Intro: Hidden Mirror Ball
I made this for a local amateur theatre.
The ball was hidden in the roof until the second to last scene, when it descended for a dance number and then retracted afterwards.
Because there wasn't enough space to lift the ball completely out of sight, a set of doors was needed to stop the ball interfering with other stage lighting during the rest of the play.
Step 1: Parts & Tools
I didn't buy anything for this project. All materials and parts were things I already had or found In the theatre.
The mecancal bit
.2 lengths of steel U section slightly longer than the distance I needed to lift the ball.
.3 wheels that fit into the U section. ( I had to trim mine down a bit to make them fit)
.3 long M6 bolts for wheels
.35 M6 washers
.2 long M8 bolts for U section
.2 M6 bolts and wing nuts to attach mirror ball and motor to plastic conduate
.1 mop bucket filled with spare wheels and pulleys to use as a counter weight.
.assorted lengths of screws
. length of plastic rope
.length of black plastic 20mm conduate
.small loop of fishing line
.2 thin strips of hard board to line U section with
.various blocks of wood
.scraps of chipboard
.pieces of thin plywood large enough to enclose the mirror ball
.mirror ball and motor
The electrical bit
.24v power supply (salvaged from an old amplifier)
.4 single pole change over relays
.4 diodes to suppress the back E.M.F. from the relay coils
.1 change over non latching push switch.
.3 normaly closed non latching push switches.
.1 24v motor with a 500 to 1 gear box and strong drive shaft for lift winch.
.1 24v motor with a 200 to 1 gear box for door winch
.1 mains voltage coil change over relay for control from stage lighting dimmer
.2 scraps of strip board
.lots of wire. I used Red, Black, Orange and Green
electric finnish nail gun
set of hole saws
set of drills
20mm spade drill bit
Step 2: The Track
I placed one steel U section in the middle of a length of wood and drilled an 8mm hole through the wood at each end of the U section.
I sandwiched a piece of wood between two U sections and used M8 bolts to secure it all together. The extra length of bolt was used to stop the saddle falling off the end of the track.
The wood sticking out each end was used to fix the track to the roof structue.
I later discovered that the saddle kept getting stuck half-way, because the inner surface of my track was too lumpy, so I cut a strip of hardboard to line the inside of the two U sections.
Step 3: The Saddle
I placed the wheels in the track and measured the distance between axels.
The saddle was made out of a scrap of chipboard flooring. I cut two rectangular pieces for the sides, temporarily screwed them together so that they didn't move when I marked and drilled holes for the wheel axels.
The 2 front wheels were now fitted using washers to space the wheels away from the side pieces.The wheels run freely because the tube down the centre is slightly wider than the wheel.
The 3rd rear wheel was fitted in the same way once the saddle had been fitted around the track.
A mounting place for the mirror ball motor was then screwed under the front overhang.
Step 4: Lift Winch
This was something I already had and all that was needed was a large pulley for the drive shaft. This was made from several disks cut out of chipboard with a set of hole saws and nailed together. The centre hole was enlarged to fit the output shaft with a 20mm spade bit. A metal collar that screwed to the base of the drive shaft was nailed to the end of the stack and fixed in place with 3 grub screws in the collar. The cut surface of chipboard was left rough to improve friction on the rope. the pulley is long enough for the rope to simply track along the pull as it turns so the rope should not get too worn by the rough surface.
2 eyes were screwed into the top of the saddle behind the track for the rope to be tied to. the rope then made one turn around the motor pull and was then continued up to a freely moving pull, across to another free pull. back down again and tied to the counterweight.
See picture ? for wiring diagram for winch relays and motor.
2 screws were used to activate the top and bottom limit switches.
Note, the top limit switch needs to be a change over type, so that the doors can be closed after the mirror ball has been lifted out of the way.
Step 5: Mirror Ball Enclosure
I had a 400mm ball so the enclosure was made about 450mm square.
because of where the ball descends from only the bottom 15mm of the front edge is visible so to save on plywood the front piece is only 25mm high. the back panel was also made up from 2 pieces to make best use of the bits of ply I had. The corners were joined with triangular scraps of wood and a finish nail gun.
The hinges were test fitted by just screwing through the plywood and leaving the screws sticking out the other side. To fit the hinge securely a strip of wood was run along the bottom of each side of the box for the screws to go into. On the doors,a block was just placed over each hinge so that the ball could get out without hitting anything. The weight of these blocks on the inside also helped to shift the centre of gravity of the doors so that they fall slightly past vertical when open.
A loop of fishing line was fixed to the back corner of each door and passed through a U bolt and up to a large wheel attached to the motor shaft. This wheel is large enough to wind in enough fishing line to close the doors without making a full revolution, because when the doors are open a nail in the wheel opens the limit switch. When the doors are closed, another limit switch on the bottom edge of the back panel is opened and as the wheel never does a full revolution the nail can't hit the back of the other limit switch.
To reduce noise from motor as the doors close I added a 12v regulator to the door closing circuit to slow down the motor.
Step 6: Installation and Finishing Off
The first part to be installed was the track and saddle. Once it was secured, the mirror ball was hauled up with a rope and temporarily secured in the"hidden" position so that the conduit length could be measured.
I cut a slot in each end of the conduit and drilled a 6 mm hole 90 degrees to the slot. The rings on the mirror ball and motor were then fitted into the slots and fixed in place by passing an M6 bolt through the holes and rings.
Once the mirror ball and motor were joined together, the rope could be tied to the saddle, threaded through the pulleys and attached to the counter weight bucket. More weight was then added until the system was balanced. After experimenting I found that it was best if the counter weight was slightly heaver than the saddle.
With the mirror ball in it's hidden position the box was then hauled up and fixed in position on the back of the stage arch. once this was done the two control circuits were wired together and the whole mecanisim was tested.
The mains coil relay used to control the drive circuits from the lighting desk was originally used to stop a hot warter pump when the boiler was switched off by a time switch. Because this relay was pluged into a dimmer circuit an extra lamp was needed to add more load, otherwise the channel would not turn off fully and the relay would buzz loudly. The extra lamp can just be seen in picture 1. I decided to fit a red filter and point it up into the roof vent to confuse anyone who happened to walk past outside.
the last 2 pictures are an animation of the ball in motion and a circuit diagram of the full system.