Sunday, October 30, 2011

Pneumatic Toe Pincher Coffin, Part 3

The 4th quality skeleton arrived from wrapped in bubble wrap. The "skeleton wrapped in plastic" look alone would make a scary prop. I left it on the dining table as a centerpiece for a few weeks while working on the coffin.
The skeleton needs to be prepared before use. Some joints need adjusting, bolts tightened, etc. I also spent a day filing down all the flashing with a Dremel Stylus Cordless Rotary Tool- it doesn't have to be perfect, but you can spend a few hours removing excess material from the chest, arms and hands.

The entire skeleton is then painted with Rosco white fluorescent paint. In retrospect white fluorescent spray paintwould have been faster.

The bucky skeleton goes in the coffin with the feet resting on the 1/4" plywood panel covering the footwell where all the pneumatics are hidden. I mounted a four foot UV fluorescent fixture to the base up against one of the side panels - although not perfect the lighting is sufficient.
Halloween 2011 Coffin Open

On to the "scream" sound effect when the lid opens. For several years I have used Radio Shack's "sound recording and playback module" in various props and wanted to find something better. Ideally, less components. The Radio Shack module requires a 9-volt battery and isn't easily interfaced to an external speaker. I had to carefully solder wires onto the circuit board and run them to an amplified speaker, which required another 9-volt battery.

Blue Point Engineering makes a large selection of audio boards and amplifiers but I could not find a simple one channel board that also included an amplifier. While surfing I came across the ELK-120 which cost less than a BPE sound board and also included a 24-watt amplifier. Even better, the board can start playing an audio channel as soon as 12vdc is applied to the appropriate terminals. This cut complexity down to 3 components, none requiring batteries: Board, speaker, and power supply.

I encourage you to save all your power supplies/transformers/adapters. Even if you have upgraded your cell phone, don't throw the old cell phone charger away if it can provide a useful output - anything DC is worth saving for these kinds of hobby projects. Laptop power supplies are a goldmine. You can probably find buckets full of them at surplus stores, or ask a computer nerd friend if they have any extras. I had three that provided the voltage and current required by the ELK-120.

Project Update: Pneumatic Toe Pincher Coffin, Part 4

Thursday, October 27, 2011

Christmas gifts for nerdlings

When I was around 11 years old, a friend of the family gave me a Radio Shack 150-in-one electronics kit. I thought it was absolutely amazing and made many of the projects in the big instruction book. A friend of mine got Capsela which I had never seen before. Similar idea, but more for the mechanically inclined. I had the Erector Set.

Elenco 130-in-1
Electronic Playground
and Learning Center

Super Construction Set



If you know a child with an interest in electronics (I demonstrated an interest by taking apart the toaster, plugging all the extra extension cords together and dragging them around the house) or mechanics, pick up one or more of these kits this holiday season.

In addition, electronics and mechanical engineering skills equal a head start on robotic science.

Wednesday, October 26, 2011

Parallel Resistor table

Here's a handy table to find three resistors you can put in parallel to achieve a more precise target resistance. Great for LED voltage drops.

View on Google Docs
(click the down arrow in the upper right corner to download)
or Download .zip

Tuesday, October 25, 2011

UV LED miniature spotlights with snoot

I hate when people can see the light bulbs! What's nice about ultraviolet lighting is the amount of visible light is much less. An object painted with fluorescent paint shifts the UV light into the visible spectrum.

Sunday, October 23, 2011

Pneumatic Toe Pincher Coffin, Part 2

On to the pneumatics. First I needed to find a quiet (and affordable) compressor to hide in the garage that won't make a racket. I settled on the GMC Syclone 6310.

I decided the best place for the pneumatic cylinder was at the foot so that it is not easily seen. The 72.5" long coffin will still have some room at the foot even after the 5'6" skeleton is placed inside.

I want the coffin to open automatically when someone approaches. I figured the easiest way to do this was to use a cheap Motion-Sensing Flood Security Light- without any light bulbs installed. Instead, a light socket outlet would be screwed into one of the sockets and an ungrounded extension cord run to the coffin. To keep things simple, as many of the components in the coffin should use 120VAC directly.

