Can I Create a Strong Enough Electromagnet for My Video Installation?

[nbontra]

I'm a photographer at the University of Houston, and upon construction of a video installation I need a device that will alternatively cover the lenses of two different projectors. The first thing that came to mind was an electromagnet that would control the raising of different arms.

Pressure plates are installed so that the current from 120v outlet is shifted from left to right, depending on where you stand in the room. I would like the current to be directed to two different electromagnets so that it will pull on the arm and cover the lens of the opposite projector.

My question is on the technical aspect of the electromagnets. My only experience has been the traditional wire wrapped around a nail with a battery attached. While I'm sure the 120volt current is definitely too much, what would be the strongest electromagnet I could construct while maintaining safety in the gallery space?

Thank you in advance for any help. A crude drawing has been attached to better illustrate my dilemma.

-Nick B

 

[FredGarvin]

I would recommend looking at linear solenoid actuators. You could have one on each projector and each switch would control each actuator independently. The only thing with what you have there is that the electromagnet would always be on to keep one projector open. That may be an issue with the solenoid actuators as well. They don't like staying energized for really long periods because of the heat build up.

If you can provide more information on what you want to do we can help narrow down the search for something economical and off the shelf.

 

[AlephZero]

There may be an easier way, depending exactly what you want to do. You could use projectors with automatic slide changers. "Cover the lens" by using a dummy slide.

A home security company should be able to supply the pressure plates (as used for intruder detection). The interface to replace the "change slide" switch would be easy for old-style projectors where it really was a mechanical switch on the end of a wire plugged into the projector - modern remote controls may make it more difficult.

 

[nbontra]

I read a little bit on the linear solenoid actuators, and it seems like that would be an ideal solution, especially with the low voltage input. They would be powered for around 30 seconds at a time, but could go as long as a constant 4 minutes.

I wish I could just go with the "dummy slide" concept, but I'm using digital projectors hooked up to dvd players. Originally I had planned to just plug the power source straight to the projectors, but then realized there is a 10 second delay for powering up.

The only other option is to use small field monitors I have, but they only have a TNC input for the video feed. I believe I would need to have an RCA>BNC>TNC connection; for which the adapters are not cheap.

Thanks so much for the quick replies, I'm having trouble finding pricing for the actuators online though, any ballpark figures?

 

[berkeman]

Could you maybe stack the projectors, and then just use a rotating wheel with half blocked off? Then it's just a problem of a small motor and position sensors.

 

[FredGarvin]

Is there any chance of hooking the floor switches to the main power of each unit? Just turn the whole thing off for a bit? That may not work since they sound like they would need to "reboot" or something along those lines. Just thinking out loud...

 

[berkeman]

I wish I could just go with the "dummy slide" concept, but I'm using digital projectors hooked up to dvd players. Originally I had planned to just plug the power source straight to the projectors, but then realized there is a 10 second delay for powering up.

I missed this the first time through. Do the DVD players have a "blank output" capability? They probably do.

Alternately, I think you can go with a rotational actuator and limit switches even if you leave the projectors side-by-side as shown in your sketch. Think along a different axial dimension. Run a bar along an axis that connects the two lenses, maybe 6 inches in front of the lenses and raised 6 inches. Then put two paddles on each end of the rod, kind of like opposing flags. The paddles at one end are offset from those at the other end by 90 degrees.

Connect up a simple DC motor and a couple normally-closed limit switches, and applying power to the motor through a closed limit switch will turn the bar and paddles 90 degrees until the limit switch is opened. Then when you want the other projector to be let through, apply power to the DC motor through the other limit switch circuit, and it rotates another 90 degrees until the switch is opened. You can use the paddles themselves to hit the limit switches. You can get creative with other ways of running the motor, but being able to use a simple DC motor to do what you want seems like it would have value.

 

[berkeman]

Hey, you could even use opposite polarity DC voltages to turn this rod back and forth, if that makes it simpler in the final implementation.

 

[berkeman]

Oh, BTW. You should gear down the motor so that it spins several times for each 90 degree rotation. You can either just buy a motor with a small gearbox on it, or make your own gear reduction with two pulleys and a rubber belt.

 

[nbontra]

I like the idea of using the rod and paddles, it seems much more reliable than my original concept of the balancing weights. Thanks so much for helping me out, I'll be sure to post the results!

 

[Danger]

If budgetary considerations allow it, might I suggest just putting liquid crystal windows in front of the lenses?