Hi, I'm a projectionist in a movie theater. I need to convert our old white light incandescent sound "exciter" lamps over to red LED.The entire movie industry is changing over to having the soundtracks on film based on cyan dye, instead of the traditional silver based soundtrack. The big difference is that the new way is cheaper for the studios to make. Another big difference is that almost every movie projector in the world will need the new red LED readers. Most have already changed over.The soundtrack runs along one side of the film, and the sound information is a tranparent wavy stripe which runs down the middle of the track. The old style soundtracks are opaque black, and the new tracks are semi-transparent cyan blue. There is an "exciter" lamp on one side of the film which shines through a barrel with lenses in it, which only lets a slit of light hit the film. On the other side of the film is a solar cell which picks up the light information.We have a movie starting Thursday which is our first cyan film. I need to rig up a red LED light source ASAP. I saw instructions on how to do this on (search using word "homebrew"), but I need more info. I need your help in choosing a proper resistor for the LED.I ordered and received 2 Luxeon Star red LEDs. Here are their specs:Part # LXHL-MD1DMax Current - 350ma (300ma or under may be preferable so I won't have to add a heat sink)Max Voltage - 3.5vdcColor - RedWavelength - 625 NMLight Dispersal - LAMBERTIANTypical Flux (Lumens) - 44The incandescent exciter lamp it is replacing is rated at 9v 4a 36w.I've checked the power going to the light, and it is only 7.5dc. That's OK because most film technicians set the power lower on exciter lamps to make them last longer. That's what I've heard, anyway. That power supply is adjustable, but I'd prefer to leave it at 7.5v so I can still put in the old lamps whenever I need them. The power supply is rated at 5-10vdc and 5a.So, what kind of resistor would I use to get this LED to...

Hi,You have a device (LED) that will drop 3.5 volts at 300 mA.Your supply is 7.5 volts. You need a resistor that will drop 4.0 volts at 300 mA. R=E/IP=EI, Closest standard values 10-15 Ohms 2 Watt.Tom

Right resistor for high power LED for sound reader?

Aug 09, 2005 22 Replies

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Rich Grise 20 years ago

Well, you _are_ the light guy. :-)

Cheers! Rich

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redbelly 20 years ago

Agreed. No disguised homework questions, just somebody trying to build a circuit and put it to use.

I'm curious as to how Mitch made out. The thing was (hopefully) to have been built and working by yesterday, Aug. 11.

Mark

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stratus46 20 years ago

I was wondering, since you're trying to use the cyan audio, does the light source HAVE to be a red LED or could you put a red filter in front of your incandescent lamp?

I've worked on the photo lamp house used on a Cintel (Rank) telecine and at least for that machine, adequate light alignment is difficult. Dolby labs makes LED replacements for the Cintel machine. GG

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