series LED string can be calculated using the linked software.

All LEDs are red LEDs/5mm/20mA/High Bright

For 4 LED series the resistor value will be = 25 Ohms 1/4 watt

If there are 3 LED series the resistor value will be = 175 Ohms 1/4 watt

If there are 2 LED series the resistor value will be = 330 Ohms 1/4 watt