It's almost spooktober, so it's time to prepare!

Scare your niece with "real mouse brain in a jar" or with a full-functioning jumpscare plush rat without torturing an actual mouse! No stinky formaldehyde is needed, so it's also safe for kids.

It's a multi-stage project. If you have the knowledge/resource/time for only some of it, no problem! This is segmented into multiple incremental steps.

Ready, set, autops- I mean paint!

Supplies

First stage

1 small cosmetic jar, 10-20 ml (should fit a half walnut)
walnut (I hope you get the idea)
brownish nail polish, but not the shiny-glittery stuff, look for matte finish
green food coloring

Second stage

a plush rat

Final stage

a microcontroller, Arduino preferably
distance sensor, ultrasonic preferably
DFPlayer + microSD card + small speaker
RC servo

Step 1: Brain in a Jar

Now, getting the materials:

Food coloring - I prefer green, but you choose. Any supermarket has this.

Walnut - if you can find whole or half walnut kernels, that's all; if not, try to be gentle with a hammer! Crack some nuts open until you find some brain lookalike to work with. We need a clean, smooth surface at the end. The skin can (and Murphy's law will ensure, that it will) crumble, so remove it. Yes, it will take some time! Soak it overnight and remove the brown skin. Let it dry.

Cosmetic jar and nail polish - if you are a woman, buy it, you know these stuff. If you are a guy, here are some easy steps:

try act tough and look for these stuff
get lost in the aisles
ask someone competent (a woman) to help, there are way too many

Step 2: Finishing the Brain

Paint the walnut evenly, you will need multiple layers. The grooves are pretty tricky to paint, apply minimal paint only and let it dry, repeat. It took me about 5-6 layers to finish.

The food dye is some seriously thick stuff, you will need only one droplet. Don't make the filler liquid too dark!

Try to fill the jar. As the picture shows, small air bubbles may get trapped, at least it's easy to remove them with a toothpick. If the brain wants to float, you can force it to stuck. Glub-glub-glub...

Close the lid, stage one is ready!

Step 3: Lab Rat Gettint an Implant

Get ready for Stage 2!

Literally no one realized the "mouse brain in a jar" thing without the mouse, so make it clear for everyone.

grab the mouse
apply some glue on the bottom of the jar
put the jar with the sticky end where the brain should be
not so optional: clamps - better safe than sorry, and no one likes brain jars rolling around

My epoxy glue cured overnight. A huge upgrade for a small price!

Note: the brain and goo is removed to show how the glue is applied and cured. You can (and should) do the process with the full jar.

Step 4: Electromechanic Lab Rat - the Servo Mount

Stage 3: the simple animatronics!

We need a fixture for our moving rat. Of course you can buy a servo mount, but it's too easy.

I used some wood, a drill and a saw. The drilled holes are big enough to turn the saw around (or show me, how to cut perfect 90° with a simple saw)

The original plans included moving hands, but there is not enough space, so just make one mount.

Step 5: Electromechanic Lab Rat - Rat Spring Board

The servo arm is too short, cut a wooden board long enought to fit the body.

Also, if you mount the servo on the side, it will be visible. Make something similar to a lap joint to hide the servo and keep it in line with the board.

And what are those holes for on the servo arm? Now you know! Fix it with at least 2 screws - and don't forget to leave the mounting hole free! You will need it in a moment. Find a box to mount the servo on. This is where bait can be hidden along our electronics like Arduino and distance sensor. After that, mound the board on the servo.

Servos can move 180°, so move the servo to 0° (or 180°, depends on how you installed it) and since they won't move under/over that degree, that will be the "resting" position - let that lie on the box. You can correct the starting position in the software!

I tried gluing the rat on the board, but it's a pretty bad solution. We know what comes next, but the rat has still no idea...

Step 6: Zombie Lab Rat, Assemble!

It's time to finish the mechanical part.

Open the bottom part of the rat by cutting along the horizontal seam and make a hole big enough to fit the board.

Insert the board gently in the middle of the stuffing all the way in. Even the servo can be hidden inside the rat. In the next (and last) chapter we can finish the work by adding some program to our parts!

(and check out the movement)

Step 7: The Code

Get the latest Arduino code from GitHub: click here!

The code is self-documenting, but have an image and some text to read here too how to connect things:

All VCCs and GNDs can be connected, every part is 5V tolerant (but no VCC-GND short...)
DF Player RX - Arduino pin 1 TX with 1-10k resistor
DF Player SPK_1 and SPK_2 - the speaker
HC-SR 04 ultrasonic sensor Echo - Arduino pin 2
Ultrasonic Trigger - Arduino pin 3
Servo signal - Arduino pin 5

For best performance, the Arduino can be powered from a phone charger, or from a powerbank to remove suspicious cables.

Now hide the electronics inside the box and remember: soundwaves spread! They will bounce back from anything solid, avoid false triggers. And we are ready! Thanks for reading and sharing!