Secret Knock Detector

Difficulty

Summary

We saw this project on Instructables, and had to try it for ourselves. Of course, we ended up using all the Duinotech parts we had around. It uses a piezo transducer as a microphone to detect knocking, and then activates a relay if the correct knock sequence is detected. The original project was designed to unlock a door, but our version has a relay output which you could use for just about anything.

Materials Required

1	Arduino Compatible Nano Board	XC4414
1	Arduino Compatible Mini Breadboard with 170 Tie Points	PB8817
1	150mm Plug to Socket Jumper Leads - 40 Pieces	WC6028
1	Arduino Compatible 5V Relay	XC4419
1	4.2kHz Audio Transducer	AB3440
1	2.2M Ohm 0.25 Watt Carbon Film Resistors - Pack of 8	RR1656
1	Arduino Compatible RGB LED Module	XC4428

Table of Contents

Connections

Connections

Most of the connections can be seen on the photo above, but note that D2 is connected to ground with a small loop of wire (we shortened one of the resistor legs and used an offcut). This keeps the Secret Knock Detector from entering programming mode. The resistor itself is connected between A0 and ground like the piezo, and can be just seen on the left of the Nano board between A0 and GND.

Duinotech	Relay Board	RGB LED	Piezo	Function
5V	5V			Power
GND	GND	-	Black	Ground
A0			Red	Signal from transducer
D4		R		Red LED
D5		G		Green LED
		B		Blue LED - not connected
D3	S			Input for relay board
D2				Connect to ground

Compile and upload the code to the board, making sure the board type is set to Nano. Once the green LED comes on, knock on the table next to the sensor. It should blink off to indicate it is detecting the knock. It’ll probably then go off for a second- this means it didn’t detect a correct code. The default code is ‘Shave and a Haircut’.

You can also open the Serial Monitor in the Arduino IDE and monitor it’s output to see what the detector is detecting.

Future Improvements

The Instructables page has some good suggestions on how the software can be changed, including printing out the code and changing the default code.

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