Introduction: Animatronic Rat

This is a prop you can use on Halloween to greet the kids. And maybe give them a surprise to boot. Put it in a scene or just by the front door. You can leave it plain or dress him up.

This rat can move about in place, talk with a wireless microphone and spit acid (really just water). Watch the videos to see how he moves.

The star of the show is a latex rubber rat I found at Wal-Mart for $7. It's about 14" tall and was filled with some kind of poly stuffing. The goal is to build a movable skeleton that conforms to the inside of the latex rat. You don't have to use a rat, any kind of hollow latex creature that is flexible can be used like a cat, gargoyle, snake, bat ect... But look for something that can be easily animated. For example if the mouth has no depth to it you really can't animate that.

I had no plans drawn up for this project and created it as I went with stuff I scrounged up around the house and at home improvement stores. This is a crudely built prop with rusted bits and popsicle sticks, but hey it actually works! You will have to use your imagination to come up with substitutions if needed.

The most expensive part of this project is getting hold of a 4 channel radio control. I used one from an old RC airplane that I never could fly. It doesn't have to be fancy or programmable, look on ebay, there are deals to be found there. You could even use a RC car radio with 2 channels and just animate the mouth and something else.

If you want to see more follow the steps to see how it was done.

Here are some videos to give you an idea of what he does.

Demo of Rat in motion Video YouTube messed up the audio sync on this one. The mouth is supposed to be synced with my voice.

Internal Skeleton video

Rat shooting water Video

Putting the skin on Video

This one below is a little dark but it was from this Halloween.

Step 1: The Rat

To start you need a willing subject. Rubber creatures don't complain much. Whatever you use make sure its plyable and has areas that can be animated.

Here is a general parts list, but you will have to use your imagination and substituted what will work for you I cant give you a detailed list to take to the store and shop as every creation will be very different.

Rubber creature
3 or 4 channel radio with standard size servos
water pump. (out of an electric squirt gun or you could probably buy a small pump)
Micro switch
Battery Case for 4 1.5V D cells ( or a transformer to supply the power)
Plastic tubing
Aluminum angle and u stock
Wood for the base and wood scraps for other things
Sports water bottle
2 red leds for the eyes
Electical wire
Piano wire for linkages

I cut the tail off and slit the rodent up the back to behind the head and halfway across the bottom. I removed the stuffing but didn't throw it away as it was needed later to pad out the arms, head and some of the body. The internal metal skeleton doesn't support the latex shell well and it droops in spots if not supported.

Step 2: The Base

The base was made out of some wood scraps to make an 12"x15"x4" box with an open ended bottom. Make sure its large enough to allow for whatever you want install in it. I wanted speakers but I ran out of room.

I located the center and drilled a hole to provide a pass though for the pivot post of the skeleton of the rat. I also cut a small arc in the top to allow the tubing and wires to slide around when the rat rotates.

You need to find or make something to mount the internal skeleton to that will allow the prop to rotate freely about 45 degrees. The bottom part of the metal frame is attached to that. I used a flywheel from a very old VCR but you can make something similar with a circular peace of thin wood or hard plastic and a thin bolt passed through and some washers. Think lazy susan. This provides a rotating platform with center bolt that can be used to rotate it from below, with a servo. The picture shows the bottom part of the frame already attached to the rotating part and passed through the top of the box.

Inside the base is mounted the radio receiver and 2 servos for the rotation of the rat and activation of the water pump, the water pump was ripped out of an old leaky squirtgun. The power is provide by a battery pack with 4 D cells to provide 6 volts for the radio receiver and the water pump. The water reservoir is a sports water bottle with some pluming installed. Make sure you drill a small hole in the cap otherwise you will get a vacuum lock when the pump starts running.

Step 3: The Internal Skeleton

I added two pieces of L bracket to the rotating part to make up the lower half of the body. I kind of gauged where the rat would bend at the waist and drilled a hole between the two brackets and passed a bolt through to make a pivot point.

I cut a piece of U channel to reach from the pivot point to the head of the rat. I passed the bolt through the two L channel and U channel holes creating the basic frame. The upper part can pivot forward and back.

