Spooky Whispering Prank




About: My name is Randy and I am a Community Manager in these here parts. In a previous life I had founded and run the Instructables Design Studio (RIP) @ Autodesk's Pier 9 Technology Center. I'm also the author ...

The Spooky Whispering Prank is a device which whispers scary things when the lights are out and it senses someone is present. This joke device tells whether or not someone is there based on the amount of noise being made. If the lights are out and there is a loud noise, it assumes someone is present and intermittently begins to whisper until the lights are turned on. This is a great gift for anyone who is at all scared of the dark, monsters, and/or ghosts. It particularly goes over well with children.

Note: Albeit this should go without saying, do not remove, replace, disable or take apart any existing fire alarms in your home or the home of anyone else. Get a brand new smoke detector from the store for this project.

Step 1: Materials

For this project you will need:

(x1) Smoke Alarm
(x1) Arduino Micro
(x1) Audio FX Board***
(x1) Mono Amp Board***
(x1) Electret Microphone Amplifier
(x1) Photocell
(x1) 2.2K resistor (or similar)
(x1) Adafruit 1/4-sized PCB "proto board"
(x1) Female header pins
(x1) Assorted shrink tube
(x1) Assorted stranded wire
(x1) Double-sided tape squares

***Since making this project, Adafruit seems to have temporarily discontinued the Audio FX board with the built-in amp. In the meantime you will need to use the non-amplified audio board with this additional external amp board.

To do this, wire the audio output of the audio board to the audio input of the amp. Next, connect the 5V power pin from the Arduino board to the V-in pin on the amp board, and wire both grounds together. Finally, attach the speaker to the amp board as appropriate.

Step 2: Open

Open up the smoke detector.

Step 3: Remove the Board

Remove the smoke detector circuit board from the smoke detector's enclosure.

Step 4: Attach the Headers

Slide the female header pins onto the Arduino micro's male header pins. If the headers are too long for the board, break a few off using pliers until they fit into the board.

Insert the pins from the female headers into the PCB and solder them in place.

Once complete, remove the Arduino from the headers.

Step 5: Attach Wires

Trim the leads of the photocell shorter and solder wires to them.

Insulate any exposed metal connections with shrink tube.

Step 6: Wire the Photocell

Notice on the Adafruit PCB that the rows are numbered and the columns are marked with letters.

Find the pin in 'row 14' and 'column i.' Wire a 1K resistor between there and ground.

Connect one of the wires from the photocell next to the resistor in 'row 14.'

Connect the other wire to the power rail.

Step 7: Connect the Microphone

Solder a wire between the microphone board ground connection and ground on the PCB.

Next, do the same for the power connection.

Finally, wire the output on the microphone board to row 13 on the PCB (the row right next to the photocell).

Step 8: Power and Ground

Solder a red wire between the power rail and row 6 (on the same side of the board as the sensors).

Solder a black wire between ground and row 4 (also on the same side of the board.

Step 9: Upload

Upload audio to the Audio FX Board as you would if you were copying audio files to a hard drive.

The audio should either be in formatted to .WAV or .OGG file types. More audio can be fit on the board with .OGG, but .WAV sounds better.

You can add as many clips as will fit on the board.

I recorded my audio files on my computer using Audacity (free audio editing software) and the computer's built in microphone. You can use whatever works best for you.

When creating your audio files, it is best to name them sequentially (i.e. 00SCARE.WAV, 01SCARE.WAV, 02SCARE.WAV, etc), but ultimately you must live your trust.

I numbered my files ##SCARE.WAV. If you break from my naming convention, remember that you need to update the code to match your file names.

Step 10: Attach Wires

Solder 5" wires to the following pins on the Audio FX Board:


Step 11: Connect the Boards

Now is time to connect the audio board to the PCB.

To begin, wire the reset pin to row 15 on the opposite side of the board from the sensor.

In row 14, next to the reset pin wire, connect the audio board's TX pin.

In row 13, just next to that, connect the RX pin.

Finally, wire the audio board's V-in pin to the PCB's power rail, and connect ground on each board together.

If you are uncertain, you can double check all of your connections against the above wiring diagram.

Step 12: Attach the Speaker

Attach the speaker to the amplified audio output terminals of the audio fx board.

Step 13: Program the Arduino

Program the Arduino with the following code:

Step 14: Plug in the Arduino

Insert the Arduino into the breadboard sockets such that pin A0 on the Arduino lines up with the row on the board connect to the photocell.

Some pins from the Arduino will not be inserted into a header socket. That is okay.

Step 15: Trim the Battery Terminals

Trim the battery terminals inside of the smoke detector enclosure so that they are shorter.

Step 16: Solder Wires

Solder a red wire onto the lead terminal which connects the positive contact on the battery.

Solder a black wire to the other lead terminal.

Step 17: Trim

Use diagonal cutting pliers to trim away any unnecessary support tabs inside of the enclosure. The goal is to make the enclosure as open as possible to fit the circuit boards inside.

Step 18: Drill Holes

Drill a 1/4" hole in the top of the smoke detector enclosure to be used with the photocell to sense light.

Drill a 3/8" hole in the side of the enclosure to insert the electret microphone through to detect sound.

Step 19: Attach Power

Solder the 9V power wire to the row which line up with the Arduino's V-in socket.

Solder the ground wire from the battery to ground on the circuit board.

Fully attach the battery to the terminals if you have not already done so.

Step 20: Mount the Circuits

Normally I am not a fan of hot glue, but this is a case in which I think it's the best tool for the job. It gets the job done quickly and efficiently. This device does not need to last a million years, but just long enough for your victim to discover it and smash it over your head.


Hot glue the microphone board in place.

Next, hot glue the photocell flush with the 1/4" hole that was drilled to let light in.

Finally, hot glue the speaker to the grill already present in the smoke detector.

Don't worry too much about the PCB or audio fx board. They can 'float' on the inside of the enclosure, and will just sort of shuffle themselves into place when you close it up.

Step 21: Close It Up

When all the glue has set, shut the enclosure.

Step 22: Stage the Prank

Mount the double sided tape to the bottom of the enclosure and stick it high up on the wall or on the ceiling.

The prank is now in effect.

Microcontroller Contest 2017

Participated in the
Microcontroller Contest 2017



    • 1 Hour Challenge

      1 Hour Challenge
    • Colors of the Rainbow Contest

      Colors of the Rainbow Contest
    • Sensors Contest

      Sensors Contest

    9 Discussions


    2 years ago

    This is really great, but I am still looking for a cheapter alternative for the Audio FX Board.

    And another thing, what is the microphone for, if it reacts on light and you upload your whispers?

    1 reply

    Reply 2 years ago

    The microphone listens for noise and then it assumes someone is there if it hears noise with the lights off

    Anirudh Ralhan

    2 years ago

    That's so frigging Awesome!!! And well explained. Loved it :-)


    2 years ago

    Use a cheapo photoelectric sensor for the case, that way you wont have to deal with the Radioactive chamber, that would need special disposal


    2 years ago

    see, this is what this place needs, more FUN, projects. This idea is BRILLIANT!.

    For those of us that are not Adafruit literate, there are plenty of solid state audio recorders out there that have speakers, some with built-in PIR sensors.


    2 years ago

    Your pictures are amazing!


    2 years ago

    Evil. I love it.

    Out of interest, where do you get the preset whispers from?


    2 years ago

    Just as an aside, a lot of smoke detectors use a radioactive element in the detection circuit and should be disposed of carefully.