Want a larger than life grim reaper costume with fully mobile giant arms and (the best part) a skull that opens and closes at the sound of your voice? This one's for you!

Bill of materials

Note: I had a goal when I set about making this costume: other than the skull, I wanted to buy as little as possible and upcycle or recycle as much as I could. I recycled most of the pvc and fittings from an older project I no longer used. I took many of the skeleton pieces from various Halloween lawn decorations I've collected over the years. I realize this parts list looks enormous, but most of it is either super-common stuff you probably have in your kit already or stuff you can recycle from somewhere.

For the skull:

Plastic hollow skill with articulating jaw
servo
heavy wire for servo rod
small screw in loop
various wood scraps to secure servo

For Scary Terry's analog audio servo driver:

0.1 uf 50 volt capacitor
10uf 35 volt capacitor
0.1 uf 50 volt capacitor
0.01uf 100 volt capacitor
470 uf 35 volt capacitor
0.1 uf 50 volt capacitor
1k 1/4 watt resistor
47k 1/4 watt resistor
5k trimmer pot
47k 1/4 watt resistor
10k 1/4 watt resistor
1k trimmer pot
10k 1/4 watt resistor
470 ohm 1/4 watt resistor
220k 1/4 watt resistor
50k trimmer pot
4.7k 1/4 watt resistor
10k 1/4 watt resistor
10k 1/4 watt resistor
2.2k 1/4 watt resistor
10k 1/4 watt resistor
1458 dual op amp
4066 cmos quad switch
555 timer
1N914
LED
2N2222 or 2N3904 npn transistor
3 pin male jack for servo connection
Two 8 pin IC sockets
One 14 pin IC socket
1/8" (3.5mm) female audio connector
3x dual screw terminal connector
Circuit board (3 X 5cm works)

For the servo power supply:

USB power bank
Old sacrificial USB cable
2N2222 transistor
2x 180 1/4 watt resistor
22μf capacitor
2N3906 pnp transistor
47k 1/4 watt resistor
Dual screw terminal connector

For the microphone with preamp:

electret microphone
2N3904 npn transistor
10k 1/4 watt resistor
100k 1/4 watt resistor
10k 1/4 watt resistor
2x 0.1μf capacitor
9V battery clip
9V battery
small circuit board
1/8" (3.5mm) male audio connector
Dual screw terminal connector

For the body:

1/2" PVC pipe
1/2" PVC T fittings
4 1/2" PVC elbow fittings
(optional) 2 x drawer rail cartridges
PVC cement
Strap or rope for securing to body
Various prop bones (I used 2 forearm pieces, two hands, and part of a spine)
Heavy wire (I upcycled some wire hangers from the dry cleaner)
Black cloth for cloak
Hack saw and/or PVC cutter, drill, needlenose pliers

Step 1: Assemble the Skull

Open the skull

Open up the back of your skull in a way that will be as hidden as possible in the final product. I used a dremel with a cutting wheel, but a utility knife will probably work just fine.

Mount the servo

Construct a mount for your servo. I cut a notch into a piece of scrap wood, and then curved that scrap wood so that it would fit the arc of the top of the skull, while centering the servo axle above the inside of the lower jaw. This will put the bulk of the servo to one side of the skull.
Attach the servo to the mount. I used a combination of screws and hot glue, positioning the servo properly, and holding it until the glue cools.
Reinforce the mounting with glue and spare wood if needed. This servo will be generating a fair amount of torque, so take the time to make it strong.
Attach a lever arm to the servo control horns. I used a piece of scrap wood taken from of all things a clothespin. I attached it with hot glue.
Drill a hole in your lever for the control rod that will open and close the jaw.
Cut a hole in the bottom of the skull for the control rod. This hole will have to be elongated to the front and rear of the skull as the control rod will move back and forth when the servo turns and the jaw swings open. You'll probably need some trial-and-error here.
Screw a small loop into the center back of the lower jaw. Mine pierced through, so I filed the sharp point off.
Attach the servo horn and measure the distance between the end of the control arm and the loop.
Size your control rod from the measurement. I'm using an actual servo push rod, but any stiff wire will do.
Attach the control arm. I bent loops into the ends of mine using needlenose pliers.

Mount the battery (if using a

This part really depends on your choice of battery. I'm using a rechargeable 5V USB power bank, and I do not want to permanently mount it. In my case, I created a wooden ledge with a wire clip that I created by shaping some stiff wire from a coathanger. The clip is just held in place by bends in the wire and tension from the battery. I hot-glued a small support near the back of the skull and used the top of the eye socket as another support. I hot-glued the ledge in place opposite the servo to better balance the skull.

