Introduction: Monster Catcher Night Light

About: I'm the Maker in Residence at the University of Washington where I prototype electronics and fabrication projects.

This nightlight doubles as a trap to catch monsters under the bed! Before you go to sleep at night, place it under your bed and it will catch any monster under there. It even keeps a light on all night to make sure new monsters stay away.

This Instructable makes a great project for parents of a kid who’s afraid of monsters under the bed. The trap can help them sleep easy knowing that any monsters are locked away in the trap and can’t get them.

The enclosure body of the Monster Catcher is made with laser cut acrylic (though this could be 3D printed if needed) and a few 3D printed parts. The electronics are built using an Arduino.


  • 3D printer - dual extruder
  • Filament - PLA and PVA
  • Sheet acrylic - 12” x 24 “ x ⅛”
  • Adhesive vinyl etching - can be replaced with sanding, sandblaster, or spray paint, or some other diffuser
  • Sharpie
  • Laser cutter - with a bed large enough for 12x24 piece of acrylic
  • Glue gun
  • 2 L brackets & screws
  • Arduino Uno, Leonardo, or equivalent
  • Jumper wires
  • 2 hobby servos
  • Assorted resistors
  • Trimmer potentiometer, aka trimpot
  • 9V battery pack with on/off switch
  • 9V battery
  • 2 small breadboards

(Note the breadboard and jumper wires can be replaced by perfboards and stranded wire, if soldering the circuit together)

Step 1: Write the Code

Clone or download the Monster Catcher code from the colleeninboots Github repo. Upload it to your Arduino Uno or Leonardo.

The firmware is designed to open the trap, i.e. the teeth, by moving the servos when the board is turned on, showing that it is ready to catch a monster. As the user turns the trimmer potentiometer (or trimpot), it changes the color of the light. Once a color is chosen, the trap snaps shut after 4 seconds, capturing any nearby monster. If the knob is turned all the way to 0, then the LED shuts off and the trap opens back up.

The program is coded for a Common Anode RGB LED. If you have a Common Cathode RGB LED, just remove the "255 -" from each of the digitalWrite lines in the setColorRGB method. If you aren’t sure, then check the data sheet for the LED that you have.

Step 2: Connect the Circuit

The next step is to get the circuit working. Connect the components as shown in the schematic above. The servos will open and shut the mouth of the trap - one servo controls the top teeth and one the bottom. The RGB LED will be the light source of the night light, and the trimpot controls the color. The 9V battery powers the whole thing, while the Arduino controls all the components.

I find it helpful to build one functional element at a time and then test. For example, connect only the RGB LED and then turn the Arduino on to make sure it is connected correctly.

I even added some buttons and modified the code slightly to test the function of the servos (one to move the servos to the “open” position and one to move it to “shut”).

The video above shows how you can use a piece of notebook paper to keep track of the components and their function while testing.

Step 3: Laser Cut the Enclosure

The monster catcher electronics are enclosed in a box with a cutout in the top for the “teeth” of the trap. All the sides of the box can be cut on a single sheet of 12 x 24” acrylic using the attached adobe illustrator file, including an insert for the inside of the box to hold the servos in place.

The file includes a few test pieces for gauging the power setting of the machine and the tolerances of the finger joints. Settings vary a bit by machine, but I was able to get good cuts using the manufacturer settings for 1/8” thick acrylic on the Epilog Universal Laser.

I recommend cutting just the rectangles in the top first, one by one, to test power settings on your machine. Then cut just the finger joint gauges to make sure you’re happy with the fit. Finally, cut the rest of the pieces.

Lastly for this step, we'll add a light diffuser. I used adhesive etching vinyl. This is a roll of stick-on plastic that has the look of etched or frosted glass. I found it to be an easy way to get an evenly frosted look and diffuse the light inside the box. Just cut the vinyl in the same shapes as your pieces and stick it on. Two layers gave me a level of diffusion I was happy with.

Step 4: 3D Print a Few Parts

A few of the more complex parts of the design can’t be laser cut, so we rely on 3D printing. Files for the teeth, the rails that the teeth slide in, and the knob that you will attach to the potentiometer are attached below.

Note that the teeth are meant to printed with dissolvable supports. The teeth plus the hinges, pins, and servo attachment will all be printed in one go. Then you leave the print in water overnight to dissolve the supports. This will leave a relatively smooth finish so that the hinges will work smoothly.

To print with dissolvable supports, you need a dual extruder printer and PVA for the support structure. The design might work as is for breakable supports (i.e. if you only have a single extruder head or no access to PVA) though I haven’t tried it. If you try it out or make some design tweaks to get it to work, please share!

Step 5: Assemble

Start by attaching the servos to the hinges of the teeth. There is a small hole on each of the teeth hinges so that you can press fit the servo shaft into it. If they aren’t fitting, try using a drill to make a slightly bigger hole until it fits.

Next hot glue the rails to the face. The rails should be just the right distance apart so that the teeth can slide in and out. Place the servos into the rectangle cut-out in the servo insert and screw them in.

To prep the box, screw in L brackets to two sides of the box. Then either hot glue the box together or screw it together with more L brackets, but leave off the face.

Place all the electronics inside the box and re-attach the servos to the right pins. Next screw in the servo insert (complete with servos and attached teeth) to the L brackets it is attached to the box.

Finally, slide the teeth from the servos into the face and attach the knob to the potentiometer inside the box.

Now there’s room for finishing touches. Draw eyes on the face, attach monster designs on the side, name it -- There’s lots of opportunity to decorate the box with fun designs so have fun :)

Step 6: Catch Monsters!

Sleep tight knowing that all the monsters under the bed are boxed up thanks to your cool new night light.