LED Juggling Knives

This project was inspired by one of my friends, who is an avid juggler. He was looking for glow in the dark juggling balls and I wanted to try my hand in building it. As I worked through the design I realised that it would be much easier to build knives instead of juggling balls and I moved forward with that. With my knowledge of the 4017 IC, I decided to add a bit of flourish to the lights, which are now in the form of LEDs, and that is how I ultimately ended up with this project.

A set of juggling knives that can be also used in the dark for better visuals.

The LEDs are lit in a sequence, just like a led chaser. This is done by a 555 timer circuit in Astable mode combined with a 4017IC. The housing is relatively simple, and it was 3D printed.

This is a relatively challenging project to build with a lot of components required.

Note: The following supply list covers the making of 3 juggling knives, as all juggling knives usually come in sets of three. If you want to build only 1, you can divide the quantity by 3.

Circuitry BOM:

1) 3 x NE555 IC + 8 pin IC holder (link)

2) 6 x IN4007 Diodes (link)

3) 3 x 4.7k Ohm resistor (link)

4) 3 x 3.3k Ohm resistor (link)

5) 3 x 4.7uF 10V electrolytic capacitor (link)

6) 3 x 40mm by 50mm Perf Board. (Any size bigger than this is fine. You can trim it down later) (link)

7) 2 x Male to Male Jumper Cable 40pcs. (link)

8) 3 x 4017 IC + 16 pin IC holder (link)

9) 3 x 10k Ohm resistor (link)

10) 25 x 5mm LEDs in Red colour (5 are extra just in case) (link)

11) 25 x 5mm LEDs in Blue colour (5 are extra just in case) (link)

12) 25 x 5mm LEDs in Yellow colour (5 are extra just in case) (link)

13) 3 x 9V Battery + connecting clips (link)

14) 12 x 3mm Dia 20mm long bolts (link)

15) 3 x 3mm Dia 6mm long bolts (link)

16) 3 x Tactile latch switch (link)

Tools and Equipment:

1) Soldering Iron + Soldering wire

2) Desoldering pump

3) Pliers

4) Wire cutters + Wire Strippers

5) 3D printer or access to a 3D printing service provider

There are two parts of this circuit. 555 timer and 4017 IC. I will be exploring the former in this step. You can ignore this step if you are familiar with the 555 astable circuit.

555 timer circuit

This circuit is a very common favourite of all electronics geeks. The schematic for the circuit can be seen in the first image. Here is a link to the website where I got the circuit image from https://www.electronics-tutorials.ws/waveforms/555...

For this project, the circuit is wired up in an astable configuration. This means that the voltage output of the chip alternates between high and low at a constant frequency. The voltage output vs time graph of this circuit can be also seen in the image.

It is a square wave. Essentially the circuit produces pulses at a constant rate. This can be used to make an LED blink. The frequency of the pulse can be controlled used the values of the two resistors and the capacitor. These are respectively R1, R2, and C1 in the image. The equation used to calculate the frequency from these values has been given in the equation.

The final term is the duty cycle. It is the percentage of time for which the output is high. This can be also calculated with the equation in the image. Unfortunately, if the duty cycle is too high it leads to bouncing (link) or the connecting circuit can't even detect the pulse/low period.

To rectify this issue, this circuit has an additional 2 diodes compared to the conventional circuit. This is to make the duty cycle more even 50%-50% between on and off. (link)

The 4017 IC works with the 555 timers.

The 4017 IC has multiple output pins: pins 1-7, 9, 10, and 11. Each of these output pins is set in a sequence as shown in the image. When the chip detects a pulse at its clock pin (pin 14) it makes the next output pin the sequence high and the current one low. This way with a continuous pulsing input each output pin on the 4017 will light up and turn off in sequence.

The input is provided by the 555 astable circuit's output. If LEDs are connected to the output pins of the 4017 IC in this state, they will light up in a sequence or a line, which is exactly what is desired.

More details about the 4017 circuit can be found here: https://www.electronicshub.org/ic-4017-decade-cou...

Because the circuit has to be small to fit inside the knife, the components were all soldered onto a perf board.

The next 10 steps will (steps 4-13) show a detailed guide for how to solder the 555 circuits. However, if you want a summarised version you can find that below here, while also looking at the schematic.

The pin number refers to the pin number of the 555 IC. It is numbered in a U shape starting from the top left and ending at the top right.

Pin 1 Connections: Connected to Ground directly.

Pin 2 Connections: Connected to pin 6 directly, connected to R2 through D1

Pin 3: It is the output. Connected to the clock pin (pin 14) of the 4017 IC directly.

