Thread: New prop for this year - Need help with design ideas.

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So I have a new prop that will be my centerpiece for my yard haunt this year. I know what I want it to do - I'm just not sure how to best do it, mechanically.

I want to have a huge scarecrow mounted to a pole. When people approach, it will 'lunge' at them by falling forward off the pole (stopping before too far, obviously).

I've seen this gag on a terrible vampire prop at Spirit, it actually made me jump and shocked me last year. If it makes ME jump, it's a good effect

So, first, NO PNEUMATICS! I posted about this prop a few months ago then life took over and I never came back to the forums. But a few people kept suggesting pneumatics. No. Cant do it. Wont do it. The reasons are irrelevant, but I will not be using penumatics.

The fall is not the issue. What's the issue is pulling it back up into place. Every system I can think of to pull it back into place basically leaves it unable to fall back down again.

If I use a wiper motor to wind it back up, I cant then release it back down again. I thought of a geared system, but then I need to disengage the gear from the wind up mechanism.

I'm trying to do this as simply as possible, as the rest of the prop will be kinda complex (I'm hoping to animate the head on at least a 1 or 2 way axis, as well as make the arms come forward when it falls).

All that I've got taken care of - I'm a programmer by hobby and with some electronic equipment I have, I can take care of all that.

I just cannot, for the life of me, think of a way to pull the scarecrow back up to a vertical position and then release that tension so it can fall again. Well, I CAN think of a few ways, but they are extremely complicated, like a gearing mechanism where a gear is removed. Thats too much mechanics.

I've access to a lot of tools for fabricating parts, just cant think of a mechanical way to do it.

So, brainstormers, anyone have any thoughts?

Thats why you use pneumatics...Just kidding.

It's a tall order but I have seen similar props that use motors. Someone did a guillotine that wound the prop back up and released it back down. You could maybe wrap you head around the way it was done.

Here is the thread, the videos start on page 4. Too many to post:

mechanical guillotine

I would just attach the scarecrow to this and make his feet swival forward like a hinge. Use a pully overhead. The weight of the prop should make him decend forward when the rope is released. Then he will lean forward and be pulled back up when the rotine ends.

Havent looked at the thread yet - but yes, thats basically the action I want. Gravity makes it fall forward, mechanics recall it back up.

I'll check out your thread and videos, hopefully it gives me enough to get this working.

The hardest part of this for me is, pull the body back up, but then release the tension on the cable so it can fall, then pull it back up again. Nothing I can think of (within reason) will work.

I'll let you know how it goes. Thanks for the link.

Just to be sure I'm seeing that right...

The motor winds the cord on a cylinder... to trip it, the motor rotates so the cord pulls off the cylinder... then it snaps back into place, and winds?

Brilliant. Now I gotta see if I can do this with my setup... it WOULD work...

How about this scenario? Use a motor with some sort of arm, and a piece of aircraft cable attached to the end of the arm on one end, and attached to the scarecrow on the other end. Also have some sort of spring-loaded latching mechanism that locks the scarecrow in place when it's reset.

Here's how it works:

The prop is triggered. First, an actuator (perhaps this one):

http://www.sciplus.com/singleItem.cfm/terms/16526

opens the latch, allowing gravity to drop the scarecrow. Next, the motor turns the arm (which pulls the cable) to pull the scarecrow back up until the latch catches, then returns to it's previous position (thus putting slack in the aircraft cable to allow the scarecrow to freefall when retriggered), after which the the prop's main trigger is reset for the next scare.

You could use some sort of spool with the cable instead, kind of like a winch. It would be the same basic function; pull the scarecrow back up until it latches, then unspool to allow slack in the cable before the prop trigger is reset.

Spool + an electromagnetic clutch?

Where will the scarecrow be hinged, at the waist or the feet, and how far forward do you need it to fall? How much weight are we talking about?

Well my issue is that I'm LIKELY going to use a wiper motor, which means I wont have fine control over it, which means it needs to turn one direction til it's tight, and thats about it... I wont have enough control to unwind it and stop at the right spot, its why I'm investigating this method.

If that doesnt work I'm going to see about getting a stepper motor with enough torque to wind up the 'crow, and if thats the case, I can wind it a specific amount, then unwind it the same, with perfect precision, and release it with an actuator.`

But I really like how you die the guillotine... very elegant. I have a wiper motor, and some car door lock actuators... I can use a spring to hold the wiper motor in the wind up position, then pull it out so it unwinds with the actuator. Wouldnt even need a release for the scarecrow, as it would just release as the cable were released.

As for the clutch - I'm not real sure how they work. Anyone got a diagram or reccomend a website? I'd google it, but god knows how many different clutches I'd need to sort through to find the right one...

An electromagnetic clutch is basically like the clutch on a car, except that it's actuated by magnetic force, instead of hydraulic force or a cable. Basically, when the clutch is engaged, your spool would be connected to the motor shaft. When it's disengaged, the spool can spin freely, with just the friction of the bearing it's on (not having to drag the motor with it).

So, the idea is: have the motor spool in the rope/wire, then when you want to have the scarecrow fall toward people, you disengage the clutch. The spool is free to spin on its own, and the scarecrow falls toward its victims.

As far as controlling the wiper motor, you could use a limit switch. Picture a spring-loaded switch similar to this, but probably a bigger one. Put a hole in the lever arm, and thread your cable through the hole. Now put a bead on the cable, near the scarecrow. As the motor spools up the cable (behind the limit switch), it pulls the bead closer and closer, until the bead hits the limit switch, and shuts off the motor (once the scarecrow is upright, since that's where you put the bead).

You might need to add some flexibility into the limit switch lever (a spring, perhaps), since the wiper motor probably won't stop the instant current is removed.

- Hook

Electromagnetics are pricey, though. That's why I'm curious as to just how far the 'crow needs to fall forward. This could be done with a properly sized armature, I think. Again, it depends on the throw and the weight. The wiper motor can be controlled to do two 180° rotations to drop and raise the prop.