Light Saber Igniter

Home made Light Saber projects can consist of one bright led in the hilt or a row of LED's in the shaft that all light up at the same time. To look a bit more authentic a sequential lighting of the blade can give the visual appearance of the blade extending.

This project produces an "igniter" that will turn on a bank of LED's in sequence. It has a high current driver capable of driving up to 500mA per stage (that's a lot) and has an adjustable power up / power down speed. This project requires no prior knowledge of electronics or programming ability. All the components should be easily sourced from any electronics shop that sells Integrated circuits and resistors.

This project is not designed to be the be all end all design for lighting LED's. In fact it is about as simple has you could get. If you moved into micro-controllers you could probably light each individual LED and in RGB. This project is designed for anyone to build and have a good result, lets call it Meccano electronics.

This project does not cover building a blade, this might be covered in another project. There are many videos on youtube cover the building of Light Saber blades using LED's for example here and here.

As mentioned any store that calls themselves an electronics store (not a store that sells blenders and TV's) should hold the following inexpensive parts.

Components

ULN2003 This is an IC that lets us switch large currents. A an IC of micro even can drive 500mA per output. If we were to put a lot of LED's on the output of a micro it would probably damage the chip. Do prevent this we will use the ULN2003 on the output stage to drive, well, more LED's than you actually need. This chip has been around 40 years so is well tested.

LM3914 This IC is used to turn on LED's based on the input voltage. It can turn on up to 10 individual LED's based on the voltage on it's input pin. It can be configured to display in a single dot or bar mode. Youtube has several tutorials on using this chip have a look here.. We will be using it in bar mode, this chip has been around 30 years.

OP-Amp (LM358) OP-Amp's have also been around for decades and are widely used. We will be using it in a bootstrap circuit that slowly increases or decreases it's output voltage based on it's input. What type you use is optional but you will need one that states it is a rail-to-rail op-amp. I have had an acceptable result with the LM358, you should check the pin out of this chip if you intend to use a different type as the circuit is configured for this layout.

Miscellaneous

8 x 10kΩ Resistor

1 x 1kΩ Resistor

1 x 3k3Ω Resistor

1 x 2k2Ω Resistor

2 x 1N4148 Zener Diode

1 x 10uF Electrolytic Capacitor

1 x 10kΩ Trimpot or Variable Resistor

1 x Variboard or Stripboard

1 x SPDT Switch

1 x 9v Snap On Battery Connector

1 x 9v Battery

Optional - ic sockets 8, 16 and 18 pin. Plugs and some LED's.

Aside from the above you will need the following tools.

Soldering Iron

Solder

Stanley/Box cutter knife

Fine Side Cutters

As mentioned you need no electronics experience to complete this project. As long as you assemble the components as described you will be able to build the igniter. You should practice soldering if you have not done so before and watch some tutorials on Youtube to get some tips on technique.

Circuit Board

The circuit will be assembled on strip board or some times called Variboard. This is a non conductive material like fiberglass that has strips of copper to build our circuit on. The piece you have been supplied will almost certainly not be the correct size so we need to cut it down. I have tried to reduce the size of the circuit as much as possible so it will fit in a hilt. Count 31 holes along the length of one side and score it with a sharp knife. If you bend slightly it should snap cleanly. Count 12 holes in the other side, score and snap here also

Using the schematic start adding components to the circuit board. Start with the lowest profile parts first like wire links and Zener diodes. Note the Zener diodes are polarized (need to go in a specific direction). When you insert the parts turn the board over and solder them in place. If you put all the parts in at once some will fall out when you turn it over so just build the circuit up.

Part List

D1 Zener-diode 1N4148

D2 Zener-diode 1N4148

R1 10k
R2 1k

R4 2k2

R3 3k3

P1 Trimmer 10k

IC3 ULN2003

IC2 LM3914

IC1 OPAMP

Warnings

The Zener Diodes and capacitor must go in the correct orientation or the circuit will not work. The capacitor is marked with a white strip or - mark. The Zener Diodes have a black paint mark. The IC's have a notch at one end and this notch needs to be at the top for each chip. The gap between each track on the circuit board copper side is very narrow and it is quite easy to accidentally bridge across. In the photos I have used IC sockets so I can pull the chips out if I want to use them in other projects. This is optional but for a few cents it is worth the expense as it is easy to damage an IC if you try and desolder it.

