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Accurate Homebrew Blue

About Those BLUE LEDs...     October 5 2018

As you can see on this page I have tried both pure blue and warm white LEDs over the years. I have come to prefer warm white for various reasons, but one that I cite often is that they are "easier on the eyes". Knowing that this sounds subjective, I decided to do a little research to see if there is any scientific basis for this assertion. This is what I found. Copyrighted images are used under Fair Use Copyright provisions and remain the property of their owners.

Eye sensitivity diagram
Image source: https://en.wikipedia.org/wiki/Color_vision

The chart above shows that human color vision is not very sensitive in the blue region of the spectrum. As we go up in frequency toward mid-band the sensitivity increases markedly and then drops off again as we approach red. The blue, green, yellow, and red lines represent the center frequency of common LEDs. Note that by the time we hit blue the sensitivity is down to 20%.

Spectrum of a common blue LED
Image source: https://commons.wikimedia.org/wiki/File:Blue_LED_Spectrum.png

Here we see the spectrum of a common blue LED. It is a very narrow banded emission centered around a frequency of about 480 nM with virtually no output throughout the rest of the band.

Relative spectral emissions for Cree white LEDs of varying CCT
Image source: https://www.digikey.com/en/articles/techzone/2013/apr/defining-the-color-characteristics-of-white-leds

The above image shows the color spectrum of a "white" LED at various color temperatures. In this case "warm white" is defined as a Correlated Color Temperature of 2600K-3700K. This shows significant optical power across a much larger area of the visible spectrum.

Spectral power distribution of a 25 W incandescent light bulb
Image source: https://en.wikipedia.org/wiki/File:Spectral_power_distribution_of_a_25_W_incandescent_light_bulb.png

This is a typical warm and friendly incandescent lamp. What a difference running a wire at QRO makes!

Differences between daylight, incandescent, fluorescent, halogen, cool white LED and warm white LED
Image source: https://articles.mercola.com/sites/articles/archive/2016/10/23/near-infrared-led-lighting.aspx

And finally, a side-by-side comparison of various different types of lighting.

So now that you have seen more than you really wanted to know on a flippin' radio page, what does it mean? My take away from all this info is that pure blue LEDs simply make the eyes work harder even if they have high output. It is harder to distinguish fine detail and the optical power has to be significantly higher to produce the same relative brightness as warmer and more broad-banded light sources.

Supporting links on the hazards of blue:


July 21 2012, Oct. 5 2018

Although I did for a while quite like the "ice blue" look of the TR-7 with blue LEDs, the monochromatic light from the LEDs rendered the red dB-over-S9 numbers in the S-Meter black. A post from another ham on the Drakelist reflector alerted me to a "warm white frosted" LED pilot lamp replacement from Coin Taker, a supplier of parts for pinball machines. Click HERE to see it. They were cheap enough ($0.69 each, as of this writing), so I ordered a bag of 10. The cost was well under $10, including shipping, and they arrived promptly.

The LEDs are intended as #44/47 replacements, requiring 6.3 VAC @ 40 mA. They are non-polarized, which means that they probably have an internal bridge rectifier. As such, they can easily run from a higher DC voltage with a series dropping resistor. I initially chose to use a 180 Ohm 1/2 Watt resistor in series with each pilot lamp socket but this seemed to make the lamp life unreasonably short. Around 300 Ohms seems to be a workable value with the newer and more efficient lamps (2018). Also added was a strip of bright white plastic diffuser material cut from an empty shampoo bottle behind each lamp, to brighten and even out the illumination. (Note: Drake 4-Line and earlier rigs have a white background behind the lamps which, according to K4OAH, yellows over time and alters the color of the display illumination). Below are some pictures of the results; it seems to be very difficult to accurately capture the color on camera, sometimes they look a bit on the green side, sometimes a little overly blue, but you get the general idea. The red is back! The overall coloring seems much closer to Drake original, which should make the purists happier as well.

Note the shadow on the right from the lamp base, which is normal with the stock lamps.

Still a hot spot in the middle, but not as much as with the homebrew lamps. There's that shadow on the right again!

The TR-7 next to the RV-7, which still has the homebrew "ice blue" lamp. Hotspots are exaggerated by the camera.

Full front shot of the TR-7/RV-7.

Smile for your closeup, Mr. Meter.

You can compare the coloring with the Homebrew Blue photos below to get an idea of the difference. Although there are various opinions of what "Drake original" blue is, the general consensus among restorers is that Lee Filters #172 "Lagoon Blue" gel is very close to the original. You can see a sample of this color HERE. It is also roughly equivalent to Rosco E-Colour 172, which can be seen HERE. The background color of this page is generated by the Hex color codes for the Lee 172 swatch on the Lee web page. Rosco E-Colour 172 is noticably different from Lee 172, as can be seen below (generated by Rosco 172 Hex codes) and the page background color. This may just be differences in the way the colors were rendered for internet presentation, but is probably close to the actual gel color.

Update 10/29/2014! Thanks to Dave, KD2E, who has provided information to the effect that Roscolene #853 is the ORIGINAL Drake filter material. I have added Roscolene #853 right below Rosco E-172. Thanks, Dave!

