Creative Technologies > Block 3 > Projects > Knock Knock Box

Knock Knock Box

Knock on this coffin to awake the dead. Do not worry, the skeleton will not get out and come after you but it will reply from inside.
Ok, we know that you know that there is not a real skeleton there. It is really a piezo used as a knock sensor. After you have made this project you might think of a handy way to use this sensor in other ways. Check out this secret knock detecting lock for some inspiration.

Materials


  • 1 Arduino Uno board
  • 1 Education Shield
  • 1 breadboard
  • 2 piezo speakers
  • 1 1M ohm resistor
  • 3 black jumper wires
  • 3 colored jumper wires
  • 1 9V battery
  • 1 power plug
  • Knock Knock box kit
  • Masking tape

Instructions

  1. Attach the shield onto the top of the Arduino board.
  2. Connect a piezo speaker across the breadboard gap and connect one leg to digital pin 8 and the other to GND.
  3. Using a separate breadboard, connect a 1M ohm resistor between analog pin A0 and GND.
  4. Make a piezo knock sensor by connecting the second piezo speaker to the analog pin A0 and GND.
  5. Build the Knock Knock Box from the kit provided.
  6. Connect the Arduino assembly to the computer and upload example ‘KnockKnockBox’.
  7. Disconnect the Arduino assembly and connect a 9V battery with a power plug to the Arduino’s power socket.
  8. Place the assembly in the Knock Knock Box. Make sure the piezo serving as sensor is in contact with the lid of the box before you close the lid.

Code

Find the code in File>Examples>EducationShield>Block3-Magic>Projects>KnockKnockBox



How it works


    

  • The EducationShield library is included.
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  • The constant MAX_KNOCKS is defined to hold the value 30.
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  • The PiezoKnockSensor object is declared, sensor.
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  • The variable speaker is declared.
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  • The variables that will be used to keep track on knocks are declared, timer[], started, timeoutBase, timeout, currentKnock.
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  • In setup(), the knock sensor is configured.
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  • The knock variables are assigned values.
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  • The program jumps to the function clearArray().
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  • In clearArray(), a for loop loops through the array timer[] and 0 is assigned to each element.
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  • The program jumps back to setup().
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  • In loop(), if a knock is detected the following happens.
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  • If started is false, it means that the first knock in a round was detected. started is assigned true.
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  • The variable currentTime is declared and assigned the value of millis(). millis() returns the amount of milliseconds that has passed since the start of the program.
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  • timeoutBase is the variable that will be used to check how long time has passed since the last knock. Therefore, every time a knock is detected timeoutBase is reset by assigning it the value of currentTime.
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  • The value of currentTime is also assigned to timer[] of the index of currentKnock. The first time a knock is detected currentKnock is equal to 0, therefore the current time is stored in the first element of timer[].
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  • currentKnock is increased with 1.
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  • If no knock was detected, the if statement is skipped.
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  • If started is true an additional if statement is used to check if too long of a time has passed since the last knock.
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  • If too long time has passed the program jumps to the function playBack().
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  • In playBack(), a for loop loops through the elements of timer[] as long as an element is not equal to 0.
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  • For each loop, a tone is played and the program is paused for as many milliseconds as is was detected between the knocks.
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  • The delay time is calculated by timer[i+1]-timer[i]. This means the time between the start of the program and the next knock, minus the time between the start of the program and the current knock.
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  • When the for loop is finished, the program jumps back to loop().
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  • The parameters are reset by setting started to false, running clearArray() and setting currentKnock to 0.
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  • If not too long time has passed, nothing happens and loop() keeps looping, checking for knocks detected.
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Troubleshooting


    

  • Refer to the illustration and double check your connections. Make sure the shield and jumper wires are firmly connected.
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  • Make sure the battery is connected properly so the ON LED on Arduino is on. Test with Arduino connected to the computer and 9V battery disconnected. If it works with the USB cable but not with the battery, there could be something wrong with the battery, change for another one.
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  • Remember to knock on the right wall of the box (the one with the piezo knock sensor).
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  • See piezo knock sensor reference for debugging.
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Experiment further


    

  • Change the sound of how the box knocks back.
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  • Add LEDs to the skeleton’s eyes that blink, either when you knock on the box or when the piezo is playing.
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