Tag: Mechanics

Gentle Safe Cracker

The problem:

Misterious Safe Box
Mysterious Safe Box

I found a little safe box in the garbage some time ago and I would really like to open it. The box features some dents and scratches that show some people tried to open it in the past but did not succeed. Also, when shaken, it produces some interesting noises that indicate it is not empty and may contain a valuable treasure some cool junk.

The Plan:

Of course, there are many ways of achieving this (e.g. cutting holes into its walls, removing the hinges, hiring a locksmith) but I do not want to break the safe, nor do I want to hire anyone to do it in my place. Inspired by so many movies, the first thing I tried was to listen to the (nonexistent) clicking of the dial as it turns: it did not work at all.

Very quickly, I decided that the appropriate way of doing it (given my set of skills) was to have a machine do it in my place. So, I decided to build a little manipulator that will test all possible combinations of the safe until it opens up.

Since this requires precise positioning, I thought a servo motor would be the best choice of actuator (and also because I have some other project ideas involving servos).

The Materials:

  • Pololu Micro Serial Servo Controller
  • Hitec HS-425BB Servo Motor
  • Male and female headers
  • Power adaptor (4 to 6 VDC)
  • Wire
  • Heat shrink tubing
  • USB to Serial cable
  • 2 gears and a spindle (one four times larger than the other, I got mine from an old photocopier)
  • A project box (in my case, an old computer power supply case)
  • A bottle cap and an old heat sink
  • Steel wire (e.g. form an old coat hanger)
  • Magnets (the stronger the better)
  • Various screws

Essential Tools

  • Screwdrivers
  • Rotary tool (Dremel)
  • File

The How and the Why:

The Electronics
Electronic Parts

I chose the Pololu servo controller since it is easy to interface by either using a computer or a simple microcontroller. Also, I already own a USB-to-serial cable (that I normally use to program the Arduino) that I can use to send commands to the controller.

The choice of the servo motor was based in getting the maximum torque at a reasonable price.

In order to control the servo motor, I devised a simple python module as seen in my previous post.

Since, usually, servo motors have a motion range slightly greater than 180 deg, I decided to use gears to be able to produce a motion range large enough to operate the safe (at least two full turns).

Mechanical Parts
Mechanical Parts

I used an old heat sink and cut it with the Dremel in order to produce a bracket for the servo and a mounting hole for the secondary (smallest) gear axle.

I attached the larger gear to the servo directly using the brackets and screws that came bundled with the motor. Also, in order to hold the shaft in place, I used e-style retaining rings and spacers (other dummy gears and shoulder washers) in order to match the grooves already present in the shaft. Of course, I got all these handy mechanical parts from an old photocopier I found in the garbage.

Dial coupling attached to the small gear
Dial coupling attached to the small gear

In order to couple the small gear to the safe dial, I used a bottle cap which fitted perfectly over it. The cap has some child proofing which provided a firm grip for the dial.

The Project Box
The Project Box

Once the assembly was done, I mounted it into an old power supply box. Although the box required some drilling and cutting, it was very easy to adapt. I would recommend using this type of boxes for other projects since they are sturdy and easy to machine.

Electronic Assembly
Electronic Assembly

So to include the electronic parts into the box, I used a small plastic capsule (that must come from the time when my young brother liked those little toys they sell in a dispensing machine). In the capsule, I placed the Pololu servo controller, the USB to serial cable and the servo motor cable, and the power supply cable.

I needed to adapt the USB to serial cable in order to match the pin-out on the controller. For this, I used a male and a female header, and some wire. I connected the GND and the V+ pins to their respective counterparts, and the TX pin to the Serial-in pin.

In order to supply the appropriate power, I used an old cellphone charger and I replaced its original connector with a female header. As always, it is good practise to isolate the electrical connections with some heat-shrink tubing.

The (disappointing) outcome

Very gentle safe cracker
Very gentle safe cracker

Once everything was done, I fixed the new little machine to the safe and got ready to get it trying codes. I used a coat hanger and some rare earth magnets in order to hold the cracker firmly in place.

To my great disappointment, I realized that the gears I used provoked a (much expected) torque reduction. This meant that the system is not strong enough for turning the dial to a set position reliably. No matter how much lubricant I used, the dial was too stiff for the little robot.

This represents a (temporary) victory for the safe, but the war is far from being over.


