Hello everyone! Welcome to my blog. Today, I’m excited to share how I built a robot that successfully hides food from your siblings. This idea came to me one day after school when I discovered all of my snacks had been stolen by my elder sister. Determined to prevent this from happening again, I came up with a solution: a secret safe that only I could open with a secret code.

The safe needed to:

1. Look like an everyday item to avoid suspicion.
2. Be large enough to store 3-4 packets of chips.
3. Open discreetly.

After many failed attempts, I finally landed on this concept: a wooden bookshelf with a hollow box behind it. A small sensor that could detect vibrations would be placed discreetly inside the hidden space. This sensor would listen for a specific knock frequency. When the correct knock pattern is detected, the sensor would output a signal that triggers an unlocking mechanism, opening the bookshelf and revealing the hidden space behind it.

Here is the list of materials required:

• 1 Arduino Duemilanove (or compatible, or any microcontroller with at least 1 analog input and 3 digital outputs)
• 1 servo motor
• 1 piezo speaker
• 1 SPST momentary pushbutton (normally “off”)
• 1 red LED
• 1 green LED
• 1 NPN transistor (P2N2222A)
• 1 rectifier diode (1N4001 or similar)
• 1 2.2k ohm resistor (1/4 watt)
• 1 10k ohm resistor (1/4 watt)
• 1 1M ohm resistor (1/4 watt)
• 2 560 ohm resistors (or whatever will run your red and green LEDs at 5v)
• 1 small piece of perf board (5×15 holes or longer)
• 1 9-volt battery clip and 9v battery (or any other way to get 7-12v to the Arduino; a wall adapter is a great option so you don’t have to worry about batteries running out; 6 AAs would be another option for longer-lasting power)
• Connector wire (20 gauge or narrower flexible wire in various colors for connecting the electronics together)
• 2 wood planks (34cm x 24cm)
• 2 wood planks (34cm x 20cm)
• 2 wood planks (24cm x 20cm)
• 2 hinges

I will leave an Instructables link that further details how to make the build step-by-step. You can follow that tutorial as it is quite similar: Secret Knock Detecting Door Lock.

This is how the final build looked and functioned:

This is the code I used to make the circuit work, you can see how to build the actual circuit using the instructables link above.

Overall, the build was pretty straightforward, except for some issues with the code. You need to configure the code depending on your specific build. I also made the bookshelf that houses the circuit and the secret space behind it using 2-inch thick plywood. I used a jigsaw to cut out the exact dimensions and then fastened them together using L-shaped brackets. This was a really fun build to make and, in the end, it served the purpose of keeping my snacks safe from my siblings. I would highly recommend this build to anyone facing the same problem!