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Old 06-30-10, 04:20 AM   #1
marada
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Any way to achieve 10 watt output from a dynamo?

I will be doing a lot of camping on my bicycle in the winter and am looking for a way to charge my laptop by a dynamo. My laptop is an ultra efficient Asus model and only uses around 8.5 watts per hour of use. Given that no solution is 100% efficient I gather I will need around 10 watts of power per hour to charge the laptop for each hour of peddling. I know I could charge it over a longer period of time however I want to aim for 1 hour of peddling equating to 1 hour of charge.

Is there anything out that that would suit my needs, or maybe something I could adapt / make? The highest power dynamo I seem to be able to find is the Busch and Muller Dymotec S12. This outputs 12 volts but only at 6watts. If I went for something in the 12 volt range would I be able to use a 12 volt car charger with the laptop to take it up to the 19 volts that the laptop charges at?

Any ideas would be much appreciated
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Old 06-30-10, 07:18 AM   #2
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It is principally doable, but you should ask yourself whether it is a reasonable thing to do. A bad bottle dynamo might be 20% efficient and thus would tax you at the level of 3W*5=18 W. A hub dynamo might be far more efficient, but taking out 10W you would be working away from its normal operating point. My guess is that you might make it 50% efficient in your application, i.e. taking you at the comparable 10W*2=20W. Would you want to ride at all times with an old bottle dynamo operating? This is the question you need to answer for yourself and there is no problem testing it out. Just make sure that the dynamo works under load, i.e. actually powers a 3W system.
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Old 06-30-10, 08:50 AM   #3
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Thanks for your reply. I don't know why you are talking about 3w as B&M have a dynamo that will output 12v at 6w. They claim this is just as efficeint has a hub dynamo. Please see the following link:

http://www.dotbike.com/productsP3027.aspx

What I am asking is there any dynamo that will output 10W every hour?
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Old 06-30-10, 10:23 AM   #4
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marada, the issue that I am raising is whether your legs will be happy when asked to deliver extra 20W every second of every hour.

I am not sure about S12 because of its complicated electronics, however Schmidt can be certainly made to deliver high power. In fact, I think that it may not be possible to get a high power from a bottle because it might just start slipping under too high load.
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Old 06-30-10, 10:36 AM   #5
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So is something like the Schmidt SON the best thing to go for if I want high power output? Or are there better choices? Just out of interest how fast would I need to go to get something like the Schmidt SON to output 10W, as I note the SON is advertised at 6v 3W
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Old 06-30-10, 11:23 AM   #6
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I'd look for a solar charger.
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Old 06-30-10, 11:44 AM   #7
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Solar charger is a solution in bright sunlight. The issue is whether the winter riding is going to take place in some southern areas or at moderate northern latitudes. Cloudy skies will not be able to power solar cells to the needed degree.

A dynamo acts as a current source at high speeds, providing ~0.55A of current. For 10W, the dynamo needs to drive a load with ~40 Ohms of impedance, at ~20V. However, this is rms AC and the laptop needs smooth DC.
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Old 06-30-10, 05:58 PM   #8
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Yes, it's technically feasible but 10W is a lot of power on a bike. According to a quick web search, the 'average' biker puts out about 125W while pedaling so a 10W generator will be a very noticeable load. Modern hub generators are apparently up to 80% efficient but even so, it will still be a significant drag.

As to whether you can use a 12V output to charge your laptap, it all depends on the electronics. Most likely you won't be able to keep the dynamo output at max all the time and it will occasionally drop below the ~14V a nominal 12V automobile system maintains. Best case, the charger will just shut down until the voltage comes back up. Worst case, it will be damaged and/or damage your laptop (that would be a truly worst case, especially if the Li battery in the laptop exploded!). The charger may also have a minimum power requirement in addition to a minimum voltage requirement. Same caveats apply but now the dynamo needs to supply a minimum power instead of a minimum voltage to keep things humming along. All in all, I would recommend against it unless you can find a charger designed by someone who really knows what they're doing (not me, BTW) specifically for this type of application.

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Old 07-01-10, 09:47 AM   #9
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Solar charger is a solution in bright sunlight. The issue is whether the winter riding is going to take place in some southern areas or at moderate northern latitudes. Cloudy skies will not be able to power solar cells to the needed degree.

