Secret Knock Unlocks Door

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 [IsmailSan’s] electronic lock, the secret knock automatically unlocks the door. You can see a video of how it works, below.

(Ed Note: Grrr…GitHub repo got pulled between writing and publication. Go check out the in-links in the bottom paragraph if you’re interested in knock-detectors.)

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!

12 thoughts on “Secret Knock Unlocks Door

      1. Would be the heavy duty metal doors which only lock and unlock from the outside?

        They usually come with an eye-level viewing hatch which also only locks and unlocks from the outside…

    1. Actually, I remember sharing this with him and he said “If I ever need advice for taping something to something else, I’ll give you a call.” lol. It was college and I didn’t have a lot of tools of the trade

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