Thursday, October 30, 2008 Pop Up Corpse

Here are a few pictures of our first motorized prop. The project started with a frame from an old stationary bike, a broken weedeater, a wiper motor from a Buick Skylark and a $20 'talking bride' skull. The bride skull would cycle through three corny songs that were useless for scaring tots, but was equipped with an articulated jaw driven by a 5-volt motor. We wanted a corpse that would pop-up fast and say/scream something with synchronized jaw movement, did not want to spend more than $50, and did not have an air compressor. And so the design criteria were set.

This is the frame of the corpse in the rest position. The mechanical elements and hands are completed. His spine is the weedeater - the curve of the aluminum frame provided a good offset to attach the linkage from the drive arm. The main frame is the stationary bike, chosen because it already had good bushings where the pedals were mounted. The bushings were easy to re-use as the pivot point below the corpse's pelvis. The rest of the armature is old aluminum from other projects. Also visible is the wooden frame of the toe-pincher coffin, to be finished with cardboard (I know, I know) stapled to this frame.

Here is the corpse in the upright position with his scare full on. The theory is that once he reaches this position, an evil laugh (courtesy of Vincent Price) will be played from CD, complete with synchronized jaw movement and flashing yellow eyes. We will see...
His hands are the classic coat hanger wire, duct tape and glue-soaked paper towel technique. The fingernails are cut from a milk jug.

Raising the left arm was a simple matter of mounting it on all-thread, then adding a length of aluminum angle stock parallel to the spine with a pivot point slightly offset from the spine's. The right arm was to move sideways, and required a bit of clunky engineering. There are six pivoting joints in the assembly that transfers the vertical motion to the lateral motion that we needed. No way this could fail under repeated use, right?




Two end-of-cycle switches were required to stop the motor when the corpse was fully reclined and then again when he achieves the full upright position. Initial tests of controlling the motor directly through the switches showed that the cam would not park at the right spot to keep the switch open. The result was the corpse most often thrashing up and down continuously, nearly beating itself to death.

This was solved by using two 12-volt relays: one wired to 'latch' when it receives a negative pulse (until either switch is actuated) and in turn activating the second to provide 12-volt power to the wiper motor. The configuration worked quite well and was cheap. NOTE: the 12 volt power feed needs to be fused for safety!

A final treatment of glue-soaked paper towel has been applied to the head and hands, tinted with food color. The cardboard neck and a little of his chest received the same. The torso and arms have been filled out with chicken wire. We dressed him with a t-shirt to hide the mesh pattern, then a muddy dress shirt, and finished out the coffin with cardboard, foam and some old curtains. The foot of the coffin is left incomplete because we will install a chicken wire mound and cover with leaves and debris. This will hopefully create the impression that the coffin is jutting up out of the ground.




He moves pretty quick from activation to full upright position with the whole cycle taking less than 2 seconds. The motion has considerable force, which caused me to abandon motion-triggered activation, as there would be no barrier between him and the tots. No point in risking injury to anyone! Besides, I would have just sat and watched him scare people anyway.




Sorry that there are no photos of the inside of the head. After researching the normal circuits and endless douglas fir tree hacks I landed on a simple solution to synchronizing the jaw to audio: driving the motor directly from an amplified audio signal. How can you drive a DC motor and DC LEDs from an AC audio signal? By connecting the speaker outputs of one channel from a car CD receiver to a bridge rectifier (essentially four diodes, much like a car alternator uses to convert its AC output to DC) and then the outputs of the rectifier to the 5-volt DC motor and lights. This does present an odd (low impedance) load to the amplifier, and initial testing with an 8-ohm home receiver caused it to pop into protect mode at higher volume. The 4-ohm stable CD receiver that we used Halloween night performed well. Adjusting the balance control allowed shifting more power to the left (jaw motor) channel without overpowering the actual audio output of the right (speaker) channel. If our audio system had not included a balance adjustment, we could have increased the gain of the left channel before burning the disc.