A dynamo acts as a current source at high speeds, providing ~0.55A of current. For 10W, the dynamo needs to drive a load with ~40 Ohms of impedance, at ~20V. However, this is rms AC and the laptop needs smooth DC.
This is not quite correct. The solar intensity typically decreases about 20% when the sky becomes overcast. If you build a system capable of your 10 watts, it just needs to be upsized 20% to perform as you want on a cloudy day. Or, just count on getting your recharge within 90 minutes.

Another more knotty issue in sizing a solar array is that it is only maximally effective when the sun's rays are falling on the solar array in a perpendicular direction. At any other angle there is a loss of incoming radiation based on the cosine of the incident angle. At 45 degrees off-axis the incoming energy, and hence the outgoing electrical power, will be 30% off of the peak value.

Regarding DC and AC, use a rectifier and a capacitor filter to convert the AC to smoothed DC. You might need to look at what voltage you'll get. You might need a DC/DC converter with a boost/buck function, a boost/buck or SEPIC type, to deliver the right voltage over a wider range of speeds. Figure on 10% to 15% loss in the converter.
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Old 07-01-10, 09:49 AM   #10
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The best modern generators, such as the (surprisingly) out of production B&M S6 is around 80%, but the still-current B&M 6 is more like 50% efficient, and some lesser bottles are not that good. I can't say about the hubgens.
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Old 07-01-10, 11:37 AM   #11
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This is not quite correct. The solar intensity typically decreases about 20% when the sky becomes overcast. If you build a system capable of your 10 watts, it just needs to be upsized 20% to perform as you want on a cloudy day. Or, just count on getting your recharge within 90 minutes.
I built a system and it was 90%+ difference, depending on sky and time of the year. In upsizing you assume that you have no constraints on the surface facing up. That is not true, unless you indeed use a trailer. Otherwise, you need to struggle to get enough surface for your power under the best conditions. With solar irradiation power of the order of 300W/m^2 and say 5% cell efficiency, you need 80cm x 80 cm facing up to get your 10W, under best conditions.


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Another more knotty issue in sizing a solar array is that it is only maximally effective when the sun's rays are falling on the solar array in a perpendicular direction. At any other angle there is a loss of incoming radiation based on the cosine of the incident angle. At 45 degrees off-axis the incoming energy, and hence the outgoing electrical power, will be 30% off of the peak value.
This 30% sounds like theoretical value based on the cosine. In reality there are further reflection losses adding another ~30% on top.

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Regarding DC and AC, use a rectifier and a capacitor filter to convert the AC to smoothed DC. You might need to look at what voltage you'll get. You might need a DC/DC converter with a boost/buck function, a boost/buck or SEPIC type, to deliver the right voltage over a wider range of speeds. Figure on 10% to 15% loss in the converter.
Your reasoning seems to refer to the situation of a source that can supply more than the load demands. The reality is that of a source that has hard time meeting the demands of the load.

Last edited by 2_i; 07-01-10 at 01:38 PM. Reason: wrong word used
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Old 07-01-10, 11:42 AM   #12
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The best modern generators, such as the (surprisingly) out of production B&M S6 is around 80%, but the still-current B&M 6 is more like 50% efficient, and some lesser bottles are not that good. I can't say about the hubgens.
Hub Gens like the SON and SRAM are around 65% eff, Shimano's a little less so but not by much.
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Old 07-01-10, 12:19 PM   #13
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I have no useful info about bike generators, but I do have a science/engin background like many of you, and this topic interests me.

If you have a device that uses say 9 watts, that's power, and is instantaneous. If it is on for an hour, that is 9 watt-hours, which is energy, not power. To recharge one hour of use, you need to recharge with 9 watts for an hour, or 3 watts for 3 hours, etc... assuming 100% efficiency.
In fact there is the conversion efficiency of any generator for mechanical motion into electrical power, so to get 3 watts of electrical power, you might need to put out 6 watts of extra mechanical power into the pedals. There is also the recharging efficiency of the charger and battery itself, which is not 100% either.
I've seen 3 watt generators available, so if your are camped with the computer on for an hour a day (round it to 9 watt-hours of energy used), you may need to ride the bike for at least 4 or 5 hours a day to recharge it. Maybe someone makes a higher power hub generator, but the market for it may be small due to extra exertion needed on the bike.

