1
\$\begingroup\$

Electronics newbie here and looking for a bit of advice on a project.

I’m trying to build an LED light panel that will serve as a light source for a 4x5 large format enlarger. After being let down by a commercial product I waited for over 3 months to arrive only to find it didn’t even fit never mind work properly (I won’t mention any names).. I’ve decided to try and build one myself. I’ve done a few Arduino projects with various components, so I would say I have a basic understanding but having read loads recently on LED circuit layouts I must admit I’m still thoroughly confused!

So what I’m trying to build is an array of R,G and B LEDs with the 3 colours on individual dimmable channels so I can mix the colours (to be honest it’s only the blue and green that are critical as only these are needed for black and white negatives). Also, I’ve gone for individual colours rather than a combined single LED as the wavelengths (especially of the blue) has to be fairly specific in order to be able to use multi-contrast paper, so I’ve now got my LEDs which are fairly typical values:

Red: 2.2V drop @ 20mA

Blue and Green: both 3.3V drop @ 20mA

One other thing is that the panel would be powered by an external timer (not from an Arduino) so a transformer of some sort would be involved, my main timer outputs 240V AC - I’ve put 9v in the circuit below as I think this should be enough, but could be changed if not.

I’ve considered various options, but the one I’m leaning to at the moment is to build it in clusters with each cluster having 2 of each R,G and B LEDs in series, then linking then all the clusters in parallel (see below)

LED Circuit layout

Ignoring the fact that there’s no dimming ability in the above, does this approach seem right or would there be a better way of doing this? With 20 clusters (so 40 LEDs on each channel), my thoughts/calculations would be that 9v would be supplied on each channel and the total circuit would use 1.2A. I may be barking up the wrong tree here and got it completely wrong.. apologies for the very long question but any advice of thoughts would be greatly appreciated!

\$\endgroup\$
1
  • \$\begingroup\$ Seems about right. (I didn't bother to check the resistor values). Though note it's easier to control the negative side of the LEDs for dimming and wire all the positives together. \$\endgroup\$
    – user253751
    Nov 4, 2021 at 15:56

2 Answers 2

2
\$\begingroup\$

One other thing is that the panel would be powered by an external timer (not from an Arduino) so a transformer of some sort would be involved

This is not going to work because you'll be using an AC-DC switching power supply or driver for your LEDs, and these have enough storage capacitance to stay on for a significant time when mains power is cut off. So if you use a standard darkroom timer with that, the switching power supply will add some unspecified and potentially variable delay both at turn-on and turn-off, and that would suck.

On top of that, I assume for multicontrast paper you will want to control the power of green and blue LEDs separately.

So if you use one timer, that means both colors will be on for the same amount of time, so then you need two dimmers, for example PWM, with one dimmer per color. So you set the time on the timer, chose a contrast setting, which decides the PWM ratio of both colors. In this case, if you use the timer to control the main power supply, then when it turns on and off its voltage will ramp up and down. The red LEDs and the green LEDs will turn on at different voltages, so the reds will shine for longer... the dimmers may also do funky stuff at power-up, screwing up your contrast settings... and the turn-off time of the power supply depends on the discharge time of the high voltage capacitors, which depend on the current drawn by the LEDs, which depends on the contrast settings. So it will be a mess.

If you have RGB LEDs, you can have a nice feature: by enabling only the red ones, you can even check focus and framing with the paper in the easel. Could be useful if you want to do multiple exposures... Then you need the power supply to be always on (not controlled by the timer) so the red LEDs will light when you flip a switch or turn a dimmer knob.

So, if you have a darkroom timer that you like then the simplest option would be to use it to drive a relay, or an AC optocoupler, that either controls a MOSFET to turn on the LEDs, or simply sends a signal to the PWM dimmers so that they turn on. That way the timing of the light would be exactly what you set on the darkroom timer, and the power supply is always on. That also saves a power supply for the arduino if you have one, or for extra circuits.

Next question is the optics: you need uniform lighting on the film.

Usually in an oldskool condenser enlarger that would be an incandescent point source with a big condenser lens in front that produces parallel light rays.

To emulate that with LEDs you would need a powerful yet small LED, so it would have to be a COB, but that is only available in standard RGB wavelengths, and you need nonstandard wavelengths.

So you'll probably end up with lots of LED chips soldered on a 4x5 PCB instead. This will not produce parallel light rays, so it will be more like a diffusion enlarger instead of a condenser enlarger, and it will produce different looking prints. I've never used a diffuser enlarger, so I'm not the one to ask about the difference between diffuser and condenser, but I heard , purists really care about it, and if you're using large format I'll assume you care about every little detail that affects quality, so I should mention it.

Anyway, because individual LEDs will produce different light outputs, at the same current, there is a risk of hotspots or non-uniform lighting on your film. So you should try to get binned LEDs, then make a really good diffuser. Perhaps a mirror box with diffuser sheets on top and bottom. You can take a picture with a digital camera at low exposure to judge uniformity of lighting. If you use resistors, you could add several footprints for resistors in parallel, so if you notice some of the LEDs are a bit weak, you can easily adjust their resistor values.

If you need lots of power I would recommend a metal core PCB with a heatsink on the back, you can get them at nice prices from jlcpcb for example. I'm not sure it is a good idea to use a fan for cooling, if it vibrates that could reduce sharpness. In any case thermal design is important because light output of LEDs depends on temperature so if they heat too much, both your timings and contrast settings will be off.

Another issue is led Vf (forward voltage). Unless the LEDs are accurately binned, it doesn't match very well between LEDs, and it also depends on temperature. So if you use resistors to set the current, it will increase with temperature. LED efficiency decreases with higher temperature. Don't count on these two compensating each other. So if you do a series of prints, the first one will be different from the last as the LED panel heats up. Since red and green have the same Vf temperature coefficient but different resistors, they will drift differently, so the first print and the last will also have different contrast. Temperature also changes the wavelengths.