Step 2: Build the Audio Servo Controller

All credit for the servo driver goes to Scary Terry. The assembly instructions for this circuit can be found at: http://www.scary-terry.com/audioservo/audioservo.h...

The only modifications I made were to include two additional dual screw terminals that were directly switched by the 4066 switch IC. The circuit itself is a pretty high part count due to its analog nature, but it's pretty straight forward. It's an op-amp stage with adjustable gain that feeds into a six output switch, which drives a 555 timer based astable circuit. That is, the switch switches on the 555 timer, which provides the square wave PWM input the servo signal wire needs. There's some adjustments to set the max and min PWM duty cycles so that you can put limits on how far the servo goes. Then the servo itself is just switched on with a transistor.

I assembled this all on a 5cm x 3cm protoboard, and attached the 3mm audio jack to board directly. I wanted to try various power sources, so I left the power input as another dual screw terminal.

Next, I attached the board to the the piece of the skull I removed in the previous step using enameled wire (for easy removal later). I drilled a hole through the skull piece and mounted the audio jack. Ignore the square hole in the photos. Initially I was using the skull as a stationary prop, and I had a barrel jack for a 5V power supply mounted there.

Step 3: Build the Microphone With Pre-amp

There are so many pre-amp circuits floating around the web. Some are more complex than others. Some require op-amp chips. Not this one. This is a simple 3904 transistor based pre-amp. It would probably sound pretty bad, but we don't care. We just need to boost our mic signal.

I've included the parts list for this simple pre-amp, which is quite similar to the one I used: https://www.instructables.com/id/Pre-amp-to-electr...

I don't know the origin of this pre-amp. The image in that instructable seems to show up everywhere. Regardless, it's simple, and it works. In addition to the circuit as shown, I made the mic detachable with plenty of wire for the leads so that I could position it as needed in the costume. I added a 9V clip and I used a battery. Honestly, I used an old 9V that I took out of a beeping smoke detector, and it was just fine. It doesn't take much power to run this pre-amp. For the output, I used a dual screw terminal (yes, I have a huge ziploc full of these--they're dirt cheap on eBay) to which I attached a male 1/8" audio cable I salvaged from an old pair of headphones. Since the input to the audio driver is mono, I attached the left and right channels (often red and white wires) both to the + output, and the ground (usually a black wire) to the ground output of the pre-amp.

I taped up the pre-amp for protection and attached it to the battery with tape. You may want to put a small bag around the mic, because there's a good bit of moisture in your breath.

Step 4: Build the Servo Power Supply

Servos are power hungry, and we're going to be working them a lot. They also need a specific power around 5V. USB power banks, like those used to boost the life of a cellphone, satisfy both of these requirements. You can pick one up with a 3600 mAh rating for next to nothing. I've even seen them at Dollar Tree. If your servo on average uses 300 mA, that's 12 hours. The servo will probably use a bit more than that at times, but in any case, you should be good for a party.

Here's the problem

Those things are almost always built with an automatic shut-off that turns the thing off if there's no load for more than a few seconds. How much load depends on your battery. 50 mA seems to be reasonably standard. So we need to add a load. You could use a resistor across the terminals, but that would a) get really hot, and b) consume a lot of power. 50 mA on constantly will drain that 3600 mAh faster than you like.

And the solution

One common trick is to use an oscillator to pulse a brief 50 mA pulse every second or two. We can do that with a two transistor oscillator. There are really good instructions on how to build one here: https://www.dorkbotpdx.org/blog/paul/battery_pack_...

You need to figure out the right resistors for your battery pack, though. Paul's won't necessarily work for you, so prototype on a breadboard first. The 22 ohm resistor in the diagram above will give you a 5V/22ohm = 227 mA load. That was too high for my battery. I needed about 50 mA, so 100 ohms works for me. Actually, 90 was fine, too. I tried a resistor in that range, and even with the oscillator, it still got uncomfortably warm. So I used two 180 ohm resistors in parallel instead, halving the power both saw (which still got pretty warm). You'll also need to adjust the 100k ohm resistor in the diagram. That controls the pulse timing, and how fast or slow you need depends on the shut-off time of your USB battery pack. 100k was too slow for mine, but 47k worked perfectly. Experiment.