BTW, I have heard of pro riders who can put out 1-2 horsepower, that's 750-1500 watts for 10-20 minutes and more! If we all had to do real work to replenish all the energy we use in our lives, we would be amazed I think.

If you do find a 10 watt hub generator, please post about it.
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Old 07-01-10, 12:38 PM   #14
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The ewerk is what you want. This is a widget that hooks up to a standard bike dyno and outputs electricity at the voltage and amperage you choose. Looks like it'll dial up to 12.6 V.

I haven't done the math on this, but if you've got a low-capacity battery on your laptop, it's conceivable that you'd fully charge it in less than a typical day's ride. In which case it would make more sense to dial back the amperage.
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Old 07-01-10, 01:32 PM   #15
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Where are you going to be where you can't plug the laptop in once in a while?
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Old 07-01-10, 01:41 PM   #16
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camping...
perhaps some campgrounds have power outlets? .. probably not too many.
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Old 07-01-10, 02:40 PM   #17
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I built a system and it was 90%+ difference, depending on sky and time of the year. In upsizing you assume that you have no constraints on the surface facing up. That is not true, unless you indeed use a trailer. Otherwise, you need to struggle to get enough surface for your power under the best conditions. With solar irradiation power of the order of 300W/m^2 and say 5% cell efficiency, you need 80cm x 80 cm facing up to get your 10W, under best conditions.




This 30% sounds like theoretical value based on the cosine. In reality there are further reflection losses adding another ~30% on top.



Your reasoning seems to refer to the situation of a source that can supply more than the load demands. The reality is that of a source that has hard time meeting the demands of the load.
Yes, because that is usually the case when you design a real-world power system. I understand the realities quite well, and note that I did not say a design was possible. What I started to do was to outline a few of the key degradation factors that are present.

BTW, I measured insolation difference with cloud and sun, and found a 20% difference just due to that change. I am curious how you tested to see 90%, i.e. what the best and worst conditions were, and what efficiency function you actually measured.

I think that in bright sun there's more than 300W/m^2 available, that 8% efficiency is a more likely cell efficiency value, and that the required array area is still quite large. A 1-hour charging system is tough to get. But to charge the laptop in 4 or 5 hours is not hard at all.

When I design power systems I analyze the source requirements based on the load, the system design, the environment, and the usage profile, and see what is the minimum source that will do the job - it's always bigger than the load input power. If that can't be build for the application, the application is a no-go. This one might be, at least as a 1-hour system.

I think the OP needs to reduce his expectations. A 1-hour charge is quite a challenge.
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Old 07-01-10, 03:42 PM   #18
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Yes, because that is usually the case when you design a real-world power system. I understand the realities quite well, and note that I did not say a design was possible. What I started to do was to outline a few of the key degradation factors that are present.
Granted.

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BTW, I measured insolation difference with cloud and sun, and found a 20% difference just due to that change. I am curious how you tested to see 90%, i.e. what the best and worst conditions were, and what efficiency function you actually measured.
It was a while ago. I built a test system with a solar panel charging batteries that drove the lighting system. It worked OK in bright summer sun but I needed lights during early winter nights. The panel produced nearly no power under overcast winter nights. Even in summer, the power from the panel was only decent for a few hours if the sun was shining. In the end, it turned out to be a complete failure. However, for someone touring under bright sun, a panel good be a good supplementary source of power. Somewhere on the net there is an elaborate system described that combines dynamo and a panel for the charging batteries. I doubt OP has 3% of the capabilities needed to build that particular system, but, after all, one needs to start somewhere.

Regarding the 20% reduction, I bet you have used a meter that measures the visible spectrum. The cells are driven by the short-wavelength part of the spectrum. If there were just 20% reduction, people would be getting sunburn w/o sunlight.


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I think that in bright sun there's more than 300W/m^2 available, that 8% efficiency is a more likely cell efficiency value, and that the required array area is still quite large. A 1-hour charging system is tough to get. But to charge the laptop in 4 or 5 hours is not hard at all.
I suggest you try it using a panel that you can hope to mount on a bike.

