Adafruit’s Secret Knock Activated Drawer Lock is designed to conceal traditional lock mechanisms – unlocking only when a secret pattern of knocks is detected. Essentially, a solenoid locks secures the drawer while a piezo buzzer “listens” for knocks.
Meanwhile, an Atmel-powered Trinket (ATtiny85) compares the knock pattern to the stored secret knock. If they match, the solenoid latch neatly retracts and the drawer can be opened. As Adafruit’s Steve Hoefer notes, setting your own custom knock is as simple as holding down a button and tapping the new rhythm. On the software side, the project uses straight-forward Arduino code and the standard Arduino EEPROM library.
In terms of wiring, Hoefer recommends (initially) building the circuit on a solderless breadboard to make sure everything works before soldering it into place.
“Solder the piezo buzzer to the back of the PCB so it can be installed flush against the desk/drawer/etc and hear knocks more clearly. [The] length of wire between the lock and the knock sensor depends on where you plan to mount it,” Hoefer writes in a recently published Adafruit tutorial.
“Measure your distances and be sure to allow some extra for movement of drawers and doors. You should be able to put the lock several feet from the detector without any problems. If you’re installing them close to each other then simply cut the connector off the solenoid lock.”
According to Hoefer, the circuit uses the Trinket’s pin #3, so in some cases, Makers may need to disconnect the 2.2KΩ resistor from pin #3 to program the circuit.
“Note that my sample programs just fine with pin 3 connected, but your mileage may vary. If you want to be extra sure of being able to program it after it’s soldered in place consider using female headers to mount the Trinket, or put a jumper between pin #3 and the 2.2KΩ resistor,” Hoefer added.
Interested in learning more about building a knock drawer lock with Adafruit and Atmel? You can check out Steve Hoefer’s detailed tutorial here.