How it works

If you're curious about how this works, it's just an oscillator. When you turn it on, the 3906 is connected to ground and opens all the way, opening the 2222 and the load is on. The capacitor charges through the base of the 3906, and since there's little resistance, it charges up very fast, bringing the base of the 3906 up to +5V and shutting everything off. The capacitor discharges through the 100k resistor. This is your classic RC timer, with time constant determined by R and C (2.2 seconds in this case). Once that drops sufficiently, the whole thing starts over again. The on time is brief.

Assembly

Build the circuit on a small piece of protoboard.
Cut the male USB-B end off of your sacrificial USB cable.
Strip back the insulation and clip off everything except the red and black power wires. Everything else is ignored.
Strip the ends of the red and black wires and attach them to the +5V and GND rails of your oscillator respectively.
Plug into your USB bank and turn it on. If everything is correct, the power bank should stay on. They usually have an LED indicating this.
If everything works, solder a dual terminal screw connector onto the protoboard
Connect the power to the servo driver board by connecting wires between the correct screw terminals. It always helps to label the + and - screw terminals with a permanent marker.

Step 5: Assemble the Costume

Assemble the body frame

Cut the following lengths of PVC, adjusted for your body size:
1. 4 3/4" neck
2. 2x 4 1/2" collarbones
3. 2x 5 1/2" shoulders
4. 2x 13 1/2" risers
5. 4x 3 1/2" spacers
6. 2x 31" back braces
7. 2x 18" chest braces
8. 10 1/2" cross support
9. 2x 28" upper arms
10. 2x 15.5" lower arms, inner
11. 2x 17" lower arms, outer
12. 2x 4" handles
13. 2x 1" couplers
Cut a hole in the bottom of the skull. I used a 7/8" spade bit because that matched the outer diameter of my 1/2" PVC.
Hot glue neck piece in place in skull.
Cut piece of spine to disguise visible part of neck
Attach T to bottom of neck.
Insert collarbones into either side.
Attach side of T to each collarbone
Attach risers to bottom of each T
Attach bottom of T to bottom of each riser
Insert spacers into both sides of both Ts
Attach Ts to rear of each spacer.
Insert couplers into rear of each T
Attach elbows to other side of couplers, facing inward
Insert crossbrace between elbows
Attach back braces to bottom of rear Ts
Attach elbows to front of spacers, facing downward
Attach chest braces to bottom of elbows
Place dry-fit assembly over your shoulders and adjust until comfortable and symmetric.
Have partner mark the joints
Carefully glue assembly together with PVC cement.

Assemble the arms

It is best that you adjust the lengths of the upper arms and the inner lower arms to your size if you want everything to work naturally. The arms are puppets. The upper arm pieces should hang from the shoulders of the frame built in the previous step to your elbows. The inner lower arms should go from your elbow to the center of your palm, because they end in handles that you will be holding.

Drill 1/4" holes through the both ends of the upper arm piece, the outer ends of the shoulders on the frame, and one end of the inner lower ams.
Fashion a ring out of stiff wire. I used old wire coathangers and tried several different ring styles, which I'm including photos of. They all are fairly robust.
Insert rings into drilled holes, linking the rings together at the joints as you do so. Arm bone connected to the shoulder bone (sing it with me).
Assemble the lower arms (2 options)
1. First, I used drawer rails from an old Ikea dresser, using screws to attach one side to the inner piece and the other to the outer piece. This made extendable arms that locked in place (see video). I dropped this design for two reasons: 1) I could just see hurting someone, and 2) the real reason is that the rails aren't very strong outside of a dresser, and I ended up slinging them apart, sending ball bearings everywhere.
2. OR attach a T to the end of the inner pieces. Insert the handles into the perpendicular T socket.
Add bones and hands to the outer arms. I used old decorations here. I had grand ambitions to create large mechanical hands, but that will have to wait for next year. Instead, I cut holes into the ends of hollow plastic arm bones and slipped them over the outer arm pieces. I drilled pilot holes through the hands and into the outer arm PVC piece and screwed the hands into place.

Attach the cloak

You can be as fancy as you like here. Get some cloth from a fabric store, or buy a pre-made grim reaper costume an drape it over the frame. You may need to bulk things out with some stuffing, but remember, the guy is made of bones.

Attach the microphone

I simply taped the preamp into place on one of the risers with electrical tape, and ran the mic near my mouth using stiff wire. It has some range, so perfect placement isn't critical.