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Mr.RC-Cam

Halloween 2016: Talking Skull Project

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I've decided to create a talking skull for Halloween. Anyone that wants to do this will find that the internet is full of DiY instructions on building one. But I prefer to roll my own designs, so I started fresh. After all, the adventure in creating such things is all part of the fun.

First I needed a skull (with or without a body) that had a hinged jaw. These are easy to find this time of year (Halloween season). The local big box hardware store was advertising a five foot posable skeleton for only $30US that would have been nice to use. But they were out of stock, so instead I purchased a four foot animated Grim Reaper prop from them.

The Grim Reaper already had a animated jaw (with a spooky audio voice) and LED eyes. Overall a fantastic deal for only $20. Too bad its clever technology will be ripped out and replaced with entirely new hardware.

Here's what I got:

grim_reaper3_800.jpg

grim_reaper1_400.jpg

grim_reaper2_800.jpg

 

I bought it at my local Home Depot store:
http://www.homedepot.com/p/Home-Accents-Holiday-48-in-Hanging-Animated-Grim-Reaper-with-Lights-and-Sound-4302-48222HD/205828002

 

 

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This project will allow anyone to talk into a microphone to control the Skull's mouth. To help scare the kids on Halloween night I'll be using a electronic voice changing device to mask my voice.  There will be speakers in the Reaper and a wireless link to broadcast the audio from a remote hiding place. Along the way there will be soldering, Arduino programming, and 3D printing.

Let's get to it!

After removing a few screws the Reaper's guts were laid out on the work bench. A main control unit holds the batteries and speaker. Attached to it is a motorized jaw actuator.

grim_reaper_guts1_800.jpg

 

The jaw actuator is a primitive affair. It's simply a geared motor with spring return. When powered, the jaw opens until the motor stalls. Without power a spring returns the jaw to the closed position.

skull_jaw_motor2_800.jpg

 

With careful manipulation of voltage it is possible to achieve variable jaw movement / position. But after some experimentation I decided an R/C servo would offer more precise control over jaw position. After all, the goal is to make the Skull's "mouth" movements look realistic while it talks.

That means that all of the Reaper's electrical guts will be buried in the local landfill. May it all rest in peace.

 

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The new brain in my skull is a Arduino Pro Mini ATMEGA328 microcontroller board. These sell for as little as $2 USD on eBay (or about $5 on Amazon.com). I've used them in other projects with great success. Some eBay examples: https://goo.gl/pp1lbt

The skull needs to process an audio/voice source so it can control the jaw movements. The Arduino can't do it alone, so some circuitry was added to the Arduino controller that connects to any common audio amp's speaker wires. It is transformer coupled for compatibility with amps that use a BTL output driver (as well as any other type).

Audio processing involves DC rectifying the audio signal into a variable voltage. Rather than a simple diode solution my design uses a op-amp based rectifier. Here's a white paper with details on the precision op-amp rectifier: https://goo.gl/BcQtwy

Custom software on the Arduino performs peak detection on the rectified audio that is sampled on a analog (ADC) input. The voltage is software converted to a variable PWM servo pulse. I wasn't sure how well this would work but in retrospect it all turned out to be very good design choices. The jaw movements look quite realistic while my skull talks.

I used a piece of phenolic proto board to hold the Arduino, op-amp circuit, and other bits. It was a quick build especially since I did not care about making it look pretty.

Here's a photo of the skull's custom control board:

arduino_pcb1_1000.jpg

 

By the way, one issue I've seen in other talking skulls was excess servo noise (spectators hear the motor which spoils the effect!). The Hitec HS-225 I'm using is quiet and has more than enough torque, so it is a great pick for the job. Unfortunately it cost more than the typical eBay servo choices.

 

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Instead of hacking the existing jaw bracket I decided to create a new one that would hold the servo and control board. So my 3D printer was put to work. The black plastic pieces seen in this photo are the new 3D printed parts.

skull_jaw_assy3_1000.jpg

 

A pushrod connects the servo to the lower plastic jaw. A Dubro "servo saver" (spring shock absorber) was used to protect the servo from stalls or other mechanical mishaps (prevents broken gears):

skull_jaw_assy2_800.jpg

 

BTW, the jaw's articulation axle is a long 6-32 machine screw with both ends squared off using a file. The squared ends lock into the jaw's original retainer caps:

skull_jaw_axle1_800.jpg

 

This rear view shows how the Arduino based controller PCB is mounted:

arduino_pcb2_800.jpg

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A pair of speakers are located in the Reaper's breast area. A 1/2" PVC cross pipe fitting from the hardware store, along with some 3D printed brackets, were used to create a compact assembly.

Speakers1_800.jpg

 

With the help of a 3D printed bracket, the speaker assembly attaches to the existing plastic shoulder frame in the Reaper. Here's a peek under the skirt:

Speakers2_800.jpg

 

Another 3D printed bracket was created to hold the battery and a $13 wireless audio receiver module. Some Velcro straps provide a snug fit for the 3S LiPO battery pack and wireless audio receiver.

Speakers5_800.jpg

 

There's a 10 watt stereo amplifier module to drive the two speakers. It is protected by a 3D printed plastic case.

amp_assy1_800.jpg

 

A couple screws are used to mount the Audio Amp on the back side of the speaker area.

amp_assy2_800.jpg

 

The wireless audio receiver is mono so both stereo Amp inputs are ganged together. The Amp's left speaker output connects to the left speaker as well as the Hi-Z Audio Input of the custom Arduino board. The right Amp output only connects to the right side speaker.

Most importantly, all the wiring in the skull and body is inter-connected using small connectors. The plug-in connectors allow me to easily disassemble my chatty Reaper when it has a problem.

 

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At this point the talking skull is fully functional but there's some minor stuff to finish up. However there is plenty of time to complete it before Halloween night.

Here's a quick & dirty demo video that shows the skull talking. It's just me with a voice changer, rambling on and on, so you can see the mouth move about. So far, soooo gooood.

 

 

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Great work as always! If you manage to make a time expander I will be first to build it as I dont seem to find time to do anything these days.

 

Terry

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Thanks for the nice feedback. I know what you mean -- free time for hobby projects is a precious commodity.

 

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Ok, Halloween is done! We received a lot of compliments on our display. The talking skull was cool but our holographic witch illusion stole the show. Here's a walk-through video (3 minutes long):

 

 

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