Working from the size of the bottom panel, 11-3/4" x 11", I had to find a pneumatic cylinder that would fit in the foot of the coffin. McMaster-Carr has the best selection of pneumatic components. I purchased the following items:

QtyMC Part #DescriptionEach
16534K18Quick Disconnect Hose Coupling 1/8" NPT female x 1/4" Industrial Plug$1.84
15644K91Assemble-Your-Own Retracting Air Hose, Red, 1/8" NPT, 25'$12.68
65644K51Easy To Install 1/8" NPT Male brass fittings$4.11
14414K32Air Filter$7.38
16692K16In Flow Control Valve, 1/8" NPTF to 1/8" NPTF$20.22
17889K22Directional Control 3-Port SS Solenoid Valve, 1/8 NPT Female, 200 PSI, 120 VAC$113.86
16498K5371-1/16" bore cylinder, 5" stroke, 1/8" NPT$36.54
19066K12Exhaust Flow Control Valve, Male Thread to Push-to-Connect Tube, 1/8" NPT For 1/4" OD$23.27
18457T31Push-to-Connect Muffler Plastic, 1/4" Stem OD, 52 SCFM$7.38
16498K72Pivot-mount cylinder bracket set$3.05
16498K43Rod clevis with pin$3.88

I could not find any 1/8" NPT rubber flexible air hose and ended up ordering some rigid retracting air hose. McMaster-Carr sells this along with brass fittings as part of their "Build Your Own Air Hose" solution. I cut the rigid hose into shorter lengths and attached the fittings to make three separate hoses.

To make sure everything works correctly, I "breadboarded" the components on the workbench to test them out.
I recommend using pipe joint compound on the threaded connectors before screwing them into the components to prevent air leaks. A squirt bottle filled with soapy water can be used after the system is pressurized to locate leaks.

The 1/8" NPT to "Industrial" Quick-Disconnect adapter allowed me to use standard air hose QD connectors - treating the system like any other air powered tool for use with the compressor. Home Depot did not have any black air hose so I found one made by Goodyearhere in the U.S.A.

One end of the 50' black air hose connects to the compressor. I set the regulator to 90 PSI. The other end of the hose connects to the quick-disconnect connector which goes to the air filter. The air filter removes any dust or debris from the air supply so it does not get into the components. The filter connects to an inline flow control which allows you to adjust the rate at which the coffin door opens. The flow control connects to the normally closed port on the valve. The cylinder connects to the port that is switched between the normally closed and the normally open ports. A 90-degree flow control is screwed into the normally open port on the valve. This flow control allows you to adjust the rate at which the coffin door closes. An exhaust muffler is plugged into the flow control to keep the noise of escaping air to a minimum.

Since the stroke of the cylinder is 5", the rod end will need to be attached approximately 3.5" up the lid from the edge.

An X10 RC6500 Key Chain Remote Starter Kitallows manual override of the coffin while the motion sensor light is tripped. This particular unit retains its on/off state even when unplugged. Leaving the unit "on" lets the motion sensor control the coffin. Since the motion sensor may be tripped prematurely, or may stay on too long, the remote control provides a means to manually close and open the coffin if multiple groups of kids are walking past the motion sensor.

Continue to: Pneumatic Toe Pincher Coffin, Part 3

Tuesday, October 18, 2011

Pneumatic Toe Pincher Coffin, Part 1

This year's challenge was for me to learn a bit about pneumatics and try to build a coffin that would automatically open when kids walk past it. Spooky skeleton inside, maybe some cool lighting and sound effects.

After reading Animatronics: Guide to Holiday Displays, I felt ready to begin this project.

First, I needed a "toe pincher" coffin plan. I searched the internet and found a few designs but they did not look quite right. I knew I was going to place a 4th quality bucky skeleton in the coffin, and that skeleton is 5'6" tall. Adding 3" all around for "padding" I knew approximately what the size of the coffin needed to be. I decided to write a little trigonometry program to solve for a set of measurements where the side panels would require no more precision than .25 inches.

The program produced 10 designs which I drew up and compared. I selected one design as most visually appealing. Click the image below to view the final design.
While all the panels are of easy-to-cut dimensions, the angles are not so straightforward. I decided to make angle templates (download: 48, 54, 76) to help align the miter saw.
The only other dimension I needed to determine was the height, or depth of the coffin. That is, if the coffin is laying flat on the ground, how high it comes up from the ground. I decided on 11 inches.

Cutting and assembling the coffin is a 1-2 day project. The panels, base and lid are 1/2" birch plywood. I decided to use small 2" hinges with removable pins to connect all the panels together and to the base. That way I can take the pins out and break the coffin down into pieces for storage if space is tight.

After attaching the frame to the base and adding the lid, I painted the entire coffin black, inside and out. I then started to wipe down the sides with green fluorescent paint mixed with water. It is my hope that this will give the coffin a spooky antiqued look when lit with a blacklight.

Continue to: Pneumatic Toe Pincher Coffin, Part 2