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Greetings. Please bear with me as this is my first Instructable. However, I have built this effect for this last Halloween and have had some pretty good results with it. So I thought I'd share.

Videos and pics of my test setup for this effect, as well as my entire Halloween setup for 2014 are at the following link: https://goo.gl/photos/aEG3HtFUVeJmWEsz9

UPDATE: This is also the project that was mentioned in the October 2015 issue of Popular Science (page 67).

Introduction

The video above demonstrates the effect. The basic setup uses a projector, a PIR motion detector, raspberry pi, and a piece of plexiglass with a thin spray coating of reflective paint. A loud, startling transparent image appears when a person walks in front of it. I will go over the steps and shell script code used in creating this effect.

The following steps will outline the creation of each of the components of the effect and some possible alternatives.

Step 1: The Transparent Screen

Materials

Plexiglass or Lexan (as large as you need). May be purchased at a place like Home Depot or Lowes.

Reflective spray paint (such as Reflect-All, Rust-Oleum 214944 Reflective Finish, or Night Brite). These are a little harder to find in most of the brick and mortar stores and you may have to go online. Alternatively, you can try a very thin/sparse coat of glass frosting spray paint, which is easier to find.

Wood pieces for frame. I happened to have some left over baluster pieces from a deck project.

Build

Screen - Lay the clear screen down on a tarp and spray a very thin layer of paint over one side of the screen. You do not need much paint at all. I essentially tried to mist on the paint from a good height in order to get a nice sparse, relatively uniform layer. Try practicing first on some cardboard or scrap pieces to get the technique down before spraying onto the screen. Ideally, your screen should remain transparent and nearly clear.

Frame - A large piece of plexiglass is likely too floppy to stand well without a support frame. Lexan (polycarbonate) is stiffer but may be more brittle (and is typically more expensive). You can either build a standing frame or hang the screen from an overhead support. I had to build a standing frame, since I was using this effect outdoors.

I bolted together some leftover deck rail balusters that I had laying around, as shown in the picture above. The screen is sandwiched between pairs of supports at the bottom and side(s) and tightened with bolts drilled through the support pairs. The support legs at the bottom are also drilled and bolted to one of the bottom supports, as shown. Right angle brackets have been screwed into the side and bottom supports.

Alternatives

Theatrical scrim - or any type of thin semitransparent gauzy material hung from a doorway or other support. In the right light (ie. backlight), this should be pretty transparent from the front but still act as a good medium to project a ghost onto. This is used to good effect in this YouTube video. More detailed video instructions are here.

TransScreen - Used in movies and advertising. This is the type of screen my project is trying to simulate at a much cheaper price.

Fog Screen - If you are really ambitious, this is a great effects screen and several people have posted their own builds:

<p>That is REALLY cool. It's amazing how you seemed to get just the right amount of paint for the projection to be highly visible while the glass still seems pretty transparent. Great first time Instructable - you should enter it in the First Time Authors contest if you haven't already.</p>
<p>Thanks. I took your advice and entered the contest.</p>
<p>really bro nice use of projection and raspberry pi... i really got scared </p>
<p>oww very nice</p>
woe WO WO WO that is scary
<p>OMG ! This is so <em><strong>Cool and Scary!</strong></em> I would have made it if i would have an projector.</p><p>it is best for pranks! my friends would just be screaming! and <em>( i think it could be made on an glass with 2 way privacy film, with a LCD at Behind?)</em></p>
<p>This is so cool and spooky! Even when I was just watching the vid, I jumped out of my skin. Nice work, and congratulations on the feature!</p>
<p>That is freaking cool!</p>
Great one man...... Transparency is a must for total spookiness! Great work.
<p>This is a great effect! That would totally freak me out. Nice work!</p>
<p>Hello, the following is a response to the question I have received &quot;I don't have a PiFace, can you please kindly show me the workaround?&quot;. My response is bellow: </p><p>You would access the Pi's GPIO (General Purpose i/o) pins directly or use jumper wires to connect the pi to a protoboard. A brief tutorial about using the GPIO pins is here:</p><p><a href="https://learn.adafruit.com/adafruits-raspberry-pi-...">https://learn.adafruit.com/adafruits-raspberry-pi-...</a></p><p>To simplify things you only have to connect the three wires of the PIR to the GPIO pins as follows:</p><p>PIR sensor trigger -&gt; GPIO 7 pin on the Pi</p><p>PIR sensor VCC -&gt; +3.3V pin (we are using 3.3v power ONLY and the sensor should be set to use 3.3v logic, NOT 5v)</p><p>PIR sensor Ground -&gt; GND pin</p><p>If you want to add a manual button trigger connect the button, then you connect the button between GPIO 0 and GND. A mode change button connects GPIO 3 to ground.</p><p>The appropriate changes to the shell script are:</p><p># Define PiFace Pins </p><p>MotionPin=7</p><p>TriggerPin=0 </p><p>ModePin=3</p><p>Please note that it doesn't matter which particular pin numbers you use, so long as you are consistent in the script code. Also please note that there are different numbering systems for the GPIO numbers on a Pi, the script accesses the pins using the &quot;gpio&quot; command, which uses the Wiring Pi numbering of the pins:</p><p><a href="http://wiringpi.com/">http://wiringpi.com/ </a> and <a href="http://wiringpi.com/pins/">http://wiringpi.com/ </a></p><p>I will also post this message to the general comments under my project to help anyone else with this question.</p>
<p>After looking at the specs for the various PIR motion sensors, almost all of them require +5v on VCC, but many output +3.3v on the output. Some give you a choice to output <strong>+5v or +3.3v</strong>, depending on a jumper setting. Therefore, make sure the the VCC pin on the PIR is connected to <strong>+5v</strong> and make sure that the output voltage of the PIR (which connects to the GPIO pin) is at <strong>3.3v</strong>.</p>
<p>I meant to write http://wiringpi.com/pins/ as the second site for the Wiring Pi pin numbering </p>
<p>If you could what is the pin layout you have on the costum connectors?</p>
<p>Holy S*** dude. Amazing.</p>

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Bio: At various points in my life I was an engineer, programmer, biophysicist, and physician. Now I look at people's insides without having to cut ... More »
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