Using discrete components to create a retro display for your bedroom door, or your desk! - by Daniel Koch
BUILD TIME: 30 MINUTES
DIFFICULTY RATING: BEGINNER
The phrase 'having your name in lights' dates from a time when the theatre was one of the main forms of entertainment, and specifically a time when most theatres had brightly-lit Marquee signs out the front and neon all over. The names of the lead actors in a performance were usually placed on these signs, and the more famous an actor was, the more their name would draw a crowd. Having 'your name in lights' is a way of saying 'being famous'.
We're going to do this with a variation of a circuit we have used many times: The CD4017 decade counter driven by an NE555 timer circuit. However, we have not used this circuit to do what we are going to do. We came close with the starry night project, where each LED was partially lit, then the decade counter lit each one fully in turn. This time, we have incandescent filament light globes, turned off and on fully in turn, to make it appear as though they are 'chasing' each other around the sign.We're going to make our sign from household materials. We used foam-core board but corrugated cardboard will work too, or even a small framed stretched canvas. We will build this to be powered by four AA batteries, so that you can use a long and narrow battery pack to better fit behind a framed canvas or be hidden behind cardboard. The sign we make can be for a bedroom door, or for a desk. Accordingly, we will show both methods.
SOME HELPFUL HINTS
We encourage you to read all the way to the end of the article before you build. Not only will you then have a better feel for the overall picture as you build, but we sometimes discuss options or alternatives that you will need to have decided on. You will need some basic hand tools for most builds. Small long-nosed pliers and flush-cut side cutters meant for electronics are the main ones. Materials like tape or glue are mentioned in the steps, too. We always produce a tools materials list if you have to go shopping, but anything that is lying around in most homes is just stated in the steps.
As always with Kids' Basics, we avoid soldering to make the build more accessible to more people, but having an adult around can still be helpful. You won't need any particular skills besides being able to identify components at a basic level, and even then, we help as you go along. If, for example, you don't already know what a resistor is, you'll probably be able to work it out from the photos and description in each step. We do provide a schematic or circuit diagram but this is just helpful if you already know how to read one. Don’t stress if you have never learned, but take the chance to compare the digital drawing of the breadboard layout (which we call a 'Fritzing' after the company that makes the software) to the schematic and see if you can work some things out. You can make this project from the Fritzing and photos alone. You might also like to check out our Breadboarding Basics from Issue 15.
|1 x Solderless Breadboard
|1 x Packet Breadboard Wire Links
|36 x Plug-to-Plug Jumper Wires*
|6 x 1kΩ Resistor*
|R2, R3, R4, R5, R6, R7
|1 x 200kΩ Resistor*
|1 x 100nF Capacitor*
|1 x 470nF Capacitor*
|1 x NE555 Timer IC
|1 x CD4017 Decade Counter IC
|6 x BC337 NPN Transistors
|Q1, Q2, Q3, Q4, Q5, Q6
|18 x Grain-of-Wheat Incandescent Globes, 6V
|1 x 4xAA Battery Holder
|S5028 + P0455
|4 x AA Batteries
* Quantity shown, may be sold in packs. You’ll also need a breadboard and prototyping hardware.
† The mounting hole for this switch may be smaller than the specified 20mm.
We have designed this circuit with six channels, and they each need the same number of lights to work properly. We are using eighteen globes, but you could use twelve or twenty-four as well. Think about the size of your sign, and read ahead to the 'How It Works?' section for more details before you build. Even if you don’t want to know the electronic details, we have a section here about the chasing pattern of the lights and how it is created. You'll need to know this to plan your lights properly.
Place the breadboard in front of you with the outer red (+) rail furthest away from you, and the outer blue (-) rail closest to you. Add two wire links, one to join the two blue (-) rails and one to join the two red (+) rails.
Insert the NE555 with its notch or dot to the left, and six wire links. One joins pin 8 to the upper red (+) rail, one joins pin 1 to the lower blue (-) rail, and one joins pin 4 to the lower red (+) rail. Three connect to join pin 6 to pin 2.
Install the 200kΩ resistor (RED BLACK BLACK ORANGE SPACE BROWN) to the right of the NE555, and two wire links to join it to pins 6 and 3. Also, add a 100nF (104) capacitor between pin 5 and the upper blue (-) rail, and a 470nF (474) capacitor with one leg in the lower blue rail and the other leg to pin 2.
Place the CD4017 into the breadboard, and add three wire links to connect its Clock pin 14 to the output pin 3 of the NE555. Add three more wire links to connect pin 8 to the upper red (+) rail, pin 13 to the upper blue (-) rail, and pin 8 to the lower blue (-) rail.
