This is a post mortem report to serve as inspiration for fellow makers, with some notes on what went right and what did not go so well.
To summarize, I built a laser obstacle course leading to an ante room with a secret door which our Halloween guests had to brave before being allowed access to the buffet. The two images above show the guest experience and I will point out construction details in the next steps.
Step 1: Scaffolding
The ante chamber is a 6' x 6' area on the patio area. My plan was to cut the studs so they are just a little bit shorter than the aluminium roof. Then add some pipe insulation on top and have a nice friction fit wall.
This worked out really well.. for about 12 hours. The next morning the metal roof had expanded or the studs contracted, so now there was a 1/2" gap causing the studs to fall over. That's when I started adding crossbeams and corner braces so the wall panels could support each other. I also added some diagonal supports behind the panel sections. These were screwed to some weights (buckets filled with sand & debris) to further stabilize the panels. With these modifications the walls were now sufficiently stable (though leaning against them is still not a good idea).
As can be seen in the other pictures, I then covered the inside with lots of cardboard. My first thought was to use Home Depot's 4'x8' foam insulation panels for $8 but that seemed too expensive. Instead I went to local dollar stores and bought all their $1 project displays (sorry science fair contestants :-[ ). Some of the stores were nice enough to also give me their excess freight cardboard. My small nail gun did an admirable job of quickly stapling the cardboard to the studs. Again, the project looked great for a little while. After a couple days, however, the cardboard started bulging outward. I still do not know if this was caused by afternoon winds or humidity, but it ended up being really annoying. Those foam panels probably could have prevented a lot of expletives in the long run.
For the walkway along the side of the house I used the same 8' studs + foam insulation, spaced about 6' apart and just resting against the house. I was concerned that they might move or fall, but oddly enough that part of the project was extremely reliable.
Step 2: Decorations
Decorations were mostly bought from the local dollar store and then stapled / taped to the walls. Any surface not showing Halloween items was covered with black plastic sheeting. Since that plastic forms the "roof" of the walkway I had plenty of it left. Best deal that I could find: $21 for Husky 10' x 100' 2 mil plastic
Above the door frame you can see a fluorescent black light fixture. I tried an incandescent black light which promptly got exchanged for the large tube. The tombstone was another thrifty Home Depot purchase ($10) with the riddle added using black light chalk. Unfortunately the chalk is not as bright as black light paint and the text turned out to be hard to read. Printer paper on the other hand glows like crazy giving an eerie feel to the arcane book.
Step 3: The Book
In order for guests to open the motorized door they first need to find and push the hidden button. The button is a microswitch similar to the model shown above. It is about 1/2" long and tall.
Preparation: Find a sufficiently thick book that is worthy of destruction (in my case Wolfgang Hohlbein's "Das Buch"). Open the book to the middle and clamp it down firmly. Search the internet for a sufficiently creepy book page (I found mine on pinterest), and modify it as necessary (I added the key clipart). Print page and hold it up to the open book so you can make markings where the button needs to go.
Using a 1/8" drill create a hole all the way through- this will be for the wires. Then using a 1/2" hole or forstner drill bit create a hole for the micro switch. With enough trial & error you can find the right height so that the switch lever is flush with the page.
At this point remove all debris from the book and make sure everything is nice and flat. Use regular craft glue and brush it all along the edges, wiping off any excess. This will prevent people from flipping through your modded book. Use a generous amount of hot glue to keep the microswitch in place. When everything is dry, glue the decorative page on top to cover the switch. I used regular laser printer paper which was sturdy enough to cover the lever but flexible enough that you could push it. When using an ink jet you might want to pick a slightly thicker paper.
The wires from the switch then got routed underneath the table cloth to the micro controller (see Step 6).
Step 4: Friggin' Lasers !
I still remember the first laser pointers being extremely expensive and now you can get them for less than $1 each. So go on ebay and buy one right now. Never mind- buy a few dozen right now! The downside to low powered lasers is that they do not have visible beams. The photos were taken with a 400W fog machine providing a medium to scatter the light. Without that fog you would only see the end point of the beam.
I ended up using 11 lasers in my little obstacle course. I took the laser pointers apart and short circuited the momentary switch so that the laser would stay on. After some quality time with the soldering iron I had about a dozen lasers wired in parallel with ca. 300mA current draw which can easily be powered by a small DC adapter.
Applying a generous serving of hot glue I glued the lasers to the wooden beams so they would point up or down. This too was surprisingly reliably with only very minor drift.
Since I wanted people to dodge the beams the next step was adding some sensors. 20 pieces will set you back around $5 (Photoresistors). Through trial and error I found out that these resistors have an On resistance of around 3k Ohm when hit precisely with the laser beam. This quickly drops off to >2M Ohm when not illuminated.
