Picture of Secret Knock Detecting Door Lock
Protect your secret hideout from intruders with a lock that will only open when it hears the secret knock.

This started out as a bit of a joke project, but turned out to be surprisingly accurate at judging knocks. If the precision is turned all the way up it can even detect people apart, even if they give the same knock! (Though this does trigger a lot of false negatives, which is no fun if you're in a hurry.)

It's also programmable. Press the programming button and knock a new knock and it will now only open with your new knock. By default the knock is "Shave and a Haircut" but you can program it with anything, up to 20 knocks long. Use your favorite song, Morse code, whatever.

Maybe a video will explain it better:

Important Notes:
(I hate to even have to say this, but since someone's going to say it, I'll say it first:)
1) This is for entertainment purposes only. Really. This decreases the security of your door by adding another way to unlock it, and it makes your unlock code known to anyone who can hear. If you put this on your door, be sure to carry your key too. The batteries might die, the suction cups might fail or you might forget your knock. Don't complain to me if someone imitates your knock and steals all your stuff, you've been warned.

For obvious improvements to safety, security and whatever, see the final page of the Instructable.

2) This is not a project for a beginner! Read through it carefully and be sure you understand it before you start! I will not take time to answer questions that are already in the instructions or from people who have gotten in over their head.

(If you think this project is too complex you might go here and sign up for the kit mailing list. The kits will be much more simple than this.)

Sorry about that. Now that that's out of the way, lets get to work.
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KGuy10 months ago

Here we are! Finally done after months of procrastination! I added a buzzer and a third led (not in the picture). The code was changed very slightly but it works perfectly. A latch in the back hangs it on the door. I must thank you, however for the genius of this invention.


hi sir, how did you do this one. where do i put the new code for the led and the piezo.. please, needed for a presentation, our prof. did not give us enough time to master this. and it serves as our final exam.. thank you in advance..

Grathio (author)  KGuy10 months ago

Beautiful work! Thanks for sharing!

Matienzo10 months ago

here is my little one in an iphone box

vinny033 years ago
I have modified the Arduino code to add the functionality of ''Users''. What it does is that it let you program a secretCode for four users. For example: If you push the button X times, after 4 seconds the green LED will blink X times and then you enter program mode (simply do your secret code). I also added the functionality to take a picture when your sequence is bad with LabVIEW and a WebCam camera. Then the picture is saved in my folder and pictures are incrementing with numbers.
Circuit sur porte.JPGMontage.JPG
Grathio (author)  vinny033 years ago
Looks great, and really nice added features! I had done a version with his & hers knocks, but this goes well beyond that!

Fantastic job!
bserrato3 years ago
I saw your project some time back and I thought it was great. I decided to take it and improve on it, the result was a system that locks and unlocks. Also if a person inputs an incorrect knock, the system will send a signal via bluetooth to a computer terminal to snap a picture and save it.

Check out the vid. Hope you like it.
Grathio (author)  bserrato3 years ago
That really made my week! It looks fantastic and works great. And the bluetooth camera is a great idea of added security.
josiasfilho5 years ago
Hi Steve!
As you know, I already started to do mine, with a few modifications. I uploaded a new video with a new option:

As soon as I finish my project I'll post the code online
Did you finish yet?  Can't wait to see the code.
Grathio (author)  josiasfilho5 years ago
Fantastic work with the servo and the unlock option.  (And an off switch!)

It looks like your setup is really sensitive.  Really nicely done! Your achievement patch is on the way!
TheBestJohn4 years ago
I have been playing around with this code for the past hour or so and found out how you can hard code in your own rhythm if need be.... at the part that looks like this
// If we're recording a new knock, save the info and get out of here.
  if (digitalRead(programSwitch)==HIGH){
      for (i=0;i<maximumKnocks;i++){ // normalize the times
        secretCode[i]= map(knockReadings[i],0, maxKnockInterval, 0, 100);

add this little guy
and open up your serial monitor....
make sure you un-commented the serial begin up at the setup and then it should spit some numbers back at you... put these numbers into the array and enjoy.
Whoot finally got my relays for my motor and heres a neat little Idea for your visual feedback....

Grathio (author)  TheBestJohn4 years ago
Oh, and for your awesome work you have been added as an official Master of Secret Knocks.  Your patch is on its way!
Grathio (author)  TheBestJohn4 years ago
 This is awesome!  Thanks for sharing your code and video.  Having the door unlatch too makes it that much more mysterious.  And I like the feedback through the peephole.

Great work!

And if I ever have any questions about taping something to something else I'll be asking you. :)
Krizbleen made it!yesterday

Guys - thanks for posting this tutorial! I used the code as part of my secret sliding library door project and it works great. People giggle when I demo it to them. I owe you a beer for this!

Krizbleen made it!yesterday

Guys - thank you for this tutorial! I used the code to program a secret sliding library door project and it works great. The secret knock is the part people like the best. People giggle when I show it to them. I owe you a beer!