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I think the OP needs to reduce his expectations. A 1-hour charge is quite a challenge.
Fully agree. Good thing, though, is that he is doing some calculations and does not try to barge ahead with some perpetum mobile as many others.
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Old 07-01-10, 05:10 PM   #19
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camping...
perhaps some campgrounds have power outlets? .. probably not too many.
It's not that uncommon to be able to find a outlet at a campground or at a restaurant or ...

A friend did the GAP/C&O with mostly camping and managed.

If there's WIFI, there certainly is electricity.

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I think the OP needs to reduce his expectations. A 1-hour charge is quite a challenge.
I think the OP needs to provide more detail about what he's planning on doing with the laptop.

If he's touring, let's say riding for 8h, is he really going to want to use the laptop for another 8h?

Lots of people manage to bicycle with laptops without needing to use the bike to recharge them.

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Old 07-01-10, 10:03 PM   #20
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It was a while ago. I built a test system with a solar panel charging batteries that drove the lighting system. It worked OK in bright summer sun but I needed lights during early winter nights. The panel produced nearly no power under overcast winter nights. Even in summer, the power from the panel was only decent for a few hours if the sun was shining. In the end, it turned out to be a complete failure. However, for someone touring under bright sun, a panel good be a good supplementary source of power. Somewhere on the net there is an elaborate system described that combines dynamo and a panel for the charging batteries. I doubt OP has 3% of the capabilities needed to build that particular system, but, after all, one needs to start somewhere.

Regarding the 20% reduction, I bet you have used a meter that measures the visible spectrum. The cells are driven by the short-wavelength part of the spectrum. If there were just 20% reduction, people would be getting sunburn w/o sunlight.

I suggest you try it using a panel that you can hope to mount on a bike.
Granted, I've not built a bike system. What I've done is some sizing calculations to scope the size of a bike system, based on my space array and system experience and terrestrial spectral insolation models. It is really hard to get a packagable 10 to 20 watt source, and there aren't many places on a bike where the array is always going ot be efficient (such efficiency as they have). I may have some of it wrong. As you say, the proof is in the testing.

My meter (with the test report, it's in the hands of my former employer) measured visible and IR, and we had a filter added to isolate teh IR we wanted, which was around 850 nm. We were building an IR-based camera system that illuminated a driver and observed the pose of his head and direction of his eyes. My interest was to assess the issues with solar IR as a source of background noise, or of a dominant alternate signal at low sun angles. But I repeated most of the measurements with and without the filter, the results were not hugely different.

Did you mean to say winter and summer nights? I suspect you've mis-spoken, since you seem to understand PVs. But if so, I have to ask why do you need to run PV at night, and what latitude will you use it in? On earth-orbiting spacecraft we run electronics off the battery when the spacecraft is eclipsed by the earth.

If you really need on-the road charging, I think you may want to go with a hubgen to get maybe 7 watts of available power most of the time, and a supplemental PV to make up a total source of perhaps 14 watts. I'd want to have a battery in this case to stash energy whenever it is available, and have a very opportunistic operating strategy.

Another choice might be to add a 6 watt bottle generator to the back, a Schmidt SON to the front, and have at least an extra 6 watts of energy collection while riding at night. Some power conditioning will probably be needed to ensure that both generators' outputs can be fed to the same bus and stored in the same battery. Plus, convertng to DC eliminates any problems with phasing the two gens together, since they will run at different frequencies and phases.
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Old 07-01-10, 11:56 PM   #21
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Did you mean to say winter and summer nights? I suspect you've mis-spoken, since you seem to understand PVs. But if so, I have to ask why do you need to run PV at night, and what latitude will you use it in? On earth-orbiting spacecraft we run electronics off the battery when the spacecraft is eclipsed by the earth.
Sorry for not being clear enough. I wanted to charge a battery during the day and use it as the source for lighting when I needed to ride in the dark. The latter took place most often during winter evenings. The next idea was to rely on a dynamo with the charged battery just providing supplementary power when stopping or slowing down in the evening. This was still before going with a dynohub, see below. I am in Michigan.