Insert three wire links to join pin 5 of the CD4017 (output 6) to the Reset pin 15. Also, add a 1kΩ resistor (BROWN BLACK BLACK BROWN SPACE BROWN) from pin 1 off to the left; a BC337 transistor with its flat side facing toward you and its middle (base) leg in the same row as the resistor; and a wire link from the right-hand (emitter) leg to the lower blue (-) rail.
Install a 1kΩ resistor (BROWN BLACK BLACK BROWN SPACE BROWN) across the gap next to the CD4017, and a BC337 transistor with its flat side toward you. Add a wire link to connect the resistor to pin 2 of the CD4017, another to link the resistor to the middle (base) leg of the transistor, and one more to link the right-hand (emitter) leg of the transistor to the upper blue (-) rail.
Add BC337 transistor with its flat side facing AWAY from you and its left-hand (emitter) leg in the same row as the other transistor's emitter and the wire link. Install a 1kΩ resistor (BROWN BLACK BLACK BROWN SPACE BROWN) between the middle (base) leg of this transistor and pin 3 of the CD4017.
Insert two BC337 transistors, the one on the left facing you and the one on the right facing away. Place them so that the legs closest to each other are in the same row. These are both the emitters. Place a wire link between the emitters and the lower blue (-) rail. Add two 1kΩ resistor (BROWN BLACK BLACK BROWN SPACE BROWN), one from pin 4 of the CD4017 to the middle (base) leg of the first transistor, and one from pin 7 to the base of the other transistor.
Install a 1kΩ resistor (BROWN BLACK BLACK BROWN SPACE BROWN) between pin 10 of the CD4017 and a spot to the right of the IC. Place a BC337 transistor here with its flat side facing you and its middle (base) leg in the same row as the resistor. Add a wire link between the right-hand (emitter) leg of the transistor, and the upper blue (-) rail.
Cut the pre-soldered ends off the light globe leads, and strip about 1.5cm of each. Twist them carefully around a pair of plug-to-plug jumper wires, and tape the ends. Do this for as many globes as you have, whether that's twelve, eighteen, or twenty-four. We left one uncovered so you can see. Use pairs of light and dark colours where you can.
Cut a piece of cardboard or foam-core board to size, or prepare your canvas. Create your name sign any way you wish. You can draw or paint, or print something out. Our sign is designed to look like a theatre sign, which were made of rows that letter plates slide into.
Measure around your sign to spread your lights out evenly. We put six along the top, six along the bottom, and three along the shorter sides, for eighteen all round. Then, poke holes in the cardboard with a suitable tool. A thick barbeque skewer works well, but be careful of splinters! A knitting needle is good, too.
On the back of your sign, number the holes in groups of six. Then, slide the globes through each hole and secure them with tape, glue, or blue tac. Be careful not to cover the numbers.
WALL MOUNTING: Cut lengths of cardboard the same length as the sides of your sign, to a depth of 4cm. Make the short sides a bit shorter to fit neatly between the long sides. Glue them together so they form a frame. Add one more piece as shown, a triangle with a notch cut at the end. This can sit on the wall hanging hook.
DESK MOUNTING: Cut two triangles the height of your sign, and about 5cm deep. Cut another piece to 5cm wide, the same width as your sign. Glue them together so that the triangles form a stand, sitting on the base plate. We used hot melt glue for both of these versions.
Take all the number 1 light wires, and plug the dark wires into the same row as the transistor that is connected, via a 1kΩ resistor, to output 0, which is pin 3 of the CD4017. There should be one leg of the transistor, the collector, which until now had nothing connected to it. This is the leg to plug into. If there is not enough space, you may need to use another plug-to-plug jumper to go between the transistor and another empty row, as we have.
Plug all the light wires for the number 1 lights into the red (+) rail closest to where the transistor is. Then, go through to do the same for the number 2 lights, which connect to output 1, which is pin 2 of the CD4017. Work carefully, because the board will get very crowded, very fast. Keep going until number 3 lights connect to output 2 (pin 4), number 4 lights to output 3 (pin 7), number 5 lights to output 4 (pin 10), and number 6 lights to output 5 (pin 1).
Connect a battery pack or USB power supply to the power rails of the breadboard in such a way that you can get to it easily. Insert the red wire into the upper red (+) rail and the black wire to the lower blue (-) rail. Your lights should all start flashing. If they have been placed and wired correctly, they should be chasing each other! If not, disconnect power and check connections in reverse order.
HOW IT WORKS
The heart of the circuit is the CD4017 decade counter, but it could not function without the NE555 astable circuit. The CD4017 advances from one output to the next every time the rising edge (going from low to high) of the clock pulse arrives at the clock input. The length of the high and low time of the clock pulse has no effect, the only impact on timing is the time between rising edges. So, a clock pulse of ten seconds would advance the counter once every ten seconds, no matter if the pulse was high for one second and low for nine, or high for nine seconds and low for one.
