Introduction: Unsafe Lockbox

This instructable was created in
fulfillment of the project requirement of the Makecourse at the University of South Florida (

Do you have not-so-valuable valuables that you want to secure but not so securely?
Do you have items that you want to keep secure only by the mere security of appearing secure?
Perhaps you simply have an affinity for oddities and a desire to create?
Or worse still a course requirement to document your term project?

It matters not, keep reading, I will teach you how to construct your very own prostitute safe!
The term prostitute safe means, a "secure" containter that opens for anyone with money.

In my application, it is a lockbox, that can unlock with a combination of a 4 digit keypad code and a puzzle using an ultrasonic range-sensor or one American quarter.

The keypad and ultrasonic range-sensor are very secure, it would take a long time to figure out the keypad combination by trial-and-error, but one quarter and the machine unlocks itself, defeating it's own security.

Step 1: Step 1: Go Get Stuff

To build the lockbox, you will need the following:

(I have conveniently added url links to locations where these items can be purchased, although I guarantee you can shop around and buy them for less.)
Plywood - 5x5 sheet

1 x Membrane Keypad

1 x Coin Acceptor

1 x Ultrasonic Range Sensor

1 x Arduino UNO or equivalent

1 x SPST Momentary Stomp Switch

1 x 5V Relay Module

~6ft Hookup wire

2mm x 30mm screws and nuts

1 x Arduino Breakout Board

1 x 12V relay

1 x 12v 5Ah battery (Can be subbed with different 12v battery)

1 x I2C LCD module

2 x Box hinges

1 x Box Latch

Step 2: Step 2: Assemble Your Tools

Remember! Safety is always number 1 priority!
Always be careful when using your tools.

Okay, moving on.
The tools you will need:


2mm Allen Key

Needle nose pliers

Laser cutter

Soldering Iron

Hot glue gun (optional)

Drill with 1/16th bit

Step 3: Step 3: Your Box Design

Let's take a moment to discuss the volume of your project.
Not every box needs to be the same, nor do you need to copy my box exactly.
I have uploaded the schematics to my box, but you can make your own in a matter of minutes.
Using MakerCase you can generate your 3D box cutting plans easily.

You can determine the general dimensions of your box, and include cuts that will need to be made on the faces of the box for the pieces you will mount to it in a few easy steps. It's very easy to use.

Once your box is planned, you'll need to do some arithmetic to calculate total area.
Generally the form goes: 2(L*W)+2(L*H)+2(W^H) = total wood area.

Then you know how much square footage of wood you'll need to buy.

Lastly, you'll probably be using a laser cutter with a bed smaller than your total cut area, so you'll need to break your image into several files and load each one, one at a time into your laser cutter interface.
You'll need to use an image editor that handles SVG files because that's what MakerCase dispenses.

I used Inkscape, but you can use whatever you like.

Just open the caseplans.svg file that MakerCase gives you with your editor of choice and take turns deleting and saving entire side panels at a time to make one image for each side of the box.

Last but not least, you may need to convert the file type, I had to for my laser cutter to recognize.
I used Microsoft's XPS image printer to convert the edited SVGs to XPSes.

Step 4: Step 4: Assembling Your Box

Assembly is very cut and dried when you use a laser cutter.
First assemble the bottom half of your box by wood gluing it together.

And there's a special step I need you to see.
Because of the way the servo is mounted, it sticks out away from the lip of the box by about an inch, so the latch that will interact with this needs to be offset by an equal amount as well.
To deal with this, cut a few squares of wood, about the width and height of your latch, and glue 3 of them together.
This also gives you room to screw your latch directly into the box.
Then glue the top.

Afterwards you can go about populating the box two ways.
You can glue each part into place, or you can drill holes and mount screws to hold everything.
I opted to glue every part but the servo motor and the coin acceptor.

Once you've glued and mounted each part, it's time to install the hinges.
Just screw them in. They're pretty simple. :)

Step 5: Step 5: the Control System

Okay, let's address the elephant in the room:
How the heck does this thing even work?
It's a little tricky, so read carefully.

Be sure to read my arduino code a few times.
When the machine turns on, it rotates the servo to lock the box.

Then it goes into password input mode.
The user inputs the password, the user can also press "*" to reset the password, or "#" to turn off the box prematurely.

When the password is entered, the box goes into range-sensor mode.

The user must hold their hand at exactly the correct distance above the sensor.
When this is complete, the box unlocks.

At any time the user may roll one quarter into the coin acceptor (or other coins, you get to set up the coin acceptor yourself so do it any way you like buddy) to override all security and unlock the box.

When the box unlocks it rotates the servo away from the latch.
Then, after a small delay, turns off the relay.

Sounds straightforward but it wasn't straightforward to code!

My loss your gain, included is a copy of my code, keep in mind I am not a computer engineer or a frequent programmer of any kind, but it does work.

Step 6: Step 6: the Heart of the Machine

The breakout board now needs to be soldered together.
I did a wild job putting mine together, and it may not be the most efficient way to design this, but it did work quite well.

Included is a number of diagrams detailing the wiring for the breakout board.
I used female wire connectors and soldered pin headers directly to my breakout board so that I could easily disconnect and reconnect parts during the construction phase.
You may design your breakout board any way you like.

Let's talk about what we're actually assembling here.

The momentary switch is connected to a relay, which powers the arduino, and the coin acceptor, the arduino powers a 5v relay which powers the arduino, so it keeps itself turned on when you release the switch.
The rest of the wiring is to connect the sensors to the arduino.
I included a tact switch and 2 leds on the breakout board, you do not need to do this. It was a leftover concept.

Step 7: Step 7: Lock Stuff Inside of It!

Now that you've assembled the breakout board, and hooked everything up, all thats left to do is stash stuff in your box! I hope you find many interesting and creative uses for your lockbox!