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Solenoid for Special Effects


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I'm currently in the planning stages for our final major show at Derby University, for those that aren't aware, this is a show control exercise, we've based ours around a haunted house. I plan to have various special effects as part of this, we've got a number of these which are based around something appearing or disappearing on cue. As it's show control we can't just have people triggering these effects which I appreciate would probably be a better way to work given the option.

 

There are 3 main types of effect which I need to achieve:

  • Object suspended which drops down on cue.
  • "Books" being pushed off shelves on cue.
  • Shelves collapsing on cue.

Due care and attention will be paid to ensuring that nothing will hit people on triggering and no-one will be able to be injured by failure of these solenoids.

 

I'm thinking I'm going to need both push and pull solenoids for this? Pull for Object Drop and collapsing shelves plus a push for the book shooting?

 

Does anyone have some recommendations for nice compact solenoids to do the job? Operating voltage and current aren't a massive issue as we'll buy transformers for them too.

 

Also, being cheeky for a minute. If anyone has solenoids that would fit this purpose which are available between the 29th of April and 2nd May which I could borrow/hire for a small charge it would be massively appreciated. Even if people know where to hire from, that would be handy too as I'm a little begrudged to buy for a one off 1 day show.

 

Many Thanks

 

Josh

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For my final year show control project (5 years ago now, you make me feel old) we had a large log dropping off solenoids (in an enclosed space with a deadmans for safety).

We got the solenoids and quite a few other bits in from Hawthorn. There are quite a few Derby grads there, so you might find a sympathetic ear on the phone if you call the right person.

 

For small bits like bookshelves another good source of small solenoids is railway modelling, where they are used to remotely control points.

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My tool of choice is also a remote control car servo. They generate incredible torque for their size and whilst I know it's not needed for this project, the ability to accurately position the arm is really useful.

 

I bought a Milford Instruments DMX - Servo board from CPC which I still use to drive mine, though equally there are plenty of sketches out there for Arduino. Alternatively I've also hacked them open to just use them as motors, with or without the limit switches.

Car central locking solenoids can be ok too, though they have a tendency to spring back to the centre, which sometimes can get in the way a little.

 

We've also had some good successes with maglocks (for security swipe doors). The problem is that when you cut the power, they release, but we've held pieces of scenery together that needed to part easily on cue with them. They're great at holding 2 surfaces together and once you cut the DC, it's as if there was no joint at all. I could foresee a couple of them holding a side on a book case (perhaps in an overly chunky, but hollow, side panel) which, when switched off, would allow the thing to just fall apart.

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In a haunted house environment the props would need to be self-resetting. Having something drop and then have to be manually reset would be impractical. The common choice would tend to be a motor or for greater reliability a pneumatic cylinder with cantilever if a longer movement was needed. Others have mentioned servos, but many servos do not have built in stall protection and can be burned out easily if a prop malfunctions. Solenoids have a very short throw that gets weaker the further the core extends out the coil. They too are prone to emitting smoke if designed for intermittent use but kept energised by a programming error or system failure.
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In a haunted house environment the props would need to be self-resetting. Having something drop and then have to be manually reset would be impractical.

In this case, I can have manual resets. I realise that in the real world auto reset before the next group would be better but we have the luxury of lots of bodies to do just that.

 

Josh

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You may well have seen them but if you have a youtube for making of universal Halloween horror nights. I cant find the video I was looking for but there was a whole "we do this effect like this " etc. It might have even been a podcast.
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Josh, don't forget that the assignment brief expects you to consider being able to resource the show for the duration of the Degree Show period.

 

"One of the productions will be selected for presentation during the End of Year show (Degree Show). Therefore the production should be suitable for a general audience. Moreover, you should ensure that the production can be remounted giving due regard to equipment and personnel requirements."

 

With regards to pushing things off shelves - there is more than one way to skin this particular cat. Door closers of this type , for example, can be 'triggered' with a pull-solenoid

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I had looked into car central locking solenoids at one point for dropboxes. Cheap, easy to obtain, easy to wire.

I did make up some mechs using car locking units and these have been used once successfully as a Kabuki style drop for pantomime giant squid tentacles, once for dropping a front cloth and this year again in panto, for the automatic poo-dropper. (Details on request... :D )

 

Only caveat I'll advise is that you don't want to hold the power on to them too long else you risk burning out the solenoid - discovered that to my expense.

 

 

 

 

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Magnetic fire-door retainers are quite useful for holding things against spring pressure. For Mission 2110 in a scene where the baddies were in pods, the spring loaded doors were held shut by standard mains voltage fire door checks and simply activated by turning them off at a four way switched socket block. Super simple, but great effect and failsafe given that actors were inside the pods.
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Providing a solenoid is only being used with a quick energisation period, they can be overdriven quite substantially to increase short term power output. Do with care; if it gets stuck on the solenoid will overheat which may have catastrophic consequences.
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