Quick and Easy LED Darkroom Safelight

by rblevy in Circuits > Cameras

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Quick and Easy LED Darkroom Safelight

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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 and Materials

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.

Cutting the Perfboard to Size

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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.

Adding Power and Ground Wires and LEDs

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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.

Adding Resistors and Soldering Everything Together.

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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.

Testing It

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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.

Finishing Up

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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!