# Building a Simple 9V LED Stoplight

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These instructions will show you how to build your own LED stoplight powered by a 9 volt battery.

This LED Stoplight is a fairly simple, cheap project that can teach you about some simple, but very useful electronic components including the 4017 Decade Counter, the 555 Timer IC, and diodes. You will also be able to obtain some good practice in circuit design and be on your way to becoming a better engineer. I will start out by taking you through an introduction to some of the more complex devices used in this project, for those of you who don't have a ton of practice in the electrical engineering field. Then I will take you through the assembly process.

Estimated Time: 1 - 4 hours (depending on your skill level)
Estimated Cost: \$5 - \$15

Materials:
- 9V Battery
- Battery Snap
- 555 Timer IC
- Jumper wires
- 3 LEDs (1 Green, 1 Yellow, 1 Red)
- 10 (1N4148) Diodes
- 7 Resistors
- 2 10 kΩ
- 3 680 Ω
- 2 1 kΩ
- 2 Capacitors
- 1 100 uF (u = micro)
-
1 10 nF

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## Step 1: Where to Find the Components?

Most of these materials can be picked up at your local Radioshack or electronics component store. If you have the time for shipping and handling you can purchase these materials online for around 10 - 30 cents a piece, but if your anxious to get started you'll have to hope that you can find them at a local store. Unfortunately, some Radioshack stores may not carry the IC components, such as the 4017 Decade Counter and the 555 Timer, but they should carry the rest of the materials needed (although not at the most reasonable of prices.)

## Step 2: Getting to Know Your Components (555 Timer IC)

The 555 timer IC is a common integrated circuit used for a variety of applications, more typically as some sort of timer, pulse generator, or oscillator. For this project we will be using the 555 as an oscillator, or more specifically an astable, to provide a continuous stream of rectangular pulses. These rectangular pulses will be used as the clock input of the 4017 Decade Counter. Astables can be built in several different ways, but the 555 timer IC allows for a simple build and calculations.

More information on the 555 timer IC, including current and voltage ratings, can be found in the datasheet.

Also, check out this website to get some more detail on the 555 astable circuit.

The 4017 Decade Counter has 10 outputs which go HIGH in sequence when a source of pulses (such as those from a 555 astable) are connected to the CLOCK input and when suitable logic levels are applied to the RESET and ENABLE inputs. A good display of these pulse can be seen in the image above. At each pulse the current output pin is turned off, and the next output pin in the sequence is turned on.

More information on the 4017 Decade Counter, including current and voltage ratings, can be found in the datasheet.

Head to this website to learn about some more details of the 4017 counter!

## Step 4: Let's Start Building!

Once you have gotten a hold of all the materials listed in the introduction and learned about the workings behind the integrated chips used in this project, it's time to actually start building you stoplight!

We'll start out by building the astable with our 555 timer IC, then we'll connect our 4017 counter, and finally add the LEDs and diodes to finish off the circuit

## Step 5: Astable Using 555 Timer IC

Shown above is the schematic for the astable built using the 555 along with a figure of the sort of output you should see from your 555 if everything is connected correctly. The numbers outside of the 555 figure indicate the pin number.

The output (pin 3) will be connected to our 4017 counter in the next step, however, I would suggest that you build this astable first, then check that the output resembles the figure above.

NOTE:  If you follow this project precisely, you will have a frequency of about 4.8 Hz, which means about a 2 second stoplight. This is pretty fast so if you are looking to increase or decrease the frequency of your counter in order to change the speed of the LED stoplight, the equations for doing such can be found here.

## Step 6: Connecting the Counter, Diodes, and LEDs

Once you have your astable built and checked out to be working correctly, you can then add the 4017 counter and the LEDs to your circuit.

For the 4017 counter, be sure to do the following:

• Connect pin 8 of the counter to ground and pin 16 straight to the 9 V.
• Pins 13 and 15, or ENABLE and RESET respectively, should be each grounded through the 10 kΩ resistors as shown in the figure above.
• Pin 11 can be omitted.
• Pin 14 is the CLOCK. It should be connected to the output of the astable (pin 3)

Note: The counter does not count in the order 12345678910 in terms of pins. The correct pin order is 3,2,4,7,10,1,5,6,9,11.

Feel free to change the pins connected to each LED. Here is what I chose to do:

Red LED connected to pins 3,2,4,7,10 (on for first 50% of the time)

Yellow LED connected to pin (on for 10% of the time)

Green LED connected to pin 5,6,9,11 (on for last 40% of the time)

Each pin should have a diode coming from it in the direction of the pin to the 680 Ω resistor.

Then each LED should have its positive end (anode) connected to the 680 Ω resistor and the negative end (cathode) grounded.

Note: If your LEDs are dull you may want to lower the resistor value.

Finally, connect your battery to the circuit and test to see that everything works properly.

## Step 8: Conclusion

If you followed the previous steps correctly you should have your very own, working LED stoplight, and hopefully you were able to learn about some useful electronic components and skills! I suggest that you don't stop at this simple project. Take the skills that you've learned here and try to take on a more difficult challenge.

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