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LED Enlarger

A few weeks ago I came across  Lumileds , these are very bright LED's  Originaly for a project at work I thought 'automatic variable contrast enlarger', this turned out to be much more fun than the work thing.

Outline
Black and white variable contrast printing papers use different emulsions for different contrast ratings.  These emulsions have different colour sensitivities.  To vary the contrast blue is used for high contrast and green for low contrast and a mixture for in-between grades (as they are called).  Most enlargers employ filters or two lamps to vary the colour of the light, here I use 2 blue and 2 green leds.

Only 4 leds????
Yes they are very bright.

The electronics
The LED's take 350mA at about 3.5V, the pairs are in series, driven from a 10V SMPS, something I found in a wall cube, and PWM controlled current sink.  The PWM comes from a PIC16F84.  Display segment a, b, c, d, e, g, dp, (I will leave it up to you to see why I don't have f) multipex for the grade digit, safelight on and enlarger on from the analyser are listened to by the PIC to decide what to do.  To make life simple the +10 volt line to the leds is switched by a small relay from enlarger on, this is the ultimate in an interrupt.  The  LEDENCCT  circuit diagram is here.  I have one leg left on the PIC, sugestions please!

Software
Simple assembly program  LEDEN.ASM does all the work, not the smallest it could be, untidy and a lot of debug code left in.  It fits and works so why bother making it smaller and tidier?  Test strip mode had me thinking, it puts a number in the grade digit, hence the delay.

Optics
The leds are mounted on a heatsink, well the do dissipate about 5W at maximum.  A simple 1" x 1" x 2" mirror box topped with a piece of ground glass generates an even patch of light.  A 45 degree mirror deflects the light onto the condensors of my Kaiser enlarger.  Also the LED assembly slides on a track to adjust for an even light over the negative.  The movement needed for different lenses is small,  I am condsidering a vertical system, no mirror.

Pictures of  the parts
 LED's on the heatsink
 Leds on this is with room lights on
 Alignment hole used for setting up the mirror
 Ready for printing
 Electronics in the Analyser case
The first two  pictures  min and max contrast.

Results
Goes from grade 00 to about 4.

The future
Lumileds do a bluer led (how do you spell blowwerr?), I will get one and see if I can obtain grade 5.  I will only need one because I found for the same exposure time 50% blue was needed compared to green.  I might put a red led instead of the other blue to give a whiter focussing light, and stop the electronics getting hotter.

Update...
Got bluer leds, up to grade 4.5.  Didn't do the red led.

Getting the LED's
These are about UKP6 a piece from Future Electronics, unfortunately they want UKP100 min credit card order.  I have an account with them and might be willing to get them for anyone wanting a few. huw@huws.org.uk PCB's etc. mail me.

Credits
 RH Designs , could not ask for better service, wanted for a circuit of the analyser, got one, and offered to do desitometry on my test strips. Hell just for fun might buy a desitometer from them, or build one (next current project?).  Thanks.... 

Another update..
Bought one!

Sharon, at work, aluminiszed my mirror box.
The glass shop, at work, ground glass.
Future Electronics, the LED's

The End
Takes the hassle out of changing filters etc. from BW enlarging.  Makes using the Analyser a sinch.  All I need to do now is take better pictures.