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About Davidz90

  • Birthday 01/09/1990

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    <p> Technic </p>

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  1. The key here is that they are connected in parallel, so the total current is quite high. That high current passes through AA batteries, which have quite big internal resistance. Regarding question 2, the supply voltage must be greater than total led voltage. In principle, the 1 ohm resistor could sort of work, although that setup works a bit differently than the one with two leds. Instead of reducing the current, we rely on the fact that the voltage drop over the resistor reduces the voltage of leds so they don't exceed their specs. But as I said, trying to control leds by voltage only is a bad idea.
  2. Yes, Ohm's law indicates that 0 resistance is needed. However, by the same Ohm's law the current I=U/R, where U=9V and R would be resistance of 3 leds. Since that is very small, the current would be enormous (like, several amps). Sorry, I have no knowledge of Lego part specs so I cannot fully answer questions 1 and 3. Regarding 1 I'd say that yes, 1:1 replacement without changes should be fine.
  3. Ah, ok. Yes, if the input voltage is varying, some "intelligence" is needed to keep voltage constant. However, if the voltage variations are not great, it is sufficient to ensure that LED always gets more than minimum needed voltage. For example, let's use the above mentioned system with 2 LEDs, 3V each and 1kOhm resistor. If the supply voltage is increased from 9 volts to 11 volts, then: -LED voltage is still very close to 3V, because current increasing exponentially with voltage also mean that small changes of current make almost no difference in voltage. -Resistor voltage is now 11V-6V = 5V -Current is now 5V/3kOhm = 1.67 mA, which is perfectly acceptable in most cases.
  4. Again, just use a proper resistor in series. It does just that - cuts the voltage (in addition to limiting current). Learned about LED variation hard way when I was building a toy lightsaber. Wired 100 LEDs in parallel and powered them with 3 volts. In theory, with so many LEDs, internal resistance of batteries should be sufficient. What I didn't anticipate was that some LEDs reached full brightness at 3.1 volts and some at 2.9 volts, so my lightsaber turned out uneven. The 2.9 volt ones burned out few weeks later.
  5. Yes, and the remaining voltage is the voltage drop in the resistor. So for example, you can have two LEDs in series (6 volts) and a resistor (remaining 3 volts); to get 1 mA current, you need 3 kOhm resistor. LED calculator says the same: https://ledcalculator.net/#p=9&amp;v=3&amp;c=1&amp;n=2&amp;o=w
  6. That is likely to burn them. LEDs have a very little resistance, the current will be much more than 1mA. You always want to have a resistor in series.
  7. More specifically, they pass zero current until some minimum voltage is applied, and then the current starts increasing exponentially (as opposed to linear relation for reistors). Also, the operating voltage can vary slightly from led to led, even in the same batch so it is very hard to just set the voltage for proper current. That is why current limiting resistor is always needed with LEDs. If you don't want to bother with calculations, but have a decent selection of resistors, you can just start from the highest resistance one and keep decreasing it until LED lights up. However, the formula is simple: (Supply voltage) - IR = (LED Voltage) where I is the target current and R is the necessary resistor. Here's a good tutorial: https://www.electronics-tutorials.ws/diode/diode_8.html
  8. Yes to this, with one caveat: at the small scale/speed Lego models are operating, typical airplane-like profile as depicted is not optimal. Very thin, curved surfaces (like in first airplanes such as Wright flyer or small birds) are better. If symmetrical profile is used (for reasons you described), a flat, thin plate may still be better than "proper" airfoil, but overall zero curvature profile all the way from blade tip to root would severely hurt performance.
  9. A pneumatic motor would either need one or several pistons (like the mentioned axial piston pump; howevet, it comes with all the complications of making them airtight) or some sort of turbine (but that would be extremely high speed/low torque unless large gearbox is added). Since nothing else really uses pneumatic pistons (as opposed to electric motors which are off the shelf parts packed in Lego casing), it would not be exactly "mass" production and certainly much more expensive than electric motor. Besides, it would really be useful only as a pump, unless used with significantly larger air tank than the one currently available. Or electric motor driving a pump, but then why bother?
  10. You would need an enormous air tank. One can get more mileage out of pullback motors (better energy density than air).
  11. 32+8=40 so it would need the same spacing as other combinations that sum up to 40, like 20+20.
  12. It would easily mesh with other spur gears. Most importantly, it would result in easy 1:4 reduction with 8t gear. An example of custom 3D printed 32t gear:
  13. Amazing! The power and speed are terrifying. Overall shape reminds me of the game "rollcage"; with slightly flatter chassis it could drive upside down.
  14. Slight offtopic but that is so cool! I considered this career path, but ended up teaching physics at university because I didn't want to focus on only one field . Got into Lego Technic at the age of 12 and at first, it was all about pushing the limits of bricks, mostly with various shooting contraptions. After breaking many pieces, fev velocity and energy records and poking few holes in my home's walls (my parents had to be super patient), learned a lot about mechaniscs and structural integrity. Finally settled on building mechanical clocks. Apart obvious things like gear ratios I learned a lot about astronomy and physics of clocks in general - in fact I published two papers on the subject, one of them has experimental data measured with Lego clock. I use Lego models in my lectures as well, students love it.