I decided to publish these results in spite of my failure since too often we read reports of success (notably in science) and often forget that we can learn from failures as well. Never do we read about scientits proving their original hypothesis wrong, but very often, documenting those mistakes could prevent others from doing the same.

Finally, for those wondering how I was planning to pull on the safe lids so it opens, it is remarkably simple: since the safe does not have any handle to latch it closed, I merely need to hang it from its handle and try the codes until it opens and the bottom part goes down.

The safe hanging unaware of its fate
The safe hanging unaware of its fate

Finally (this time for real), I cannot say too much about my next plans on attacking the safe, but be sure that they involve a stepper motor.



I would like to thank the great people at RobotShop for providing the Pololu Micro Serial Servo Controller and the Hitec HS-425BB Servo Motor. As I mentioned before, they will be sponsoring a set of projects here at Carlitos’ Contraptions in the foreseeable future.

They were also kind enough to quickly ship a replacement gear set when I ruined the original one on the servo motor by hand forcing it to turn (I know, I sound brilliant).

Smoking Cyclops

For a long time now, sales I wanted to have a soldering fumes extractor. Until now I have been using a small fan but I wanted something more appropriate, illness something that would filter the fumes instead of just moving them away.

Some days ago, I decided I had to build a fumes extractor. I had a broken humidifier that I found in the garbage so I took it apart and used its pieces to build the extractor.


  • An old humidifier
  • Some assorted crimp connectors
  • Some zip ties
  • A rubber band
  • A two positions switch


  • A set of files
  • A hacksaw or rotary tool
  • A drill
  • A diagonal cutter

Getting it done

The first step when building something from a broken appliance is obviously to take it apart and go trough the parts in order to find how you could put the together in order to achieve your goal. In my case, I required the suction fan, the air filter (which is actually a sort of sponge used to provide as much surface area as possible for the water to easily evaporate), the base, and the power wire. I also ended up cutting a piece of the plastic casing that has a grid on it so I can attach the filter and put it in front of the air exhaust.

Then, it is a matter of cutting, drilling, filing, sanding and fitting together until you have the desired shape and function. I really like to use zip ties for binding things together since they are cheap, strong and can be undone (sort of) easily.

As for the electrical part, the can motor had three coils that, when energized, drive the motor shaft at three different speeds. Since I do not have the need for that much speed control, I used a three positions switching order to control it. I connected the main switch lead to the mains power and the other two leads to the first (slowest) and third (fastest) coils. Then I connected the ground and the mains power to the motor as required and voila. When the switch is in one position, the fan turn at the slowest speed (since the first coil i energized) and when the switch goes the other way, the fan spins at full speed (since the third coil gets energized). Finally, the the switch is at the middle position, all the coils are disconnected and the fan sits still.

Also, since I do not have a crimper, I used a diagonal cutter to crimp all the conenctors (that go to the switch). Using a diagonal cutter for this purpose can be tricky since, if you apply too much presure, you may end up cutting the conenctor, or if you apply too little presure, then the cable may slip away from the connector. As it can be seen in the picture to the right, crimping with a diagonal cutter leaves a very distinctive mark on the connectors.

Lunar Excavator

I was lucky enough to help my friend Stephen and his team to build a lunar excavator to participate in the Regolith Excavation Challenge, hospital sponsored by NASA.

We put lots of efforts and many hours to get the robot done in time and we managed to get it running before it had to be shipped to California (from McGill University in Montreal).

Unfortunately, treatment despite the awesomeness of the lunar excavator and the fact that it was going to completely own the challenge, the UPS shipment went wrong and the robot could net get to the competition on time. Now the fight with UPS has begun to get a full reimbursement (~2000$) and the robot back.

UPS incompetence aside, I worked in putting all the electronics system together in the electrical box. This meant, I had to build two boards: one for the power management (transforming the provided 24V into a 12 and 5V in order to power the many devices and turning the latter ON and OFF), and one for the logic (interfacing the main computer with the various motor controllers and sensors).

This task was done using perfboards and lots of solder since we did not have enough time to consider designing and fabricating proper PCBs with nice places for all the components.

Note the nice (and very classy) wood finish of the electrical box interior as shown in the picture.

I will not give away any details about the excavator since it will compete next year, provided there is another Regolith Challenge.