Home Grown 8x10 Variable Contrast Light Source

[redshift]

146941I just finished upgrading my white light LED 8x10" enlarger light source to variable contrast LED. I had 12 LEDs in the white light source and wanted to keep the same level of illumination. This time I used 12 green and 12 royal blue LEDs with a 20K Ohm potentiometer to control the color from all green, mixed green and blue, to all blue. The pot is going to be replaced with a version that has better resolution.

146940

[ic-racer]

Can you post more info on the part numbers of the leds and the power supplies. Also, two pots might be better. Depending on how you will use it. With a single pot, it might be unlikely to have the same printing time through the range. With two pots and some numbers on the knobs, one could make a table of knob settings to give the same printing time.

[redshift]

This is what I ordered from LED supply.


24VDC25A Desktop Power Supply - 24VDC 2.5AMP
DC-PA-2.5-F Screw-In Terminal Power Plugs

Plug Type : Female Plug Size : 2.5mm
0A011-D-V-700 FlexBlock Buck-Boost DC LED Driver
Options:
Output Current : 700mA 2ea.
20K-POT Potentiometer - 20K Ohm
24AWG Wire - 24 Gauge Stranded AWG Wire
Options:
Color : Red
24AWG Wire - 24 Gauge Stranded AWG Wire
Options:
Color : White
24AWG Wire - 24 Gauge Stranded AWG Wire
Options:
Color : Black
CREEXTE-ROY-1 Cree XLamp XT-E Royal-Blue LED(s)
Options:
Board Configuration : Single 1-Up 12ea
A001 HexaTherm Tape - Thermally Conductive Double-Side Tape
Options:
Sheet Size : 10 Sticker Sheet 30 ea
CREEXPE2-GRN-1 Cree XLamp XP-E2 Color High Power LED Star
Options:
Board Configuration : Single 1 Up Color : Green 530nm 12ea

I already had the aluminum plate from "cut to size" and the heat sink from another online vendor.

[Peter De Smidt]

Nicely done!

[redshift]

Thanks Peter!


Nicely done!

[Geary Lyons]

I had great success with PWM, (pulse width modulation) controls. This technology is a better match than potentiometers. Good pots aren't cheap. Cheap pots are not very long lasting as the plastic carbon tracks wear quickly in between frequently used positions.

I, too, would suggest individual control for blue and green. This will allow better control and facilitate split contrast printing.

My 8x10 head has been in use about 8 years. I added some UV just after building to get closer to 5 contrast, which the royal blue would not give. The selection of LED's today is much better. The build quality and spectrum accuracy is also much better. This especially true of the UV LED's. You can get narrow spectrum UV LED's to match the need for 5 contrast. They weren't available when I built the 8x10 head. Bear in mind that:
1) You do not need a high level of UV
2) UV losses will be high due to the diffuser glass, carrier glass and lens glass absorption.
3) Plan the UV for equal diffusion across the printing area.

I am planing to convert my DeVere 504 color head to LED in the next few weeks. New LED's on the way!!

Cheers,
Geary

[redshift]

I connected a common wire from both controllers to the wiper of the potentiometer. The other two terminals were connected individually to each controller. The idea is one color will be ramping up as the other is tapering off with both on 50/50 in the middle. I have a Vishay Spectrol 10 turn pot on the way. Hopefully this gives me some repeatability.

If someone can give me instructions on how to build a replacement for the pot that will give a proportional distribution of the green and blue LED output I'd give it a try. If there is a better item already available I'd happy to consider it as well.

thanks for the help!


I had great success with PWM, (pulse width modulation) controls. This technology is a better match than potentiometers. Good pots aren't cheap. Cheap pots are not very long lasting as the plastic carbon tracks wear quickly in between frequently used positions.

I, too, would suggest individual control for blue and green. This will allow better control and facilitate split contrast printing.

My 8x10 head has been in use about 8 years. I added some UV just after building to get closer to 5 contrast, which the royal blue would not give. The selection of LED's today is much better. The build quality and spectrum accuracy is also much better. This especially true of the UV LED's. You can get narrow spectrum UV LED's to match the need for 5 contrast. They weren't available when I built the 8x10 head. Bear in mind that:
1) You do not need a high level of UV
2) UV losses will be high due to the diffuser glass, carrier glass and lens glass absorption.
3) Plan the UV for equal diffusion across the printing area.

I am planing to convert my DeVere 504 color head to LED in the next few weeks. New LED's on the way!!

Cheers,
Geary

[ic-racer]

The paper's response to the different colored LEDS is not going to be predicted only by the LED voltage or the pulse width (not sure which method your existing system uses).
The simplest is to have two knobs and use a table that you make yourself, if you want constant printing times. If using split grade printing or if you don't care about constant printing times, you don't need anything complicated. If you want it computer controlled so you just punch in a contrast grade and have constant printing times and you could hook an Arduino and program that with your own data.

Another simple, repeatable, solution is to hook an electronic timer to each color. You can do pseudo-split grade. Rather than controlling the intensity of each LED bank, you control the time. Again, you could make a table or just wing it; more green makes the highlights darker and less blue makes the shadows lighter.

[BetterSense]

I microcontroller would be tge best way. Then you could easily re-flash the programming based in experiment or even have switchable curves for different papers. I could program one and send to you if you tell me the interface.

[barnacle]

Microcontroller's the best way, as above, because you can include a map with fixed points for the grades you want to print.

Controlling LEDs is best done at constant current, either changing the current, or PWM switching at a constant current.

To maintain a (vaguely) constant perceived light output, you should consider using a log-antilog potentiometer pair, and use the two outputs to control your brightness, whether you use voltage-controlled current or PWM.

There are many ways to skin a cat...

Neil

[Tin Can]

Looking good and all suggestions are great. Availability of these components is expanding. DIY has never been better.

[onnect17]

Allow me some comments.

The dimmer needs some protection. I did not see any components listed to do it.

Not sure how much light power you need but I would make sure is not too much. That would make the exposure times a little harder to control if are under 2 secs.

I am aware you placed a heatsink in the back, but just to make sure, you should stress test the lamp inside the enlarger for 24 hours at full power and measure the light output of each color when cool and after 1 hour. The reason is the light output of the leds changes slightly with the temperature and is different in each color.

Feel free to reach me if you have any questions related to the electronics.