Pin 4 Connections: Connected to Vcc directly.

Pin 5 Connection: Not connected to anything.

Pin 6 Connections: Connected to Pin 2 directly.

Pin 7 Connections: Connected to Vcc through R2, to pin 6 through D1.

Pin 8 Connections: Connected to Vcc directly.

There are a couple of points to note that have helped me identify issues with my circuit.

It is crucial to connect pin 4 and 8 to the Vss in parallel and not series. Each pin should be individually connected to the positive rail.

I have found it easier to leave pin 5 alone with no connections. It reduces one component from the BOM while still working properly.

It is important to orient the chip pointing upwards on the perf board and to also place it right at the top of the perf board to reduce the space used. This will make it easier for it to fit in the knife's handle.

I have not used a chip holder as I am confident in my ability to solder quickly. But, unless you are confident to solder the pins within seconds, I would recommend you use a chip holder.

The positive and negative leads were connected on the completely right and left side of the 555 chip respectively.

This gives them room to turn into almost busses like in breadboards.

Pin 8 and pin 4 are connected to the positive rail from the battery connector using short wires.

Make sure to connect them in parallel.

Pin 1 is connected to the negative rail on its left.

The two pins are connected using a wire

There are two steps here.

The R2 resistance value is 4.7k Ohms

R1 resistance value is 3.3k ohms

C1 capacitance value is 4.7uF.

Its negative pin, the shorter leg, cathode, is connected to the negative rail.

The capacitor's positive pin (longer leg) is connected to pin 2.

There is also a photo of the underside of the circuit that shows all the soldering.

The lead is the output and will be soldered to the clock/input pin of the 4017 IC.

The next 4 steps (steps 16-19) will show a detailed guide for how to solder the 4017 circuit. However, if you want a summarised version you can find that below here, while also looking at the schematic.

Pin 1-7 + pin 9-11: These are all the output pins to which the LEDs will be connected to in order.

Pin 8: Connected to ground directly

Pin 12: Is not connected to anything

Pin 13: Connected to ground directly

Pin 14: Connected to pin 3 lead of the 555 timer circuit directly

Pin 15: Connected to ground directly

Pin 16: Connected to the positive rail (Vss) directly.

Additional point:

This is something I found to be very useful when building the circuit. Using a 10k resistor as a pull-down resistor I connected the chip's pin 14 to ground through the resistor. This helped prevent any floating voltage from affecting the pulsing input. It makes it easier for the 4017IC to detect the pulses.

Again same comment regarding IC holder.

The pulsing output from the 555 circuit, is connected to the input pin of the 4017 IC.

Pin 14 of 4017 IC is connected to ground through a 10k Ohm resistor. It is a pull-down resistor.

Here is a link explaining pull-down resistors: https://www.electronics-tutorials.ws/logic/pull-up...

Pin 8 of 4017 IC connected to ground directly with a wire.

Both these pins on the 4017 IC are connected to ground by connecting to pin 8.

Male to male jumper wires, specifically 10 of them, are soldered onto the output pins of the 4017 IC.

I found this the easiest way to connect the circuit to the LEDs.

There is a photo also which shows the backside of the perf board.

A male to male jumper cable is soldered to the negative rail. The other end of the lead is left alone. Its use will come in later for the LEDs

The circuit is now complete. Two more copies of this circuit have to be built with the exact same procedure for the other two knives.

I used Onshape to design the knives. It is free I believe. Here is the link: https://www.onshape.com/en/

Points to note about the design:

- Each knife is comprised of two halves, secured together using bolts.

- The circuitry fits into the compartment in the hollow handle.

- The leads coming out of the 4017IC is held in the hollow section of the blade.

- The LEDs fit into the holes on the blade and here the leads from the circuit are connected to the LEDs.

- There are 4 holes on the handle, which are for the screws, that hold the two halves together.

- There is also 1 hole on the blade near the blade's tip, which is for another screw that helps hold the blade ends together.

- The total dimensions of a single half of a knife are 261mm long x 71mm wide x 15mm thick

1. Ultimaker Cura was used as the slicer. Ender 3 was the 3D printer used for this project. The size of the knife half was just perfect to fit into the print area, but it had to be oriented diagonally.

2. Only 1 knife half can be printed at a time. If you don't have a 3D printer, or if your 3D printer is too small to print such a large object, then you can try reaching out to 3D printing services in your community or a Design technology lab at school if you are a student.

3. The .stl and .gcode files have both been attached to this step. Since there are two parts to each knife, there will be two files in each format.

4. The .stl file can be downloaded if you want to make some edits to the design before printing. The .gcode file can be uploaded directly to your 3D printer.