Because of the focused beam it might be hard to keep the lasers aimed exactly at the small sensors. My first thought was buying some parabolic mirros used in flashlights. These turned out to be way more expensive than I would have thought so I ended up making my own. I had a 1.5" cone-shaped board game piece and wrapped aluminium adhesive tape around it. The tip was cut off and replaced by two layers of semi translucent adhesive tape. Once the sensor was wedged in the tip (without touching the metal!!) I wrapped a few more layers of adhesive tape around the cone. One might think that a cone is not a great light collector but this contraption worked extremely well and I highly recommend it!
Finally I connected all sensors in series and added a known 10k Ohm resistor at the end to get a voltage divider.
Step 5: Motorized Door
The door was built from light weight 1" lumber. At this point I have to do some product placement because the following tools were really useful and I foresee using them in future woodworking projects:
The Kreg jig and screws in particular made joining studs fast, easy, and reliable. Now with the commercial break out of the way- back to the door: I hung it with two small hinges and then added the door header on top so the door frame is flush. The goal is to minimize light leakage that would give away the secret door. There is no door latch!
As can be seen in the photo, two small aluminium pieces were joined with a machine screw. On the door frame the arm is is held by a hook that I found in my project box. On the other end is a standard servo, screwed into the door frame. The servo horns that came with the servo are too small to be connected to the arm directly.
To deal with this I created a 1" wide strip out of Instamorph, and embedded both the X-shaped servo horn and the metal arm in it. Once cold, I screwed the servo horn (now with metal arm) to the servo. As much as I dislike shaping Instamorph, it is extremely durable once set and I did not encounter any problems with this contraption.
Because this is a standard servo, operating it from the electronics is really simple- connect to +5V and Ground, then have the controller send PWM pulses to the servo corresponding to 0 and 90 degrees. Unfortunately I have not had good luck buying affordable servos. The first one I tried had tolerances that were too tight, causing it to jitter in place. The other one did not jitter but had large tolerances- setting it to 0 degrees means it would end up somewhere between -3 and +3 degrees. In the end this is what I went with, but starting over I would use a motor and drive it directly until it hits limit switches (see instructable linked in Step 1).
Step 6: Electronics
The schematics for this project are really simple. Eleven lasers are connected in parallel to a 4.5V, 1A DC converter and are not connected to the rest of the circuit.
The matching photoresistors and the 10k Ohm resistor form a voltage divider that is connected to the micro controller's analog digital converter. I could have used a potentiometer and digital input, but because there were some unknowns with regards to the total resistance, I went with this approach to adjust the threshold in software.
The door is operated by a standard size servo running off a 5V DC converter which also powers the microcontroller. That DC converter was only rated at 0.5A (servo stall current>0.7A) but fortunately I did not run into any dropouts. Servo control is done via a PWM pin with the same frequency & duty cycle as explained here: Servo sample
The microswitch is connected to a digital input and pulled low when pressed.
Whenever the switch is closed, or the photoresistor voltage drops below the threshold an event is sent by the controller to the ElectricImp server. There a small script that talks to PubNub servers which in turn send a push notification to my Android phone. The Android app I wrote is basically a soundboard that plays half an hour of Halloween ambient music in a loop. When one of the push notifications comes in, the phone will play an appropriate effect (alarm siren or creaking door sound). Furthermore the app controls whether the door should open or close. It sends the desired message back to the ElectricImp server. The advantage of doing door controls via the phone is that I could also add Open/Close buttons for remote operation and to prevent people from getting stuck.
All of this could of course be done locally using a controller with sufficient ROM space. An advantage of going wireless with an IoT controller here is that the notifications are automatically sent to all registered listeners. If you want surround sound just place a few more Android phones along the path and they will all pick up the events.
Step 7: Field Report
For our two Halloween parties we had guests of all ages and backgrounds. For some reason it proved difficult for the older generation to get into the spirit of things. After briefly being stuck in the ante room, they proceeded to push against all walls, eventually overpowering the poor servo which relented with a whining noise. I am happy to report that there was no permanent damage to equipment or visitors. With hindsight I should have informed guests in the invitation about what to do: follow the signage, avoid the lasers, search for the hidden key.
The younger visitors fared better by actually reading the hint and pushing the secret button after searching for about a minute.
Finally, we had a group of 5-year-olds which were jumping over laser beams and pushing the button. Soon games were created with some kids trying to race as fast as possible, while others dodged the lasers by crawling on the floor for the entire 70 ft. The kids did an average of 10 (!) laps through the haunted house tunnel and had a marvelous time.
Weather-wise the ad hoc roof construction held up very well to prolonged rain. The real problem was caused by wind. Even minor breezes would cause the tarp to flutter wildly, pulling out staples or getting ripped. The solution was to use 3/4" staples and reinforce the tarp with gorilla tape before stapling it to the wood.
Thank you for reading and do not hesitate to ask any build questions in the comment section below.