Looking for a reliable soft warm glow

I have 5 porch lights that are currently using 7.5W and 15W bulbs with a photo-dimmer. The photo-dimmer feeds on itself. When the bulb is on, there is some stray light that enters the light sensor and the bulb dims. As the bulb dims, more energy must be let through because it powers back on. The cycle repeats fast enough that you don't see any flicker - even from the corner of your eye.

The result is that my 7.5W and 15W bulbs probably run at 75% of capacity (guessing). All the bulbs give a nice, soft, warm glow to my porch. There is enough light to feel safe and welcome at night and there is no glare.

I am thinking of switching to LED bulbs. There are several low lumen and low cost bulbs on eBay and e-stores. Some are amber at 20 lumens. Some are warm white at 38 lumens. I realize that I cannot use the photo-dimmer here and that is fine. LEDs cannot be dimmed. I can replace my wall switch with a timer instead.

My question here is twofold. First, how reliable are these low cost, low lumen bulbs? Some say they work forever while others say they die pretty fast. The ones that die fast seem to regulate voltage while the ones that last forever regulate current.

Second, can these LED bulbs give a soft glow? Some of them have a dispersion globe encapsulating the LEDs.

Any thoughts?

-- Boris

kristiancyclist said:

I have 5 porch lights that are currently using 7.5W and 15W bulbs with a photo-dimmer. The photo-dimmer feeds on itself. When the bulb is on, there is some stray light that enters the light sensor and the bulb dims. As the bulb dims, more energy must be let through because it powers back on.

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I have seen this same effect with the LED nightlight in my son's bedroom. If you place any vaguely reflective object so as to allow the emitted light to reflect back to the photodiode, it dims palpably.

kristiancyclist said:

The cycle repeats fast enough that you don't see any flicker - even from the corner of your eye.

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How do you know that it is oscillating, rather than having simply reached a stable state? Not trying to be rude, just happen to be interested in feedback systems.

kristiancyclist said:

LEDs cannot be dimmed. I can replace my wall switch with a timer instead.

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Certainly they can, by reducing the LED current. You'd have to have a circuit to sense the ambient light and then control the regulated current, of course. But you probably know that and maybe meant something else? At any rate, the wall timer may well be the easiest solution.

kristiancyclist said:

My question here is twofold. First, how reliable are these low cost, low lumen bulbs? Some say they work forever while others say they die pretty fast. The ones that die fast seem to regulate voltage while the ones that last forever regulate current.

Second, can these LED bulbs give a soft glow? Some of them have a dispersion globe encapsulating the LEDs.

Click to expand...

I've no experience with that type of lamp, but in general LEDs should only be driven in a current-regulated fashion. You may improve your probability of getting a good response from other members by posting a link to the specific model(s) you're interested in. Also, if you don't already know what "color temperature" and "color rendering index" are, look them up, and that may help you better identify what you should buy to achieve the "soft glow" you have in mind.

I can only guess that it is switching on and off really quick vs. changing some internal resistance to dim the bulb. If it modulated the resistance then I would imagine there would be a rather large voltage drop and a potential for hear build up. But modulating the voltage with a duty cycle dependent on the amount of light hitting a photo sensor would probably not cause a heat build up inside the light controller, especially if the Ron of any internal switching FET is relatively low.

I think what people/sellers mean when they say the LED is not dimmable is that the light can go from all the way on to all the way off with very levels of "dim" in between. Once you get past that forward voltage of a diode, it's on on. The curve up to the Vf point isn't linear before it shoots up (in a Vf vs. A plot).

Of course, with the right electronics you can certainly dim a LED bulb but most common dimmers wouldn't work so well probably.

Here are some examples of bulbs, on eBay, that I was thinking about.

Bulb 1
Bulb 2
Bulb 3
Bulb 4
Bulb 5

-- Boris