5. The casing was made out of black PLA and for each knife half, it took roughly 6 hours.

6. The temperature for the nozzle and bed can be found on the filament's package itself.

I am looking to print 3 knives in total. I can only print 1 knife half at a time.

It took me 6 prints in total, timing 6 hours each, to print all the knife halves. 36 hours in total.

Using pliers and occasionally my fingers, I pushed in the LEDs into the holes with a particular orientation.

The anode/longer leg was towards the sharp side of the blade while the cathode/shorter leg was towards the dull side of the blade. This was consistent for all LEDs and helped easily identify which terminal a certain leg connected to.

To keep the LEDs in place while inserting them, their's legs were bent as seen in the images.

The cathodes/shorter legs of the LEDs were bent in a line and hooked around each other as you can see in the image.

Then using a soldering iron and some solder, the joints were carefully soldered. Make sure to not accidentally short/solder any LED's anode and cathode legs together. You can use a desoldering pump to fix it if it does happen.

I then used some wire cutters to cut off any excess metal jutting out of the solder joint. This helps prevent any accidental shorting when the knife is being thrown around.

1) Take out 5 female to female jumper cable and cut them in half.

2) Using a wire stripper strip a bit of wire from 2-3 cm away from the female connector.

3) Using the wire stripper again, strip the end of the wire as well.

4) In total there will be 10 pieces like as shown in the final image. Enough for one knife.

The 4017 IC has only 10 output pins, but there are 20 LEDs in total, 10 LEDs on each side of the blade.

The LEDs on each side of the blade are given a number from 0-9, according to their sequence. The numbering starts from the bottom of the blade to the top. LEDs that have the same number on both sides are soldered together. Look to the images for reference.

Using the wires preparing in the previous step the anodes of the LEDs in the same row are connected together as seen in the image. They are then subsequently soldered.

Tip: For the LEDs on the left side, the exposed part of the wire is wrapped around the LED's anode leg.

This process is repeated for each LED in the row. This produces the second last image.

Finally, the LED's anodes are cut shorter using pliers. This is to prevent accidental shorting when the knife is being thrown around.

A wire, just like in the previous step, is connected to the common cathode on both sides and soldered. This allows the LED's to be grounded through the circuit using the wire that was attached back in step number 21.

The male output leads from the circuits are plugged into the female connectors.

To understand the sequence of connection, follow the attached image of the 4017IC's pinout.

You would start with pin 3, which denotes output number 0, and connect it to the LEDs closest to the handle. Following that sequence, you will have pin 4 (output number 1), pin 2 (output number 2) so on and so forth.

You can find the sequence here:

1) Pin 3

2) Pin 4

3) Pin 2

4) Pin 7

5) Pin 10

6) Pin 1

7) Pin 6

8) Pin 7

9) Pin 9

10) Pin 11

Additionally, you will also have to connect the negative rail lead, soldered back in step 21, to the common cathode female connector.

Plug in one of your 9V batteries as a test to see if all the LEDs are functioning. They should light up as seen in the video.

Using some cello-tape the joints between the male leads coming out from the circuit and the female connectors of the common LED anodes are secured properly.

Using some copper coil, I wrapped up the wires and tried to save space so that all the of the circuitry could fit into the knife's hollow interior.

In my initial 3D design of the knife, I did not accommodate for a latch switch, which can turn on and off the LEDs. I realised after completing 1 knife that it is a real pain to open up the entire model to turn off the LEDs. Therefore, this is what I did to add the switch. I used a latch tactile switch because it is small.

Step 1: I cut off all three legs on one side of the latch switch.

Step 2: From the remaining three switches, I found out, using a multimeter, which two legs are connected when the switch is closed. I cut off the remaining leg that is not connected.

Step 3: I soldered a single lead to one of the latch switch's pins. I also used some heat shrink to secure the joint, but this is optional.

Step 4: I removed desoldered the positive wire of the battery from the PCB, and then soldered it to the other pin on the latch switch. Again the heat-shrink it optional.

Step 5: Using pliers and a file, I cut a groove into the thick part of the handle and pressed in the switch into the groove.

There are two areas to place the bolts.

The longer bolts, 4 of them, are screwed into the 4 holes on the handle.

The short bolt, only 1, is screwed into a hole near the tip of the knife.

The knife is now completed. The process will have to be repeated 2 times more but each time a different colour LED is used.

Unfortunately, I can't juggle, and due to COVID-19, I can't get the knives to my friend who can juggle them. Therefore, the videos only show them turned on.

There is also a video in which the knives are turned on in the dark.