6 ms·

this seems a lot like http://www.museumofhoaxes.com/hoax/weblog/comments/gravity_lamp http://www.museumofhoaxes.com/hoax/weblog/comments/gravity_l...

nohat 14y ago
With perfect efficiency a 10 kg weight lifted 2 meters would only output about a tenth of a watt over half an hour. That is indeed enough for a led, but not a very powerful one. I doubt this is worth it.
- pdonis 14y ago
  See the numbers upthread; it's comparable to a kerosene lamp.
colanderman 14y ago
Yes, and the same math applies -- Their claim is a 20 pound weight raised 6 feet (or so) in the air will generate light for 30 minutes: 20 lbf * 6 ft / 30 min = ~90 mW For reference, the LED indicators on your keyboard use about 15 mW each. So that's about two keyboards worth of light. However maybe I'm being stingy -- because of persistence of vision (a characteristic of the human eye), you can run LEDs (or any light) on a low duty cycle and still produce the same apparent brightness. So if we say they run it at a 10% duty cycle (I'm not sure how accurate this estimate is), they might be able to get closer to 1 W of LED light, which is enough for reading.
- bcoates 14y ago
  The apparent brightness of a rapidly flashing LED isn't anywhere near the peak brightness, you get roughly the same brightness per power emitted regardless of pattern. You'd probably get a lot more efficiency by just using a non-white LED and picking a color to maximize human sensitivity vs. power draw.
pdonis 14y ago
There's an interesting comment about super-efficient LED bulbs in that museum of hoaxes article. However, the numbers look marginal for this GravityLight even with super-efficient LED bulbs. Quick back of the envelope calculation, using what few numbers are given in the article. Ballast: m = 10 kg (1) Height: h = 3 m (2) Energy stored: E = mgh = 300 J Time: 30 min = t = 1800 s (3) Wattage available: P = E / t = 0.17 W (4) Notes: (1) Article says you can hang anything weighing about 20 lbs. (2) No numbers given in the article, but the pictures make it look like ceiling height (8 to 10 ft., I took the larger) (3) Article says light for half an hour. (4) With current LEDs this is the equivalent of about a 2 W incandescent bulb, i.e., pretty dim. The light in the pictures in the article looks like the equivalent of about a 40 W incandescent bulb, so the numbers come up short by a factor of about 20. That would indeed be "super-efficient" for an LED; I'm not aware of any even on the drawing boards that are that efficient. [Edit: The numbers actually are not too low compared to kerosene lamps, which are what this light is supposed to replace. See exchange downthread with xd.]
- dexter313 14y ago
  The light seen in the video is not the 30 minute-one. There the weight drops much quicker. Look carefuly at about 55 second in the video.
  - pdonis 14y ago
    Well, shortening the time by a factor of 20 makes it a minute and a half of light equivalent to a 40 W incandescent bulb. So maybe this is really a combination light and exercise machine? :-)
- xd 14y ago
  Are you aware of how little light you need to see? Just because you assume the light in the video is similar to a 40w bulb is short sighted (no pun intended). A 2W "pretty dim" incandescent bulb will illuminate a room no problem. Human eyes can adjust very well to low light levels.
  - pdonis 14y ago
ChuckMcM 14y ago
Perhaps, pretty simple to do the math though. Lets plug in some numbers and see what is what. First we'll assume that the weight changes altitude by 2 meters, next we'll assume the "weight" is 5 kG (about 10 lbs) it could be more than that if you used denser material but it looks like they are expecting you to fill a sandbag to weight it dry sand is about 1600 kg/m^3 [1] so a sandbag that was 15cm/side would be about .0033 m^3 or 5.3 kg. The force exerted by that sandbag, 2 meters up is 5 * 2 * 9.8 or 98 Newton-Meters. Now the campaign says it runs the light for 30 minutes so to find the power in watts we take 98 Newton-Meters divide by 1800 seconds (bag goes from 2m to 0m in 30 minutes) and get .0544 watts per second. Assuming the generator is 50% efficient (that is a really good generator) that is about .025 Watts to run your LED. So can you get decent light from an LED with 25 mWatts? At a forward voltage drop of 4V (White LED) that is 6.2 mA of current. (updated to be a decimal order of magnitude smaller) Given that current LEDs are seeing something like 50 lumens/watt you might see 2 - 5 lumens from such a light. Not nearly as bright as I originally estimated. EDIT: The time was wrong 1800 not 180 [1] http://www.simetric.co.uk/si_materials.htm http://www.simetric.co.uk/si_materials.htm
- shabble 14y ago
  from the caption under the house door/garage images: Hang it in the shed or make it into a great porch light, you can clip on a hanging basket or anything weighing about 20lbs. So that's ~2x your estimate, which doesn't seem impossible. I do wonder what they're suggesting using for the ballast though, that avoids excessive bulk. Another consideration is going to be finding a suitable mounting point in a typical shack/slum type dwelling.
trhtrsh 14y ago
And note that the gravity lamp was announced on physorg.com, famous for how wrong it is about science topics.

