Quick and Easy LED Darkroom Safelight

This is an LED-based safelight for black and white printing in a traditional darkroom. The virtue of this particular design is that it is quick, easy, and cheap to build, especially the enclosure. If I were a purist, I would have checked the emission spectrum of my LEDs, tested for fogging, tweaked the brightness, et cetera. I haven't done any of that, but I can attest that I get decent illumination of my small (ca. 8x10 foot) space, and I get nice prints using Ilford Multigrade paper. This safelight uses 0.3 W of power, compared to about 15 W for a traditional safelight with an incandescent bulb and filter.

****Disclaimer: you should not build this or any other electrical device unless you have a grasp of current-voltage relationships and general electrical safety. I am only documenting what I myself have done here, and this information is not guaranteed error-free -- use it at your own risk. For safety, I recommend using several AA batteries in series in place of the 5V power supply I used here.****

Equipment:

- Standard hobby electronics stuff: soldering iron, pliers, wire cutter/stripper, small screwdriver (optional), "third hand" or vise, voltage/current meter (useful but not necessary).

- Jeweler's saw, X-Acto knife, or whatever you can find to cut fiberglass board. Dust mask.

Materials:

- Diffuser from a General Electric super slim 8" under-cabinet fluorescent fixture, or equivalent.

- 10 Red, orange, or amber ultrabright LEDs

- 10 470 Ohm (or similar) 1/4 W resistors

- 22-24 gauge bare copper wire

- 2-conductor wire, such as speaker wire

- Flux-core solder

- unclad perforated electronic prototyping board, a.ka. perfboard, available at Radio Shack

- 5V "wall wart" DC power supply.

- 2 position small electrical terminal block (optional).

Notes: the peak wavelength of the LEDs should be >600 nm, with very little power below, 580 nm. Red is "safest", but orange or amber is popular because you can get a better sense of print contrast. To be on the safe side, I might have bought from a reputable supplier like Digikey and compared the emission profile on the datasheet to the spectral sensitivity of my paper. In reality, I used cheap orange LEDs from China and had no problems.

I needed a diffuser because the viewing angle of my LEDs was rather narrow. I had this extruded U shape sitting around from a fluorescent fixture I'd bought to make a light box. It turned out to be just perfect because of how the perfboard slides neatly in place and stays put by friction alone

I cut the perfboard to size using a jeweler's saw, which makes precise cuts and uses cheap disposable blades. I wore a dust mask because the fiberglass dust is nasty. Scoring the board with an X-Acto knife and breaking it might also have worked. It should be kept in mind that fiberglass will ruin just about any metal cutting tool.

I ran a length of bare copper wire across the top of the perfboard to serve as a power rail, and a similar ground wire across the bottom. I placed the LEDs in between at equidistant intervals, paying attention to the LED polarity - the long leg has to connect to power.

Each LED needs a resistor. I connected one leg of the resistor to the short leg of the LED, and the other to the ground line. I soldered everything together, and trimmed the leads.

Note: there are lots of tutorials explaining how to choose resistor values for LEDs and how things need to be connected. I chose 470 ohm resistors because that gives about 6 mA of current per LED with a 5V source, well within the stated tolerance. I could have gone a little brighter or dimmer, or I could have made a brightness control using a variable voltage regulator like an LM317, or controlled brightness digitally with PWM. The design shown here is the simplest I could think of.

I hooked up the power and ground lines to a 5V DC power source. If one of the LEDs hadn't lit it probably would have been wrong polarity or a cold solder joint. But they all lit up. I also checked the current with a meter to be sure it was as expected.

I added a nice little terminal block to make it easier to connect and disconnect the safelight from the power supply. Finally, I velcroed the light to the wall of my darkroom. All done!