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If you really need on-the road charging, I think you may want to go with a hubgen to get maybe 7 watts of available power most of the time, and a supplemental PV to make up a total source of perhaps 14 watts. I'd want to have a battery in this case to stash energy whenever it is available, and have a very opportunistic operating strategy.

Another choice might be to add a 6 watt bottle generator to the back, a Schmidt SON to the front, and have at least an extra 6 watts of energy collection while riding at night. Some power conditioning will probably be needed to ensure that both generators' outputs can be fed to the same bus and stored in the same battery. Plus, convertng to DC eliminates any problems with phasing the two gens together, since they will run at different frequencies and phases.
By now all my problems have been solved. I run a hybrid dynohub/battery system. If battery needs charging, it taps any redundant power from the dynohub. For lights, I use an LED system that I actually supply with a substandard (~2 W) power which still leaves enough illumination, far more than from a halogen lamp. Even under conditions of worst winter, when I need to ride with lights on at all times, including day, I seem to have plenty of power in that charged battery. That power is exploited at times of stoppage or slowing down. I use it for a horn and plan to use it for a radio as well.

You do not need to place a second dynamo to gain more power than the nominal 3W from a standard dynamo. You just need to load the dynamo differently. I do not do that, just take out up to 3W, as needed. In general, trying to use a bottle dynamo is a bad idea. The quoted efficiencies critically rely on the soft contact between the roller and the tire. From my experience, once water comes into play, an efficient dynamo begins to slip.
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Old 07-02-10, 05:18 AM   #22
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Sorry for not being clear enough. I wanted to charge a battery during the day and use it as the source for lighting when I needed to ride in the dark. The latter took place most often during winter evenings. The next idea was to rely on a dynamo with the charged battery just providing supplementary power when stopping or slowing down in the evening. This was still before going with a dynohub, see below. I am in Michigan.




By now all my problems have been solved. I run a hybrid dynohub/battery system. If battery needs charging, it taps any redundant power from the dynohub. For lights, I use an LED system that I actually supply with a substandard (~2 W) power which still leaves enough illumination, far more than from a halogen lamp. Even under conditions of worst winter, when I need to ride with lights on at all times, including day, I seem to have plenty of power in that charged battery. That power is exploited at times of stoppage or slowing down. I use it for a horn and plan to use it for a radio as well.

You do not need to place a second dynamo to gain more power than the nominal 3W from a standard dynamo. You just need to load the dynamo differently. I do not do that, just take out up to 3W, as needed. In general, trying to use a bottle dynamo is a bad idea. The quoted efficiencies critically rely on the soft contact between the roller and the tire. From my experience, once water comes into play, an efficient dynamo begins to slip.
I'm in Michigan, too, SE area. I'd love to see your arrangement.

I only suggested the dual-gen idea because of the OP's original 10-watt load power goal. I probably should have phrased it "For the OP's needs, perhaps a dual gen arrangement might help ... " Just a sign of how badly we've hijacked this thread.

I'm not building anything too aggressive, just a bottle-driven light pair for a tour in a few weeks. Been considering an active limiter to protect halogen bulbs, but it requires a floating driver for a power MOSFET, and that's getting complicated. Better to just play with LEDs ...
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Old 07-02-10, 12:14 PM   #23
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Originally Posted by Road Fan View Post
I'm in Michigan, too, SE area. I'd love to see your arrangement.

I only suggested the dual-gen idea because of the OP's original 10-watt load power goal. I probably should have phrased it "For the OP's needs, perhaps a dual gen arrangement might help ... " Just a sign of how badly we've hijacked this thread.

I'm not building anything too aggressive, just a bottle-driven light pair for a tour in a few weeks. Been considering an active limiter to protect halogen bulbs, but it requires a floating driver for a power MOSFET, and that's getting complicated. Better to just play with LEDs ...
I start pondering whether we do not know each other already. Anyway, I might be actually around your area in the context of the summer festival. I will not be able to share the details at this time because I still cling to the glimmer of hope that I could patent the design. In fact, I largely completed an application, but have been sitting on it lacking time to put finishing touches and file. In the meantime, big guys, B&M in particular, have moved forward. I can demonstrate how it works, though. The innards are on the complicated side.
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