VisionaryNutcases made it!12 days ago

My version is installed in a fridge and using an electromagnetic lock - keep those office thieves away!

shreyask1 made it!12 days ago

AWESOME!!!!!project......It works properly.......Can someone say me how can i put a servo instead of a geared motor........PLZZZZ.


Hey guys, how can I program a DEFAULT secret code? I means how can I determinate de values (in the array), per knock? for example if I want something like "HBD to YOU" or "We r the champions my friend", many thanks for your help.

Grathio (author)  El_Minion_Dice27 days ago
Great question! The default timing information is stored in a variable near the top. It looks like this:
 int secretCode[maximumKnocks] = {50, 25, 25, 50, …
The easiest way to figure out what the new numbers should be for your rhythm is to have the lock its self tell you. To do that find the line near the bottom of the sketch that reads:
// only turn it on if there's a delay
Right below that, add this, on its own line:
Serial.println(secretCode[i], DEC); 
This will print out the secret code values when you program a new knock.

Upload the modified sketch to your Arduino, and keep it plugged into your computer.  Open up the Serial Monitor in the Arduino IDE. (It's the little magnifying glass button on the upper right. In older versions it looks like a radio.)  On the little window that pops up, be sure to set the baud rate to 9600. (The little pulldown at the bottom right should read "9600 baud"). If everything's working correctly you should see the text "Program start." in the serial window, and it will print some other status messages as you use the knock.

Now program a new knock like normal. When you're done you'll see some text and numbers in the serial output window. There will be twenty numbers.

Go back to the sketch and find the line near the top of the sketch that starts:
int secretCode[maximumKnocks] = {
And replace the numbers with your new numbers, starting from the top of the list. Make sure there are 20 numbers, and there are commas between them all, or the sketch will break. When you're done it might look like this (but with your own numbers, of course):
int secretCode[maximumKnocks] = {72, 54, 66, 60, 100, 45, 22, 80, 25, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
After making the change, upload the modified sketch, and you're done! The new default will have been changed.
george.kaileh made it!27 days ago


I've made your project, but I have changed 2 things in it:

1- Instead of piezo speaker I have kept resistor alone so nobody can hear the knock (its now working by touch, all you have to do is touch the resistor as the secret knock and the door will open)

2- I have put the battery from outside the door as a key to power everything, by that way you won't worry about the battery dying or electricity outage (if you're using a power adapter).

justnik671 month ago
The author has stated he will not answer questions for beginners. So did you make this for people that already have the skills to do it on their own? If you are not here to teach, then you are just showing-off! I am a total beginner but managed to figure out a way. Instead of using a motor, I used a 12v solenoid lock from adafruit. It automatically locks and there is no need to make metal brackets or a PVC contraption. Use a seeed relay shield. Move motor control digital pin 3 to one of the relays on digital pin 4,5,6, or 7. Now you can control anything up to 120vac. You will also have three additional unused relays available. Move led 4 and 5, to digital pins 8 and 9. I also took apart a Gfi 120vac outlet I had installed just outside the door, and wired the reprogram button to the test switch. If you want it fail safe, rewire the reset button in the gfi to a battery. 120vac power can also be used from the outlet, to the relays if you need it. Anyway, that's how this beginner did it.

This is a great project. Can it be extended to make a multi-appliance clap control system ? like certain clap opens curtains, certain clap closes curtain, another clap opens cupboard drawer, and other clap closes it..and so on in one Arduino sketch ?

thank you sir it worked.. :)

it worked sir.. thank you very much..:)

ArvindS22 months ago

Hey Grathio

We took this idea for school project. We made the circuit connections correctly. After that we connected it to the Ardiuno ( UNO instead of a duemilanove). But the motor is not running. Can u tell us the probable errors? ( Will UNO cause problem since we didnt use Duemilanove?)


mr led2 months ago
that`s so interesting.
reyscreamz3 months ago

what type of door lock should i use?

please specify :D

thank you so much

reyscreamz3 months ago

what type of door lock should i use?

please specify :D

thank you so much

reyscreamz3 months ago

what type of door lock should i use?

please specify :D

thank you so much

reyscreamz3 months ago

what type of door lock should i use?

please specify :D

thank you so much

reyscreamz3 months ago

what type of door lock should i use?

please specify :D

thank you so much

igorandrade3 months ago
ndiaz94 months ago

im just wondering. why do you need to put the knock detector at the back of the door and not in the front door? thanks for your reply, if ever :)

longandy made it!4 months ago

Here's my build - thanks for an awesome instructable!

changes - replaced motor with relay to trigger an external motorized door opener

inverted some of the green LED states so it is normally off but flashes on when knocking

sarci6 months ago

we have a filipino who made this project? just txt me this is my # 09169033782 and help me how to do this project! please! tnx...

ell092792 sarci4 months ago

docu please :)

Roberto11t12t5 months ago

What size does the SPST momentary pushbutton have to be?

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