Also if you use a low thermal mass PCB, or if the thermal transfer between the LEDs and PCB is not very good, then it will heat quickly, so a one second exposure may output a bit more light than a half second exposure. What a mess lol.

So yeah, might be a good idea to use a metal core PCB, screwed on top of a big heat sink. Even if power is quite low, thermal mass and constant temperature means less headache, and it's not much more effort compared to soldering all those LEDs. A big chunk of aluminium will also spread the heat and prevent the center from heating more than the sides.

Make sure you have enough LEDs and enough power to have usable exposure times... it's better to make a PCB with too many footprints and not solder the LEDs, than the other way around.

Green and blue LEDs should be as close as possible and side by side to try to make them into a dual-color point source, like that:

enter image description here

Since the paper doesn't care about red, placement of reds is not critical.

Now if you want current to be constant and not depend on LED Vf, matching, temperature, etc, then it would be better to use current sources.

enter image description here

This one is nice, cheap, and temperature compensated. The bipolar transistor can also be substituted with a MOSFET, in any case it should be chosen to handle the current and dissipation. You can apply PWM to the top of the resistor (labeled "Vbatt") and control the output. TL431 sets voltage on Rs to 2.5V, so current is simply 2.5V/Rs. No need for 0.1%, a cheap 1% resistor is fine.

With a 24V power supply instead of 9V, you can wire a lot more LEDs in series, and use less current sources. You can also use a recycled 19V laptop brick.

If you use 24V, then you can wire the blue LEDs in series in strings of 6, which will give a maximum voltage of about 20V, leaving 2.5V on the current setting resistor Rs above and 1.5V Vce for the transistor. If you use 20mA current, that means Rs=120 ohm ; it will dissipate 50mW so a 1/4W resistor is fine, and the transistor can be a jellybean 2N3904, BC546/7 or whatever.

If you also group the reds in strings of 6 that will only make 10.8V so you can wire more in series to prevent the transistor from having too much Vce, which would then need to be a higher power one with a heatsink.

\$\endgroup\$
11
  • \$\begingroup\$ I almost cannot imagine using LEDs for this lighting function. I've seen far far too many spectrophotometer results. (It was work I did for Siemens-OSRAM when OSRAM was part of Siements for binning LEDs of all kinds.) It certainly cannot work well as the result of sheer ignorance, anyway. I like many of your other points. +1. But I don't see any manner of "tweeking" around with blue and green as ever being sufficiently good for professional work. That said, I'm sure folks are doing that these days, too. Doesn't mean it works well. \$\endgroup\$
    – jonk
    Nov 4, 2021 at 17:11
  • \$\begingroup\$ I've never used a LED enlarger, only incandescent, so I don't know what sort of results would come out, but yeah the more I think about it... \$\endgroup\$
    – bobflux
    Nov 4, 2021 at 17:33
  • \$\begingroup\$ @jonk the different colours are to select the contrast of multigrade black and white paper. For sure the workflow would need calibrating but people have used led lights for this before. Actually one project I've seen used a screen rather than discrete LEDs. \$\endgroup\$
    – DamienD
    Nov 4, 2021 at 17:57
  • 1
    \$\begingroup\$ @DamienD I'm no expert in photographic papers. But I have more than a decade of experience with analysis regarding human color vision and similar experience calibrating RGB LEDs for television displays and outdoor LED displays, as well as long experience with their spectrums. All of that experience makes me very wary. That said, if people are fine using RGB LEDs like this, who am I to argue with what others accept? But if I were involved in a team producing this product, I'd demand good, quantitative experimental results before I'd buy into it. And I never doubted people are doing it. \$\endgroup\$
    – jonk
    Nov 4, 2021 at 18:42
  • \$\begingroup\$ @jonk essentially multigrade papers contain two or three blue- and green-sensitive emulsions with different contrast properties and varying sensitivity to green light. You then use the blue-green balance to dial in the overal contrast of the print, exposing the two emulsions in different ratios. \$\endgroup\$
    – DamienD
    Nov 4, 2021 at 19:01
0
\$\begingroup\$

For the large film format you should have a uniform illumination over the full format. I recommend to use 40 LEDs selected for equal brightness by the manufacturer. A diffuser would help to get uniform illumination both above a LED and between several LEDs.

There is a special Poly (methyl methacrylate) (PMMA), also known as plexiglass panel material for the use as a diffuser. I used such a material with success for an image processing application, it was called Satinice.

If you want exact exposure timing, you should not switch the 230 V AC input to the power supply, you should switch the DC output voltage. The capacitors in the power supply would power the LEDs for a short time after the end of exposure timer.

\$\endgroup\$
4
  • \$\begingroup\$ The enlarger will already contain optics to diffuse a point light source though. \$\endgroup\$
    – DamienD
    Nov 4, 2021 at 17:50
  • \$\begingroup\$ @DamienD An array of 120 LEDs can't be a point light source. \$\endgroup\$
    – Uwe
    Nov 4, 2021 at 18:12
  • \$\begingroup\$ what I mean is that a large array is not necessarily required to get uniform lighting, although it is one option. \$\endgroup\$
    – DamienD
    Nov 4, 2021 at 18:15
  • \$\begingroup\$ I've done some tests with a mockup box that would attach to the back of my 4x5 and a small group of the blue and green leds, first sanded to diffuse them, then into box with acrylic mirrors on the sides and then the light passes through two layers of diffusion (Lee 216 film), the results are promising but you do loose a lot of light which is why I'm thinking I'll need 40 leds of each colour \$\endgroup\$
    – Deanm
    Nov 4, 2021 at 19:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.