A realistic way of driving LEDs.

One concept which has existed for some time is, that LEDs can produce a variable amount of light, and this will be the case, regardless of whether that amount of light needs to be constant, modulated slowly, or modulated at very high frequencies. But, LEDs have as a property which many other components do not have, that they tend to produce a fairly constant, forward voltage drop (like any diode), but that, as the voltage increases only slightly past a certain point, current increases rapidly. And, in the History of Electronics, this has often caused circuit designers to put a resistor in series with the LED, to regulate its current accurately.

One big drawback of doing this is, that power gets wasted, as current flows through the resistor, and gets transformed into heat. The amount of power that gets wasted in that way, most strongly depends on what fraction of the supply voltage appears across the resistor, instead of across the LED. Another drawback is the fact, that the current which flows through a resistor, which has simply been connected between a supply voltage and an unpredictable component – such as the LED – is itself not constant, And, when supply voltages are low – such as 5V – small changes in supply voltage are large, in comparison to the only slightly smaller voltage-drop across the resistor. And so, technology does offer as alternative, a chip, with active components to regulate the current more precisely, and often, while wasting less energy. In principle, such a chip can also be installed by the manufacturer of LEDs, into the same package as the LED.

According to the schematic below, I have demonstrated such a circuit…




What is happening here is, that A presumed control current is fed in to a presumed input pin, and this circuit actually doubles that current, resulting in an amount of current which will be drawn from the cathode of the LED. Additionally, more than one LED could be connected in series, to the current-sink.

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Modern Colored LEDs

One of the long-standing facts about LEDs (Light-Emitting Diodes), was that for any color (wavelength) of light to be emitted, a different semiconductor material was needed, each with a different gap-energy, and therefore also each with a different forward voltage-drop. (:1)

What seems to have happened in more recent times, is that in the cheap, mass-production of LEDs, the Industry is realizing cost-reduction, by always producing the famous Blue LED (InGaN), but putting a different phosphor coating onto it, which absorbs most of the blue light, and which emits a mixture of wavelengths to taste.

But this is also the reason for which, when we buy glow-sticks for camping for example, we may no longer find the traditional, deep red glow-sticks as easily as we could before. What has happened is that here too, a Blue LED is used, but a phosphor put onto it that emits red light. And then, a small amount of the blue light passes through the phosphor without being absorbed, so that a ‘pink’ shade of light emerges.

At least, if the ones I just bought are any indication.

One disadvantage to this is the fact that deep-red light tends to blind human vision the least at night, so that assuming night-vision, there used to be advantages to pure red light for signaling purposes. Because the newer LEDs still contain some light-energy at the (shorter) wavelength that looks blue, those LEDs will also have a stronger, undesirable tendency to ‘dazzle’ a person’s night-vision.

My favorite color of glow-stick for camping is actually the green one.

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Beware of LEDs that stay constantly-lit.

Today we have many portable devices, which attempt to run off battery power for a long time. At the same time, these devices can have LEDs, which indicate their status for the convenience of the user. There is an aspect to that, which some users might not fully realize, but which I think is important nonetheless.

If the LED is to stay constantly lit and bright, then the amount of current / power it consumes will be high, and will therefore be a strong drain on the battery as well. Additionally, our human eyes are less sensitive to blue light than they are to red, and less so to red light, than they are to green. Therefore, a blue LED which is to seem bright, is consuming an additional penalty of power, to appear as bright to our eyes, as a red LED is supposed to seem.

My new LG Tone Pro HBS-750 Bluetooth Headphones have all the correct behavior, in that during normal use, their Blue LED only flashes very briefly, in a code that restates the battery level, but with a duration of much less than 10%. They also have the correct behavior, in that their LED stays Blue continuously, when they are in pairing mode, which is only supposed to be a brief operation compared to their normal mode. And, when they are charging, some combination of their Red and Blue LED stays fully lit, because power is assumed to be in abundance when charging.

Now, the supposed HBS-730 headphones which I had before, would have a much brighter Blue LED, which was supposed to stay solidly Blue when pairing. Mine alternated between Red and Blue when pairing. And mine stayed fully lit 100% of the time, when in Standby, a mode in which the headphones are supposed to be drawing less current, than when actually playing music. And the LED on mine, would flash in Blue with 50% duration, when I was listening to music.

This was a design failure, and was also inconsistent, with what the LG manual for the HBS-730 explicitly states. According to that manual, their LED was also to stay fully Blue only when pairing, and was supposed to flash briefly, not 50% of the time, while playing music. This correct behavior would have been very similar to what my present HBS-750 Headphones do, but did not correspond to the real behavior of what I had been sold.