In a prior instructable I gave the history of why I was making a Lego Movie obstacle course for my 9 year old son's birthday. The first part was a Micro-Manager laser target system you can view here. The next obstacle station was a Micro-Manager sign attached to an electromagnet and pressure sensor. The aim was to have the kids throw a grenade into a basket which would promptly cause the image of the Micro-Manager to shoot down. It could then be reset for the next kid.
Step 1: Oogoo Grenade
The first thing I needed was some grenades. Thanks to Mikey77's instructable regarding making your own sugru/rubber material out of corn starch and silicone (found here), I was able to make several rubber grenades to give the kids. My son obtained a Halo Mega Block mini-figure which came in a hollow block grenade. I opened it up and vacuum formed a mold over the two halves. This made an excellent mold to make multiple rubber grenades. They needed to be small enough for a 9 year old to handle, but with enough weight to trigger the pressure plate sensor I would be using.
NOTE: make sure if you copy this step that you put sufficient mold release on the mold before pouring in the Oogoo, else the silicone will meld into the mold. This resulted in me having 3 perfectly normal grenades and a 4th with an outer shell with a striking resemblance to the mold (and the mold mysteriously disappeared).
Step 2: Support Structure
Next I needed the support structure. This consisted of a 1X6X24 piece of wood for the base, a 2X4X6 block and a 1X1X36 piece of wood for the neck. The base needed to be long enough to balance out the sign on one end, the parts in the middle, and the pressure sensor on the far end. I connected the neck to the block and base with a simple hinge so it could move up and down.
Step 3: Tension
Next I screwed a small bungee court around the neck and to the base. This creates tension to pull the neck backward to the ground. After much trial and error which could have been avoided if I had bothered to do some math, I had to raise the block up about 6 inches. This separated the pivot point from where I was attacking the electromagnet. The closer the electromagnet is to the pivot point, the more energy it takes to resist the tension. The further away it is, the less energy it takes. A pleasant physics reminder.
Step 4: Magnet, Battery, and Relay
The circuit is a combination of two power supplies, an electromagnet, relay and a pressure plate. I used two 9V batteries wired in series to give me 18V. This was sufficient to keep the electromagnet attached to the metal plate I attached to the neck. The circuit needed to be normally closed. So, I attached a relay set to normally closed so the magnet would keep the neck in place, opposing the tension until released. A second 6V power supplied powered the relay. I writed a parallel circuit to that 6V power supply so one part of the circuit powered the relay, and the other ran through a normally opened circuit tied to the pressure plate. When the plate is triggered, the circuit is complete, the relay is triggered, it switches the normally closed circuit of 18V to open, disengages the electromagnet, and the tension from the bungee cord pulls the neck and Micro-Manager sign down to the ground.
Step 5: Basket to House Pressure Plate
Next I needed a place to house the pressure plate and something big enough for a target the 9 year olds could use. I decided on a cheap, $1 basket which was about 6X8 at the base. I used four screws and washers to attach the basket to the wooden base.
Step 6: Pressure Plate
OK, now for the pressure sensor. I used an old cardboard soda case because it already had bends in the cardboard which can be naturally used to help keep the two halves of the sensor separate. This meant the Mountain Dew symbol from the case remained on the outside, but the 9 year olds didn't complaint. I used some door trim insulation to separate the two halves of the plate. I then placed foil on both sides and connected a wire for each side of the normally opened circuit to each. The plate opens and closes, with the normal bend in the cardboard making a helpful separator. When the kids throw a rubber grenade into the basket, the grenades are heavy enough they make the two halves of the plate and foil touch, completing the circuit. The completed circuit runs 6V to the relay, which opens the 18V circuit, releasing the electromagnet.
Step 7: Micro-Manager
The main badguy in The Lego Movie is called Lord Business. His minions are Lego robots called Micro-Managers. I printed off a large 18X24 inch image and clued it to some 1/8 inch whiteboard. I then attached that to the neck of the structure. And now the obstacle station is complete, or so I thought.
Step 8: Should Have Thought Like a 9 Year Old
The station mechanics work exactly as I thought it might. However, I forgot I was dealing with not just 9 year old physical bodies, but 9 year old mentalities as well. Instead of aiming for the basket, most of the boys thought it would be really fun to just hit the sign instead. This resulted in the whiteboard becoming separated from the neck several times and eventually breaking apart. So the device did not survive the party. However, it was an interesting build and for the short time the kids aimed for the basket, it worked just fine.