nohat 14y ago

With perfect efficiency a 10 kg weight lifted 2 meters would only output about a tenth of a watt over half an hour. That is indeed enough for a led, but not a very powerful one. I doubt this is worth it.

pdonis 14y ago
See the numbers upthread; it's comparable to a kerosene lamp.

colanderman 14y ago

Yes, and the same math applies -- Their claim is a 20 pound weight raised 6 feet (or so) in the air will generate light for 30 minutes: 20 lbf * 6 ft / 30 min = ~90 mW For reference, the LED indicators on your keyboard use about 15 mW each. So that's about two keyboards worth of light. However maybe I'm being stingy -- because of persistence of vision (a characteristic of the human eye), you can run LEDs (or any light) on a low duty cycle and still produce the same apparent brightness. So if we say they run it at a 10% duty cycle (I'm not sure how accurate this estimate is), they might be able to get closer to 1 W of LED light, which is enough for reading.

bcoates 14y ago
The apparent brightness of a rapidly flashing LED isn't anywhere near the peak brightness, you get roughly the same brightness per power emitted regardless of pattern. You'd probably get a lot more efficiency by just using a non-white LED and picking a color to maximize human sensitivity vs. power draw.

pdonis 14y ago

There's an interesting comment about super-efficient LED bulbs in that museum of hoaxes article. However, the numbers look marginal for this GravityLight even with super-efficient LED bulbs. Quick back of the envelope calculation, using what few numbers are given in the article. Ballast: m = 10 kg (1) Height: h = 3 m (2) Energy stored: E = mgh = 300 J Time: 30 min = t = 1800 s (3) Wattage available: P = E / t = 0.17 W (4) Notes: (1) Article says you can hang anything weighing about 20 lbs. (2) No numbers given in the article, but the pictures make it look like ceiling height (8 to 10 ft., I took the larger) (3) Article says light for half an hour. (4) With current LEDs this is the equivalent of about a 2 W incandescent bulb, i.e., pretty dim. The light in the pictures in the article looks like the equivalent of about a 40 W incandescent bulb, so the numbers come up short by a factor of about 20. That would indeed be "super-efficient" for an LED; I'm not aware of any even on the drawing boards that are that efficient. [Edit: The numbers actually are not too low compared to kerosene lamps, which are what this light is supposed to replace. See exchange downthread with xd.]

dexter313 14y ago
The light seen in the video is not the 30 minute-one. There the weight drops much quicker. Look carefuly at about 55 second in the video.
- pdonis 14y ago
  Well, shortening the time by a factor of 20 makes it a minute and a half of light equivalent to a 40 W incandescent bulb. So maybe this is really a combination light and exercise machine? :-)
xd 14y ago
Are you aware of how little light you need to see? Just because you assume the light in the video is similar to a 40w bulb is short sighted (no pun intended). A 2W "pretty dim" incandescent bulb will illuminate a room no problem. Human eyes can adjust very well to low light levels.
- pdonis 14y ago

ChuckMcM 14y ago

Perhaps, pretty simple to do the math though. Lets plug in some numbers and see what is what. First we'll assume that the weight changes altitude by 2 meters, next we'll assume the "weight" is 5 kG (about 10 lbs) it could be more than that if you used denser material but it looks like they are expecting you to fill a sandbag to weight it dry sand is about 1600 kg/m^3 [1] so a sandbag that was 15cm/side would be about .0033 m^3 or 5.3 kg. The force exerted by that sandbag, 2 meters up is 5 * 2 * 9.8 or 98 Newton-Meters. Now the campaign says it runs the light for 30 minutes so to find the power in watts we take 98 Newton-Meters divide by 1800 seconds (bag goes from 2m to 0m in 30 minutes) and get .0544 watts per second. Assuming the generator is 50% efficient (that is a really good generator) that is about .025 Watts to run your LED. So can you get decent light from an LED with 25 mWatts? At a forward voltage drop of 4V (White LED) that is 6.2 mA of current. (updated to be a decimal order of magnitude smaller) Given that current LEDs are seeing something like 50 lumens/watt you might see 2 - 5 lumens from such a light. Not nearly as bright as I originally estimated. EDIT: The time was wrong 1800 not 180 [1] http://www.simetric.co.uk/si_materials.htm http://www.simetric.co.uk/si_materials.htm

shabble 14y ago
from the caption under the house door/garage images: Hang it in the shed or make it into a great porch light, you can clip on a hanging basket or anything weighing about 20lbs. So that's ~2x your estimate, which doesn't seem impossible. I do wonder what they're suggesting using for the ballast though, that avoids excessive bulk. Another consideration is going to be finding a suitable mounting point in a typical shack/slum type dwelling.

trhtrsh 14y ago

And note that the gravity lamp was announced on physorg.com, famous for how wrong it is about science topics.