Of course, we want things to move much faster. The clock signal is generated by the NE555 connected in Astable mode, and is controlled by R1 and C1 to give a pulse of around a third of a second. This arrangement of the NE555 has not been used often in Kids' Basics. R1 is connected from the output to the Trigger and Threshold pins, which are pins 2 and 6. When power is first applied, pin 2 is low, and so the internal flip flop sets the output high. This causes current to flow through the resistor R1, charging the capacitor C1. When the voltage across it is sensed by the threshold pin 6 to be at two thirds (2/3) of the supply voltage, the flip flop sets the output to low. Now, the capacitor discharges through the same resistor until the Trigger pin 2 senses the output fall to one third (1/3) of the supply voltage. Then, the flip flop sets the output back to high. This works because the output sources (provides) current when high, and sinks current (sends it to ground) when low. The outputs of many Integrated Circuits (ICs) do either one or the other, but not both.
This results in a clock pulse with equal high and low times. It is sent to the Clock pin 14 of the decade counter CD4017. Every time the decade counter senses this pulse rising from low to high, the internal circuitry switches on the next output while turning off the current one. This happens from 0 to 9, giving ten outputs (0 is one of them), but they are not arranged around the IC in order. This is because of the way things work inside, and what fits, so always be careful to find which output is which.
The decade counter will only advance the outputs while the Clock Inhibit input, pin 13, is low. If it is high, counting stops. We don't use this feature, so the pin is tied to ground to stop it 'floating' and picking up electrical noise as a false high. We also don't use the 'Carry Out' pin. This pin is high for the first five counts, and low for the other five. It is used for cascading one CD4017 to the next, and acts as a clock pulse for the next stage so that the next counter is divided by ten (one whole clock cycle per ten cycles at the previous stage). This pin does not need to be tied to anything if unused.
We do use the Reset pin 15. When this pin goes high, the counter is reset to zero. If low, the counter resets after the ten cycles and will work repeatedly. We use it to shorten the count to six active outputs, numbers 0 to 5. We take the seventh output, output 6, and feed it to the Reset pin. As soon as this output goes high, the counter resets so that it only ever counts to six, from 0 to 5.
The outputs of the CD4017 can source 10mA at best. This is nowhere near enough to drive many LEDs, although modern ones are still quite bright if limited to 10mA even when they are rated for 30mA. But, we are using 100mA light globes! What's more, we have three or four connected to each output!
The answer is to use a transistor to boost the output. The outputs are fed to a 1kΩ resistor each, which limits the current to 5mA at the base of the transistors. The BC337 transistors are rated at between 500mA and 800mA continuous current, depending on the model (the letters after BC337, like BC337A), and the brand. Even the lowest-specified ones handle the 300mA or 400mA that we are asking of it. Also, each channel is only on for one sixth of the time.
We do this by placing the globes in parallel, so that the voltage is the same across each but the current drawn by each adds up. The rest of the circuit draws such little current that we can nearly ignore it.
THE CHASING EFFECT
The chasing effect comes from the spacing of the lights. Each output on the decade counter lights after the one before it. By making sure we place the light globes very carefully, they can be made to chase by the order in which they are turned on. In the diagram, the lights are numbered. This is the channel each is connected to. Notice that they are always groups of 0 to 5. That is why you need to have lights in multiples of (number of groups of) six. If you have an uneven number, like four outputs with three lights and two outputs with two lights, then the chase does not look even.
All output 0 globes light up first, then switch to all output 1 lights, which switch to all output 2 lights, and so on. Placement of the lights is critical!
WHERE TO NEXT
The main two changes here are to the speed of the chase, and the number of lights. Changing either the value of R1 or C1 in the NE555 section will change the chase speed. Too fast, and it looks like a flicker. Too slow, and it loses impact. Try it out, you can't break anything! The number of lights is determined by the size of the sign. If you want to go for more than four lights per channel, you will need to find bigger transistors. These usually have bigger pins that are not great for breadboards - they stretch the spring contacts. Heat may also be a problem.
The other thing you can change is the power supply. We designed the circuit to run on four AA batteries at 6V, but if running four globes per channel, the batteries will not last that long. Even running three globs, battery life is not wonderful. The circuit will run happily from 5V, so you could use a USB power supply.
You can look over previous Kids' Basics projects to find examples of where we have done that. Additionally, particularly if you have gone for battery power and wall mounting, you might like to add a power switch. Do this by cutting the ends of a pair of jumper wires, keeping a pin on one end of each. Then, twist the wires through and/or around the terminals of a power switch. This can be a toggle or pushbutton, but be careful it is not a momentary type. Cut a notch in the frame to glue it into, or mount it on the back of the desk version.