Introduction: Personal Space Defense System

About: My name is DJ and I previously made electronic whatsits, 3D-printed thingamabobs, and laser-cut kajiggers for the Instructables Design Studio; now I build and repair puzzles for Particle Industries.

If you're tired of people invading your personal space, popping your bubble, or otherwise gettin' up in your business, why not send them a message by blasting them away with a water gun? Manually targeting people with your soaker of choice can be a chore, and if there is a deluge of personal space invaders intent upon approaching you, you'll need an automated solution! You need the Personal Space Defense System, because nothing says stay away from me like an over-sized, electro-mechanical backpack with a shoulder-mounted, self-firing water gun. Intrigued? Find out more on the following steps!

Step 1: Parts and Materials

Here are the parts I used for the system. I've also attached the Arduino sketch and vector files in a .zip above.

A* Micro (or Arduino Micro equivalent)

Super Soaker Electro Storm

IR distance sensor

PIR motion sensor

laser diode

power switch

illuminated switch

2200 mAh 7.4V LiPo (two cell)


LM317 regulator

N-channel MOSFET

2N3904 transistor

10K resistor

1K resistor

3K9 resistor

220 resistor

1000 uF capacitor

100 uF capacitor

3 pin JST plug

xt connector

old back pack

scrap canvas


heat-shrink tubing

1/4" foam sheet

cable sheath

1/4" plywood

zip ties


servo with gearbox

(4x) round base plate

1" x 16" aluminum tube

1" x 6 " aluminum tube

1" x 2.250" flanged aluminum tube

(8x) tube clamp A

(5x) tube clamping hub A

90 degree hub mount A

small round screw plate

1/4x20 round screw plate

(3x) flat dual channel bracket

channel bracket B

channel bracket A

3" aluminum channel

90 degree hub mount bracket

(2x) 4.62” aluminum beam

6-32 1.25" round aluminum standoff

(50x) 6-32 x 3/8" screw

(25x) 6-32 x 1/4" screw

12x #6 washer

14x 6-32 nut

1/4-20 x 1/2" screw

Step 2: Design and Operation

So what does the Personal Space Defense System actually do?

The PSDS consists of a shoulder-mounted water gun, sensor pendant, and trigger mechanism. If a personal space invader is too close (as judged by the sensor readings) and if the system is "armed," then the water gun will blast off a few shots of water towards the intruder.

Trigger and Power Mechanism

Pressing the power button will turn on the system (as indicated by the illuminated switch lamp), pressing the trigger button will toggle between armed and disarmed modes. On arming the system, the gun with flip up and the attached laser diode will power on.

Shoulder Gun

The actual gun consists of a stripped-down electric Super Soaker. Removing the case dramatically reduces weight and allows for easy direct electrical control. The water gun/reservoir assembly is wedged into a channel bracket and actuated by a hefty, geared servo. Overkill? Yes. Intimidating? You betcha'.

Sensor Pendant

In order to determined whether or not someone has effectively invaded the wearer's space, the system reads from an analog infrared distance sensor and a PIR motion detector. Should someone move (triggering the PIR sensor) and breach the distance threshold (as measured by the distance sensor) the system will be activated.


For ease of mounting and added comfort, I designed the system to be mounted to a regular backpack. The pack provides a sturdy mounting point for the main tube and proto-board for the electronics.


The gun assembly is a bit hefty, so to balance out its inherently wobbly nature, I created a counter weight that has a mount for a camera. I ended up attaching a GoPro.

Step 3: User Control Switch: Part 1

The user control switch contains the main power switch and the arm/disarm switch. The mechanical form consists of a flat dual channel bracket, a bore clamp, and a 90 degree hub. The bore clamp fits around the arcade switch. Originally the switch had a 12V lamp, but lacking that voltage for the system, I swapped it out for an LED. The clamp/switch assembly mounts to the dual channel bracket towards the top. I also extended the wires for the switch by 30 inches.

Step 4: User Control Switch: Part 2

The power switch is a latching pushbutton switch mounted in right angle hub. In order to mount it, I created a wooden plate adapter. Although I laser cut the profile (attached as vector files above), it could easily and quickly be done with a hand drill. The power switch is connected to two 30 inch long wires as well. One goes to the positive side of the XT connector, and the other goes to the main PCB. Once I extended the wires, I braided them and used excessively large heat shrink to hold everything together.

