red light

[yetistreak]

Why do almost all digital alarm clocks use red LED's in their display?

[ghariman]

Hi there,

First there is a nice article about LED that you should read:
http://pdfserv.maxim-ic.com/en/an/AN1883.pdf

Here are a couple of guesses for an answer to your question:
1) Marketing - money does matter in the real world.
I went to digikey.com and searched for prices on RED and GREEN LEDs (just the normal type ones) and I noticed that the GREEN LEDs are more expensive than the RED LEDs (more than 5x).

2) Physics - the scientific side.
The pdf article from MAXIM shows that the eye is most sensitive to green LEDs (figure 2). So in fact if you look at two LEDs one red and one green of the same power rating, you would notice that the green is much much brighter than the red and even to the point where the eyes get a bit sore/sensitivefrom looking at it. Imagine at night looking at a clock with green LEDs from a distance. I think it would be hard to make out the numbers as the green light may be too bright. So then RED was chosen. BLUE was not as it is too dark.

3) Electrical Engineering - the good stuff.
In the same pdf article you can see that the "knee" voltage of the RED LED is lower than that of the GREEN LED (figure 10). So putting GREEN LEDs in series will require more voltage than the same number of RED LEDs. Again less voltage means less number of batteries.

Those are my guesses. Hope this helps.

[deleted-71588]

With manufacturing, the number of units produced and sold is more important than list prices for individual parts. The cost of small parts to a manufacturer is an inverse function of quantity because the distribution cost of packaging and shipping 1,000 small parts or a 20,000 is about the same. Why do individual red LED's cost less, they were the first to be produced in large quantity? Why do individual LED's of other colors cost more, I'd bet that the biggest factor is less demand for them so the stocking and batching costs are spread over fewer pieces and the inventory costs (parts sitting idle on shelves in warehouses) is higher.

But the displays in most low cost clocks aren't made up of individual LED's because the cost of assembly is a bigger factor than the cost of the individual parts. The issue is then what is the total cost of the assembly (parts and assembly).

The last low cost digital clock radio I bought has an LCD display with multi-colored back lighting with a means of picking what color you want.

I'll bet the individual cost of this LCD part is more than the sum of individual parts in other designs; however, based on the retail shelf price of the clock radio, it has to be less expensive to produce.

A large part of Engineering is economics. This is one of the things that distiguishes scientists and engineers.

[ghariman]

Well said, fellow ENGINEER !