Planet Arduino

Watch any movie about the years of prohibition, and you’ll probably see character gain admittance to a speakeasy by using a secret knock on the door. In the old movies, a little sliding door would open so the doorman could check you out and let you in. With [‘s] electronic lock, the secret knock automatically unlocks the door. You can see a video of how it works, below.

The device uses a piezoelectric speaker to detect the knocking. A speaker is a transducer and like many transducers, it will work — to some extent — in either direction. A servo motor manages the deadbolt. An Arduino runs the whole thing.

The code is relatively simple. It holds an array of projected delays between knocks and compares what it hears to those delays. If you complete the sequence, the door unlocks. We probably would add some extra security if we were locking up the country’s gold supply, but for a light-duty lock, it should be fine.

The circuit is simple, too. There is an analog input tied to a resistor and the sensor. The Arduino is perfectly capable of driving a small servo directly. Add a battery and it is done.

It’s been a while since we’ve seen knock-operated locks around here, but they used to be all the rage. Here is a nice simple one, one that uses logic chips, and of course one built from 555s. If knocking isn’t your style, try replacing the piezo with a button or even a capacitive sensor. You can’t knock it!

Pressing the power button on your computer usually isn’t too much trouble, unless your computer is stored away somewhere hard to reach. [Joonas] has been hard at work on a solution that would also impress his friends, building a knock sensor to turn on his PC.

For around $10 in parts he put together an ATTiny45 that emulates a PS/2 device, which takes advantage of his computer’s ability to boot upon receiving PS/2 input. The build uses a Piezo buzzer and a 1M Ohm resistor as a knock sensor exactly as the official Arduino tutorial demonstrates, and one of those PS/2-to-USB adapters that are most likely lurking in the back corner of every drawer in your office.

[Joonas] used AVRweb to disable the 8X clock divider so there’d be enough clock cycles for PS/2 communication, then loaded some test code to make sure the vibrations were being detected correctly. You can check out his Github for the final code here, and stick around after the break for a quick video demo. Then check out a similar hack with [Mathieu's] home automation knock sensor.

https://www.youtube.com/watch?v=FTVppOZquE8

[Paul Mandel] just finished building this knock box project. It’s a familiar concept that uses a solenoid to tap on the side of the box. The Arduino driven setup monitors vibrations on the lid. When you knock on the box, it records the pattern and plays it back using the solenoid.

He was inspired by a knock-detecting door lock. Using that code as the starting point he implemented a system that takes input from a simple push button and echos back the rhythm using the Pin 13 LED on the Arduino board. This is a great way to start as it removes the complexity of driving a solenoid and monitoring a piezo element. After a bit of success he implemented each of those hardware modules one at a time. You can get a look at the final product in the clip after the break.

One of our favorite version of this project is still the knock block from several years back.