Step 5: Backpack Base

In order to mount the cross beam, I needed a way to attach a plate to the backpack itself. Before screwing the plate into the pack, it needs to be reinforced to prevent fraying. My very talented friend Ms. Push_Reset introduced me to HeatnBond, which is a fabric adhesive that allows gluing all manner of materials together. We used this to add swatch of heavy canvas to the interior of the back pack pouch. As most backpacks are often made of synthetic materials, they're quite prone to melting, so be careful! We ended up reinforcing the rim of the pack with extra canvas, but this isn't necessary unless a tear is made.

On a flat surface:

Rip the seams of the rear portion of an old backpack

Lay out HeatNBond sheet

Trace sheet over top of backpack

Cut out HeatNBond pattern and matching canvas swatch

Set Iron to wool setting

Iron on paper side of HeatNBond to interior of pack

Remove paper

Place canvas and resume heating until firmly bonded

Step 6: Backpack Plate

With the backpack properly reinforced fabric-wise, it's time to add the base plate!

Cut and lay out the large wooden plate

take a permanent marker and poke through the top and bottom six holes of the rings

Using a awl, poke through the markings until they're just wide enough for a 6-32 screw

Attach washers to twelve 6-32 screws and poke them through the hole in the rear

Mount two bore clamps on the round bases each

Place the round base over the holes and fasten them with twelve 6-32 nuts

Step 7: Water Gun Disassembly

The gun needs to be stripped down before attaching to the system, this reduces the weight on the motor mechanism quite a bit. There are 10 Phillips screws (all on one side) that need to be removed to open the case. A final captive screw in the rear retains the battery compartment. Take that one out too.

Step 8: Water Gun Modification

The water gun was designed to be powered by 4 AA batteries in series, but this is also quite heavy, so I opted just to run 6ish volts straight to the motor (more weight savings). To keep things compact, I marked the rim of the battery compartment and dremeled away the excess plastic. Be sure to fish out the switch wires (red and orange).

Step 9: Main Control Board

I've attached the circuit diagram for reference above. Most of the circuitry lives on the mini protobard, with female header sockets for quick assembly. I used the top power rail for the battery voltage and the lower rail for the 5V output from the A* Micro. I also added a 1000uF on the 7.4v battery rail and a 100uF capacitor on the 6V output from the LM317.

Step 10: Software

The program running the PSDS is a basic Arduino sketch (attached above). I've also included the Bounce library, which will need to be installed. If you use the A* board, you'll need to install the drivers and add the hardware board file to your Arduino IDE. Pololu has a very thorough guide on how to do just that.

Step 11: Servo Arm

Here's how the base of the servo assembles:

Attach two clamps to the angle bracket

Attach two right angle clamps to the channel

Insert the servo block to the channel

Insert the riser tube to the servo assembly and tighten the clamps.

Attach a right angle clamp to the main tube

Attach a dual channel plate to the clamp

Zip tie and feed the wires from the servo, water gun, and laser trough the channel.

Step 12: Water Gun Assembly

The water gun assembly mounts up and away from the servo and main electronics to keep everything nice and dry. Should the mini reservoir spill, it won't reach the servo or the shoulder of the user. The water gun mechanism is held in place, pinched inside of a piece of channel bracket A. The reservoir is supported by two beams that attached to the channel as well. The laser is attached to the top of the spout with a pair of zip ties. Both the laser and gun power wires are extended by 30 inches as well. The pump's power wire is red (the orange wire is left disconnected) and it's ground is soldered directly to the battery (springy) ground plate.

Step 13: Camera Counterweight Assembly

In order to counter the weight of the gun mechanism, I decided a camera would make the perfect addition to the pack. A 1/4-20 threaded bolt runs though a round base plate and out the front. I used a GoPro, but most camera hardware will attach as well. I also mounted the LiPo battery on this end of the pack, wedged between two pieces of foam and zip-tied to the main tube.

Step 14: Sensor Pendant

The sensor pendant hangs on the user like a necklace and just consists of the PIR and Sharp distance sensor bolted to a couple of pieces of plywood. I clipped their connectors, tied the grounds, and extended the wires by a couple of feet. Although the JST connector from the PIR sensor will fit in the Sharp sensor, the color scheme is wrong. I certainly didn't make that mistake. Nope.

Step 15: Final Thoughts

I tried my best to make this a relatively comfortable experience for the wearer. The backpack straps need to be quite tight for the pack to rest properly and the gun mounted, but overall it came out how I imagined. This pack is certainly, well overdone, but quite effective! Originally I wanted to create a dual gun setup, but decided a single gun would be a good starting point. How would you create a personal defense system? Let me know in the comments! Thanks for reading.

Epilog Contest VII

Participated in the
Epilog Contest VII