To make the mouth open and close I made a beak like contraption that would fit inside the head and into the snout. It's easier to look at the picture. I formed some wood and attached some manipulated thin metal to make a pivot point, the top part moves up and down and the bottom is stationary. Then it was pop riveted to the upper U channel at the level of the mouth. The bracket looks sloppy with some extra protrusions that can be cut off. I left them to be used as attachment points for future improvements I am thinking of.

Now I have all the movable parts. Left and right on the bottom rotating part, forward and back bending at the waist and the mouth can open and close. This would allow me to aim it in just about any direction within a 45 degree arc.

Step 4: Servos

Now comes mounting the servos. I didn't use store bought mounting hardware and just used some scrap basewood. Its crude but it works. I mounted the mouth servo on the upper U channel so it will move with the upper body and can open the mouth at the same time. I attached the servo linkage as shown and mounted the servo using a scrap of wood.

The body servo is mounted to the lower body L channel and connected to the bottom of the upper body U channel with a music wire connecting rod and linkage connector. The forward and back travel does not need to be extreme. You only need it to help you aim up and down about 45 degrees, if that much. The servo travel was extended by adding a cut down popsicle stick to the servo arm.

To rotate the rat side to side the servo is mounted in the bottom box. An R/C steering horn is attached to the rod or bolt that passes through the top of the box and linked to the servo. If you don't have something similar you can cut a control arm out of a piece of metal and jam two nuts on both sides. On the servo end I added a popsicle stick and metal strip to it to increase the servo arm travel. They are connected together with music wire. You only get 45 degrees of travel side to side, that's the limit of the servo travel. That means you can't squirt someone directly to the side of the prop but if you arrange your rat so people can't go behind it or directly to the side then it's not a problem. You could probably come up with improvements to let it rotate in a greater arc.

I mounted the water bottle, pump, rotate servo, battery pack, and receiver in the bottom with a on off switch. Everything is battery operated so the prop can be put anywhere in the yard without having to attach a power cord. For longer run times a wall wart would be better.

Step 5: Water Pump

The water pump was pulled out of a battery operated squirt gun. The pump is attached inside the bottom and vinyl tubing was run from the water reservoir to the pump and up through the frame to the mouth area.

I used copper tubing velcroed to the frame to try and control the placement of the exit end of the tubing. There is probably a better way to do it. The water reservoir is just a sports water bottle. I cut a hole in the top and attached the top of a soda bottle with cap to provide a water refill hole. This passes through the top of the box so I don't have to turn it over to refill. I used the nozzle end of the squirt gun where the water comes out and attached it to the end of the copper tubing in the mouth area with a short piece of vinyl tubing so it's flexible. This will shoot water just like the squirt gun did. You can use any cap\plug type thing with a pinhole in it to the same effect.

The pump is activated with a 3rd servo mounted to a piece of wood. I cut the servo disc into a cam shape to activate the micro switch which feeds 6 volts to the pump motor and at the same time lights up the red led eyes. This setup allows me to operate the mouth independent of the water pump so I can open and close the mouth without squirting people. If you want you can eliminate the 3rd servo and activate the pump using the servo for the mouth. Just add some linkage to activate a switch at the same time the mouth is all the way open.

Step 6: Putting the Skin On

Once you have the internals completed the latex shell is then pulled over the frame and some stuffing re-added to fill out the droopy parts, making sure the stuffing doesn't cause the servos to bind. The red leds are just poked through pin holes in the eyes just enough to see the light. The squirting end is poked through the tongue. The back can be taped shut with black tape or use Velcro or you could also just put a shirt on him to hide the slit.

Step 7: Set Him Up

It's ready to go. Put some kind of decorations around it so it fits in and maybe hides the bottom box some. I glued a cereal and soda box on the outside of my base so it would fit in with trash and stuck him on the top of a trashcan. Last year he was a trash rat but this year he is going to be a gangster rat.

You can move it around and follow people, talk to them while actuating the mouth, get people to look closer at it and interact with it then wham you squirt them. I have never had so much fun, you can control it completely and even keep squirting people as they try to move out of the way. My only suggestion is to not squirt the very young as they tend to cry and their mothers tend to want to beat you.

I did try to part many a young kid from their candy by offering to trade some trash for it but I couldn't get any takers.

Next year I plan to make the mouth servo actuated by audio input like I have seen on some skulls on the net that way I don't have to manually manipulate the mouth to match what I am saying. Or maybe have a script that animates him unattended.

This was a lot of fun to make and more fun to play with.

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