Domenico Rota

The story:

Given the umpteenth lockdown, an old electronic project is back in my hands, useful for photographer friends who still use the darkroom enlargers, chemicals, etc.

Today, those who engage in this hobby can find cheap darkroom  magnifiers of all kinds, for black and white or for color photos, the latter usually with dichroic filters and powerful halogen lights (in many cases now unavailable). 

More together  the for old enlarger it was necessary to use big voltage transformers (and sometimes even a second 110V transformer...)  all obsolete junk that in addition to consuming kilowatts can also be dangerous! in addition dichroic filters are badly adapted to the use with black and white, the restoration of these magnifiers with lamps and glass filters and  timers can cost a lot,  the final result is then not very usable...

Expecially the older color magnifiers, on the market on sale,  as is, for parts or not working are the best deals cause their price is extremely low !

My idea is to use these kind of old darkroom-enlargers by renewing the electronics and adapting to our times!

The project is called "LE2" and it is the achronim of LED ENLARGER version 2  (yep it's an evolution of LE1!!!) 

LE2 allows you to replace the magnifier light bulb,  with high-efficiency LEDs that allow you to manage the light, contrast and density of the print that occurs simply by varying the percentages of color components  RGB and UV.

The retrofit of the old enlarger must also include a precise timer and important options such as a darkroom  magnifier exposure meter to correctly calculate the exposure time of the photo paper.

In the first version of 2012 the RGB LEDs had limited power and worked practically only at 12V, nowadays instead there are very sophisticated LEDs that use power supplies up to 48V, to facilitate the transformation of the magnifiers I have created an interface "SHIELD" for the mini controller ARDUINO UNO Rev3.

Basically it is an interface with a luminous keypad... luminous, because it gets dark in the darkroom!, A three digit 7-segment LED display and four N-channel Q-Fet that can manage practically all the power LEDs on the market.

To the initial prototipe, today I also added a high voltage LDO that allows you to use the most disparate DC power supplies (from 10V to 48V) the shield board in turn, powers the Arduino controller that contains the management program.

The news does not end there ... there is also a buzzer to emit sounds when attention is required, and, icing on the cake, also an i2c bus digital light meter.

 In short, "LE2" allows You to have:
- LED light with selection of the desired DEGREE of contrast (filtration Grade)
- EXPOSURE TIMER from 0.1 to 600 Seconds!
- Start and stop of on-the-fly exposure for masking
- LIGHT MANAGEMENT:
- Focus (with WHITE light)
-  Frame the photos without margin (using RED light)
- Selectable light output power 
- SPOT light meter with automatic exposure calculation according to the type of light / negative / magnification chosen.
- Management of an external relay to use LE2 also with incandescent light bulb magnifiers (limited to use as a timer and darkroom exposure meter)
evrything  on a 60x90mm electronic board !

LE2 caratteristics

working for real in the darkroom I studied the best way to use LEDs with magnifiers for real, not just for play !

In circulation there are different types of this projects some spartans that do not have all the necessary functions, and some, that, instead of having required functions, only focus on graphic displays or alphanumeric LCD LEDs almost all they do not use UltraViolet LEDs, the worst uses complicated controllers with junk things that require tons of coding, library and waste of memory for doing fancy sounds and graphic on little displays, with rotary encoders simulating the old age magnifier controls,  also the majority of them are limited to configurations with RGB LEDs only.

However, in this type of device you need more flexibility, you need to be able to use LEDs of every type and power, and also, why not, to use a relay that allows you to Connect a simple light bulb, for sure you need exposure meter, with digital adjustment and configuration memory,  etc. etc. my decision was to start building the LE2 !  

Original ideas 

Using an Arduino Uno Rev3 for software implementation and concentrating on a different hardware specialized for the need.

Instead of using alphanumeric LCD or oled graphic matrix display I opted on a classic 7 segment three digit RED display, first because red light don't affect the paper, second because it's more readable expecially in the darkroom, plus also the four push button are lit with two LED each one (red + green) making posibe three colour, red, green and orange ! 

I study this system to combine the seven segment display , indication  lights, push button and buzzer all together using only few wires of the controller,  to do so everything  is multiplexed using only a 74HC595 integrated circuit ! 

This serial to parallel shift register each scan pilots digit 0, digit 1, digit 2 and the 8 led (under the push button) for indications  the line that goes to chip select is also passing trough the four push buttons permitting to read each one trough a diode matrix and a pull down resistor, using only a single input line to read all. 

With this ingenious hardware, all the PWM / analog / lines are free to be used for the LED's, using four N-Channel Q-Fet that can stand current of 20 Amp, with proper dissipation it's possible to drive a load of near 100W per channel (the limit is the dimension of PCB traks, and the required dissipation...  

In the photos the output N Channel FET's are soldered all on component side without dissipator, in this configuration they can stand four 10W Led without problems. 

Mounting  this mosfets on solder side with copper blades heat spreaders permit to use a maximum of four 50W Led for a total of 200W !  (useful for very big enlarger up 10x12" and above).

Leaving a lot of resource to the little Arduino board permit to have them available for other interesting things, one is the buzzer that signals when the exposure is done, and the optional BH1750 Lux meter probe !

The exposure meter is an optional, so the software is writed to detect if it is present and adapt the menu and functions ! 

When the exposure meter is available,  at startup the LE2 emits a three beep sound, that signal the BH1750 was detected and the "rdL" and Lux functions are activated.  

Taking in mind the exposimeter usage in the darkroom for real situation there is always the need to adjust the exposure according to paper / negative combinaton are used, to do so as the first exposure read and test is made, after the exposure reading the up / down push button become green, pushing them in this condition permit the photographer to adjust the exposure timing according his needs. 

Once the exposure is adjusted it's easy to print the entire roll hassle free, with few other exposure reading only if the photo require it. 

Wich one is right for you ? 

This project permit a lot of flexibility , it's possible to have the PCB only, the PCB + parts to build it yourself, or LE2 allready made and tested,  tailored with heat spreaders / optionals etc. 

Warehouse at 13-05-2021 :

+ 1  LE2  Assembled & tested

+ 16 pieces of LE2 PCB Boards

+ 2 Set of conponents for complete LE2 with Lux Meter

+ 1 RGB 10W LED 

+ 1 UV 10W LED 

PCB + Manual + SW:

The PCB only option is the easyest, I can send  the PCB with the bill of materials needed, a little operation manual and a link for  download the binary file for the LE2 firmware.  Perhaps this requires skills in SMD soldering (hot air station) Electronics components and schematic knowledge, but it's fantastic to construct it yourself !  

I soldered my first LE2 in only 30 minutes of work !