For this circuit to work we need a network of Pull-Up resistors on the outputs of the LM3914. If you are lucky you can buy these pre-made from the local shop. Just ask for a 10k resistor array, you will have to chop of one of the legs otherwise the +ve and -ve will be on the network at the same time.

The other method is to make the array. Take one of your 10k resistors and bend one leg at 90 degrees. Collect another 6 and do the same so all bent legs are joining. Solder all these legs together letting the last resistors leg separate from the rest as this needs to connect to the +ve strip. Insert the array in the circuit board making sure the common leg is going to the correct supply.

The circuit is powered by a 9V battery, the red lead goes to the uppermost strip based on diagram, the black wire goes directly below.

To turn on and off the LED's we use a SPDT switch. This type of switch has 3 contacts the center contact, or common goes to the green square indicated at the bottom of the diagram. +ve and -ve are connected to the other two terminals. Effectively we swapping between +ve and -ve supply to ramp and ramp down the op-amp.

Before completing the circuit we need to make cuts in the tracks as we don't want all the tracks conducting. Using the diagram mark the board with a pen and then cut the tracks with a sharp knife.

To make sure you have made the project properly you should make a test jig of 7 LED's and current limiting resistors. LED's need resistors otherwise they will be destroyed by your supply. To calculate what size resistor you should use you can use this website. The circuit works fine from 5 to 9v volts, the chips used can work up to 30v either way you will need to use your proposed voltage in the led calculator.

LED's are made up of an Anode(+) and cathode (-) which means it will only work when placed on the correct battery terminal, unlike a regular light bulb that works either way. When you look at the legs of an LED one should be longer than the other this is the Cathode.

This circuit is common cathode, as shown in the diagram the lower pin is the common. All the longer legs should be joined together (careful not to touch the anode, I used some insulation) and then connected to the common pin. The anodes should have a resistor attached to each and then to an output of the ULN2003.

Once you have assembled your jig connect a power source. If the circuit is working the LED's should flash briefly to start with, switch the toggle switch and the LED's should light up sequentially and stay on until you switch to the other position when it should "power down" sequentially. If the you find the sequence to slow or fast adjust the trim pot to the desired the speed. If it is still too slow replace R1 with a wire link or add another 10k resistor between the trim pot and pin 3 of the op-amp.

Troubleshooting

Here are some tips for troubleshooting the circuit.

1. Check you have made all the cuts in the tracks.
2. Check the orientation of all the IC's.
3. Check the battery is connected correctly (+ve and -ve leads in the correct position).
4. Check the Zener's and capacitor are in the correct orientation.

The following requires a multi meter (assumes you are using a 9v battery).

1. Pin 8 of IC1, Pin 3 of IC2 and Pin 9 of IC3 should measure around 9v.
2. Pin 4 of IC1, Pin 2 of IC2 and Pin 8 of IC3 should measure 0ohm to -ve
3. Pin 5 of IC2 should be 0 volts with the switch in one position and 9v in the other position. Unless you have an analogue multi-meter you may not be able to see the latency but you should see the voltage raise or fall slowly on Pin 5 when you flick the switch.

The heart of the circuit is actually the op-amp. When a +ve supply is applied to the non-inverting input the trimmer and capacitor charge slowly producing a linear voltage increase which we feed into the LM3914. The basis of the op-amp circuit can be found here if you want to know more information. I have used a 10k resistor along with the 10uf capacitor and 10k trimmer to set how long it takes to ramp up and ramp down. If you find the base setting to slow you can put in a lower value resistor or capacitor to speed it up.

The LM3914 is configured to go full scale at full voltage, as we see the voltage ramp up the LED's turn off. Now this is where it gets a bit confusing as we expect the LED's to turn ON as the voltage goes up. The problem we have is the ULN2003 wants inputs to go LOW (off) to turn on the drivers. To get the correct operation we pull the ULN2003 high with the resistor array, as the outputs go low on the LM3914 the LED's are turned on by the ULN2003.

The LM3914 is still configured to run single LED's so if you want to run the circuit without the ULN2003 driver remove the driver chip and pull-up resistors.

Why only use 7 of the 10 LM3914 outputs?

This is total overkill for this project. Commercially available sequential light-sabers have 4 stages that have sufficient POV to look authentic. I would suggest not to use more than 4 if you use this circuit as each bank of LED's needs an activating wire running back to the igniter, at the base of the blade you will have 7 wires which will start blocking light from your LED's.

The ULN2003 as a single package can only handle 7 inputs, you could add another to make use of the spare 3 outputs but this will just add bulk.