Electric devices changing brightness - motion detectors?

Factual Questions
1

I’ve often perceived that electric devices - such as the LED time displays on clock radios - seem to brighten when I enter the room. It doesn’t seem to happen all of the time entirely predictably. But occasionally I will enter a room and glance at a radio, and the lit numbers appear to brighten.

In the back of my mind I always wonder whether I am imagining it, so that IS a possibility. But if I am not, does anyone know what might explain this? I’ve never been aware of a newly purchased device mentioning this feature - but I never looked for it.

Similarly, I have on my desk a seasonal decoration - it is a photo of a cardinal w/ blinking little LED lights. When I came into my office this a.m., it was off. Then it started blinking and has continued. Does that also have a motion detector? A sleep cycle? Is it further evidence of my insanity? :wink:

2

I presume the room you’re entering is at least a bit darker than the room or corridor you’re leaving ?

If so … it may well just be dark adaptation – your pupils starting to dilate in the presence of lower lighting.

3

That phenomenon of the blinking red lights seeming to be off when you first look at it is an example of chronostasis. It usually shows up as the stopped clock illusion, where if you look at a clock with a second hand the first jump seems to take too long.

4

Some devices, like alarm clocks, do have an automatic brightness adjustment feature that will brighten the display when ambient light is high so it doesn’t get washed out in bright light, but dims the display in the dark so that the display itself isn’t lighting up the room. Perhaps it was dark in the room before you opened the door, and much brighter after you walked in…? At any rate, not a motion sensor, but a light sensor.

5

There are motion sensors built into electronic devices, commonly Halloween decorations that move or make noise when someone walks by. Your blinking cardinal could certainly have one.

But a motion sensor that controls brightness seems unlikely. I agree it’s either your eyes adjusting or a light sensor in the clock.

6

Good point. In addition to going from a darker hall to a brighter room, he may now be creating a brigher-than-before room (eg, by opening a door) that does trigger a light-sensitive LED display.

7

The one device I notice it most frequently on is a Bose. Googling, I see that the brightness can be adjusted (but my computer will not allow me to open the sites.)

I’m really not sure what specific factors apply when I perceive this, but it seems it is possibly an automatic adjustment. Take the clock radio in the kitchen, with blue numbers. I think a common instance would have me walking into the kitchen either in the evening or morning. I’m not sure whether the kitchen is consistently brighter or dimmer than where I came from, and I’m not sure I always turn on the kitchen light. But sometimes when I look at the clock, it will seem to get brighter in a series of successive (3?) pulses.

My wife perceives the same thing. I’ll try to check from my phone or another computer. I suspect it is possible that I experience it only from this one radio and then presume I experience it elsewhere, but I think I have experienced it with different devices - not all of which are Bose.

So how about the LEDs in the decoration on my desk?

8

One thing that many people don’t realize about LED displays is that when they are on, they aren’t on. They blink.

Take an LED. It has a maximum brightness that you can run it at before it overheats and becomes a DED (dark emitting diode, aka a burned out LED). Let’s say that for a particular LED this is at 20 mA of current. Run the LED at 25 mA and the LED overheats and goes poof.

Now, instead of running it at a constant current, turn the LED on for 50 percent of the time and off for 50 percent of the time, and have it blink on and off faster than about 15 to 20 times per second or so. Also, instead of running it at 20 mA, run it at 40 mA while it’s on. The average current is still going to be 20 mA, since it’s 40 mA for half of the time and 0 mA for the other half of the time, so the heating in the LED will basically be the same. But here’s the weird thing. Even though the light output is also the same on average, that’s not what your brain perceives. To your eyes and brain, the LED is twice as bright, even though it’s off for half of the time.

This is a little trick that almost every LED display uses to make itself appear brighter.

There are LED displays that also incorporate a light sensor, and what they will do is vary the on and off times to vary the apparent brightness of the LEDs. So in other words, 50 percent on and 50 percent off might be the full brightness for the LED display, and 25 percent on and 75 percent off might be a dimmer setting, or whatever other percentages they want to use for finer control over the brightness. At long as it blinks faster than about 10 to 20 times per second, your brain thinks that it’s on constantly, so you don’t notice the blinking.

