engineer

tim   .

                                                      


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.   hunkin

                                                         


cartoonist

 

LEDs

 

THE COMMENTS

I enjoy reading all the comments but apologize that there are so many I canít answer individually. Iím delighted that my new videos have been so enthusiastically received, which is reflected in the majority of the comments. The videos arenít perfect though so below each video Iíve compiled the comments about things I omitted or people thought were misleading. A few comments added wonderful additional information so Iíve included them as well. 

Quite a few people criticised my sum for working out the value of the resistor to put in series with an LED. To get the maximum brightness, the forward voltage of the LED (part of the spec) can be deducted from the supply voltage. Personally I prefer not to run LEDs to close to their max as I have had many fail over the years, mainly because of overheating. Also their brightness does not visibly diminish dropping the power by 20%. (in fact one person wrote that hepowers his daughter's 12v dolls house LEDs from  nine volt batteries). However the criticism is very valid when the power voltage not much higher than the LED voltage. If I had used a 6v supply for my 3.3v power LED, my rough sum would have resulted in a resistance twice as high.

Apparently its not strictly true that LEDs canít be harmed by connecting them the wrong way round. High reverse voltages and even low voltages on some newer surface mount LEDs can fry them.

Donald Sayers reminded me that a single flashing LED can be used to flash a whole chain of LEDs wired in series. I wish I had included this. Even better, the wonderful Evil Mad Scientists have a simple circuit using a flickering tealight LED as the base of a transistor circuit to make a high power flickering circuit. I used this on the sign at the top of my i-Zombie machine. (https://www.evilmadscientist.com/2011/does-this-led-sound-funny-to-you/)

Mark Harriss had managed to see light with a wire touching 36grit silicon carbide abrasive. It required about 1amp and 20 volts, and produced a variety of different colours.

Several people wrote to explain the warm LED filament household lights. They are strips of tiny surface mount LEDs covered by silicone and phosphors. They require about 70vdc which why I hadnít managed to get them to light up! One person added that they were invented in China, before being copied by Philips and GE. Also that the strips have four blue LEDs and one red one in series, with the phosphors blending them to the warm yellow.

Vincent Sullivan wrote that the numbering system used for surface mount LEDs referring to their size in mm is also used for other components, particularly coin cells. So for example a 2030 cell is 20mm diameter and 3 mm thick.

Gordon Slater uses a 4 inch bit of wood with a blunted sewing machine needle on the end to rest (or lightly clamp) on surface mount LEDs while soldering. Extra flux was another suggestion.

Electra Flarefire wrote that the clear coating of waterproof LEDs strips is clear PVC, not silicone. Of course, this is why they are completely clear, not translucent. It also explains my experience of the coating becoming stiffer and brittle with age as the plasticisers in the PVC evaporate over time. Useful for school projects, 12v strips will run from 9 volt batteries without too much loss of brightness.

Tom Z added a great fact about the bonkers over-engineering of some BMW headlights which use blue lasers aimed at phosphors to create the sharp focused beam.

Plons0nard suggests Bascom Basic Compiler for programming arduinos and particularly addressable LEDs as it has a specific 'rainbow' subroutine. Another suggestion was 'Circuit Python'.

Quite few people had different ways of memorising the polarity of LEDs. My favourite was that the flat on the side is the shape of a minus sign.

 

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