Rosco E-Colour #172 Lagoon Blue

Roscolene #853 Middle Blue

To further add to the mix of possibilities, Roscolux #67 is referenced as a substitute for Roscolene #853 and similar colors

Roscolux #67 Light Sky Blue

And also recommended by Rosco is E-Color #724 Ocean Blue (Thanks to Stan KM4HQE for the info)

Rosco E-Color #724 Ocean Blue

Joe KC9LAD recommends Roscolene #854 Steel Blue as being identical to NOS Drake filter material.

Roscolene #854 Steel Blue

DISCLAIMER: How accurately these colors render on your computer screen depends on the accuracy of the manufacturer's digital samples and the calibration of your monitor. How they look in your radio will depend on the number of layers used, the color of the background, and the color temperature of the illumination source.

The Original Homebrew Blue

I was preparing to make another set of blue LED pilot lamps for my newly acquired R7A when I spotted an interesting new LED offering on Radio Shack's web site: SKU#276-023, a high-brightness blue LED in a 4-pin dip package. This package is also offered in red, green, and white, but blue was my primary interest for reasons that should be obvious. My mind immediately jumped to the possibilities....

A trip to a nearby Radio Shack netted me several LEDs and a 276-150 experimenter's circuit board. I stopped at Ace Hardware on the way back to pick up some cheap automotive lamps in the proper form factor to use as base donors.

The LEDs are rated at 20 mA maximum forward current. I decided to use two LEDs in series for each lamp, and calculated that a 390 Ohm, .25 Watt series resistor would be suitable. Measured current with two LEDs fed from 13.8 Volts DC was just under 20 mA.

The bulbs of the pilot lamps were duly sacrificed in homage to the Boatanchor gods, the bases cleaned, and the Radio Shack circuit board was cut into small strips with a Dremel cutting wheel to fit the lamp bases. The LEDs and current-limiting resistors were assembled onto the circuit board, and the assembly soldered to the lamp base. It all fit together quite nicely into a neat, compact package. Here are some pictures of the Version 1 assembly (all pictures are clickable):

If you are wondering where the resistor is, it is down inside the lamp base. The end can just be seen to the left of the left LED. This worked out very well for the S-Meter, but the center of illumination was too far away from the lamp base to work well for the PTO. In the second version, I made the circuit board strip shorter and trimmed the lamp base with my Dremel to be even with the end of the lamp socket when inserted. This worked out better, although some experimentation with the position of the lamp socket was still required for the most pleasing illumination. In both versions, the circuit board was secured to the lamp base with only the soldering to the ground point of the LED string. Here is the Version 2 assembly:

The S-Meter illumination is very smooth and even, fading slightly at the edges. PTO illumination has two slightly brighter spots, fading gently to a fairly even blue at the edges of the window. Careful adjustment of the lamp position results in what I think is a cool "cathedral" effect, which I attempted to capture in the photos below. The effect is much more attractive when seen firsthand, of course. I lightly sanded the surface of the LEDs to "frost" them and (hopefully) diffuse the light a little better, but I don't think this had a major impact.

I took the full frontal pic above with the original PTO lamp still in place to show the difference in the new lighting. The greenish tint of the original is slightly exaggerated by the camera, but the difference is striking nonetheless. Overall, I was quite happy with the results. I'm sure that something could be done to diffuse the light better, or perhaps more LEDs could be stacked together to even the glow out. But I think this is pretty darn good for a couple of hours at the bench. The LEDs are pretty bright at this level; a more subdued blue can be achieved by using a higher resistor value. I found that a value of 750 Ohms also works well but is not quite as bright. The only problem I had was that I had to buy more LEDs so I could make up more lamps for my TR7 and RV7, as the LED lamps I brewed up previously weren't as bright. At some point in the future, I will investigate adapting these LEDs to the Drake L7 as well.

One final note: as I was working on the second TR/RV7 combo, I discovered that the lamp socket in the RV7  was wired backwards compared to my two TR7s, R7A, and other RV7. The shell of the socket was wired to +13 Volts, and the center pin was wired to ground. This is undesirable for a couple of reasons, one being that it is easy to accidentally ground out the shell of the lamp socket if you are trying to fine-tune the lamp position with power on. This will blow the 5 Amp internal fuse in the TR7 instantly! The other reason is, of course, I wanted all the LED lamp assemblies to be the same for reasons of compatibility and uniformity. This was easily corrected by unsoldering the wires from the lamp socket and reversing them. This is probably an isolated fault, but it would be wise to check the polarity of any lamp sockets before LED modules are installed.

"The Twins"

If this method isn't to your liking, or you you don't wish to build your own, there are also online vendors who manufacture and sell some very nice LED lamps for Drake equipment. I do not make these for sale, this information is provided so that you can have the fun of making your own.

Check out N9OO's cool blue LED lamp replacements at The Radio Lab Works if you are interested in purchasing ready-made lamps.

Carey Lockhart, KC5GTT, also offers LED lamp replacements. You can see them HERE.

Willi Rass, DF4NW, manufactured the blue LED digital readout shown in my TR7 above. Check out his replacement LED readouts, in various colors, HERE.

For a little bit different kind of blue, give a listen to my friend Laurie McClain's beautiful cover of Kate Wolf's "Cornflower Blue" by clicking HERE.

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Updated 10/4/2020