Except when you enter a room. It takes your eyes and brain a moment to catch up to the blinking. This is the chronostasis that Bill_Door referred to upthread. It’s why LED clocks appear dimmer when you first enter the room and its why blinking lights appear to be off for a longer period of time when you first enter the room.

The really weird thing is that some of the “smoothing” of the fast blinking signal happens in your eyes and nerves, and not in your brain. Neurons have a natural reluctance to fire at first, and then once they get going, they have a bit of a reluctance to stop firing. This results in some weird signal integration (as in integral calculus type of integration) within your eyes and nerves so that the signal that gets to your brain is already distorted from reality.

9

Poking around on youtube, I found a video of someone who took a high speed video of a couple of different clock LED displays:

Note that the actual blinking patterns of different clocks can vary. Two different versions are shown in the video. Other clocks and LED displays may use different patterns.

10

The source of chronostasis is a fascinating artifact of the human eye-brain system. When your eyes make a saccade (a rapid movement from one point of focus to another), your brain ignores input from the eyes during the movement, to avoid perceiving a disorienting blur. This is called saccadic masking. But then it goes further – at the end of the saccade, it tricks you (or I guess more accurately it tricks itself) into believing that the object it sees at the end of the saccade was actually visible during the entire duration of the saccade. So whatever image you see at the end of a saccade appears to have persisted unchanged for up to several hundred milliseconds.

You can observe this by looking in a mirror and shifting your focus from one eye to the other. To you it will appear that your eyes are completely motionless and you will perceive no gaps or interruptions in your vision. A camera or someone else watching you do this will see that your eyes are actually moving back and forth.

11

Anyone interested in helping me devise a way to experiment with this? (So long as your suggestions don’t involve me dressing up like a chicken or anything! ;)) Like I said, I’m not aware of this happening consistently under specific conditions. But I could easily try to walk into the dark room from either a darker or lighter room, and either turn on the light or not.

It is weird in that it appears to change AS I LOOK AT IT. But I assume that I am just happening to notice it on those occasions that I am looking directly at it as it changes. Unless looking at the bright LEDs causes my eyes/brain to react.

This should be testable and reproducible, no?

12

Point your phone camera at the clock and take a video as you walk into the room.

If it’s a light sensor, it could depend on your shirt color. Try wearing a dark shirt and a light shirt, stand in front of the clock and see if the brightness changes.

13

The Bose site says it has adjustable brightness and automatic nighttime dimming. Also refers to it “sensing” ambient light. So I’m going to try to pay attention to it when I enter the kitchen in the early morning or evening. Somehow I got the sensation that it reacted to my movement or proximity. If it is responding to turning on the lights, that should be observable.

Nice to have a project in these dark days around the solstice! :smiley:

14

First thing, buy a 1400 fps camera…

15

Apropos of nothing … they didn’t by any chance recently legalize marijuana where you live, did they ?

:smiley:

16

To test the light sensor, just stick your finger over it. The display should dim thinking it’s dark out.

17

Maybe. Some of those things have a pretty long time constant to avoid nuisance changes in brightness.

The self-regulating illumination in my car’s instrument panel is light sensitive, but changes of minutes, not moments. Having the panel flare up brightly as you go under an overpass would get annoying quickly. Conversely entering a tunnel it may take 10 or 20 seconds to crank up the brightness.

18

I was specifically thinking about my old clock radio from like 40 yrs ago. As a kid, I messed with the sensor a lot once I discovered it.

19

Some devices have crude brightness sensors. They are essentially averaging the amount of light in the vicinity and producing an output which may drive or control something else, like a digital display. They may have only a few steps – maybe 3 or 5. If the brightness average changes up or down, they will move to the next step.

Now consider how crude the sensor is. It doesn’t focus and it probably has nothing more than “half an eye” with a wide angle to gather the light and sum it. Now a person walks into the room, and the sum of the room brightness changes, since a person may absorb or reflect light. If it changes enough to cross over a threshold, the display makes a corresponding jump as well.

20

Is that saccadic motion what is being checked in the DWI Field Sobriety Test where the officer says to keep your head still and watch the tip of my pen/finger as it moves from side to side?