Week #5
Kinetic project
[ still working on the documentation of the calculations as well as of the final product]
Group members: Jasmine Nackash, Sao Ohtake, Michal Shoshan
The assignment for this project was to convert kinetic energy into light — essentially generating light from some form of movement. We discussed several ideas and ended up wanting to use gravity to turn a motor (that is used as a generator in this case), that’ll turn on an LED (one or more).
We liked the idea of having to do some physical task in order to have light — we imagined it would be interesting to have something like a desk lamp that required you to lift some weight in order to have your space lit — essentially making you “work for it” while visualizing what it takes to turn on a lamp. Another aspect of it we liked is that, given that the light could last long enough, it would require you to stop what you were doing, possibly get out of your office chair, and move your muscles a little bit every once in a while.
While researching we stumbled upon GravityLight (+ TED talk) which works exactly like what we had in mind in principle. We talked to people at the ITP shop, and realized we have to come up with a design that integrates a number of wheels, belts, pulleys, a weight and of course a small circuit and a form of light.
Initially we wanted to use a light bulb, but while measuring the current voltage we were getting with our prototype we realized that we probably won’t be able to generate enough power to light a “normal” bulb. Instead we opted for an LED, and we managed to get high brightness with up to three of the blue ones (that need forward voltage of 3.4V and a forward current of 30mA).
We prototyped with cardboard and yarn so we could start to figure out how the final thing will need to be built. There were a lot of little things that required further adjustments, like friction (for example between the wheels, between the dowels and the frame, and between the string and the dowel as well).
We continued testing if we can actually light an LED with a DC motor and a simple pulley mechanism (yes, we can)
And then we measured how much voltage we were getting with that same mechanism:
We got to a little over 6 volts at the peak. We decided to also try doing it with a stepper motor, which required rectification, and we also added a capacitor to allow the LEDs to stay lit for longer.
This setup allowed us to generate up to 9V.
The way we imagined it in the beginning was that the pulley will be tall (around 2+ meters), and will allow the weight to fall for a substantial(-ish) amount of time. We put everything together and then realized there are a lot more things you have to account for when hanging something in the middle of the room, like the whole thing moving sideways when the weight falls down.
We improvised a way to keep the dowels aligned with what we already had, which helped, but didn’t solve the problem entirely.
After talking to Jeff we realized that it might be best to build a frame for the pulley, rather than hanging it from the ceiling. So after careful planning and prototyping we came up with the following design:
It generated about 6 volts (peak), and thanks to the capacitor was able to keep two LEDs lit for about 20 seconds — when pulling it by hand, and not using a weight.
Power budget
[ still reviewing these numbers ]
Stepper Motor
RPM =
Open circuit voltage = 8-9 V
Short circuit current = 0.3A
P = V x I
P = 8 x 0.3[300mA]
P = 2.4
J/s = 2.4
Height — Mass
P = mgh
mass = 0.5kg
2.4 = mgh
2.4 = 0.5kg x 10 x h
h = 2.4 / 5
h = 48 cm
mass = 1kg
2.4 = 1kg x 10 x h
h = 2.4 / 10
h = 24 cm
Blue LED:
Voltage: 3.0 - 3.2V
Current: 20mA (max forward current 30mA)
Intensity: 3000 - 4000 mcd
Power: 3.2V×0.02A = 0.064W
per minute: 0.064W x 60(sec) = 3.84 J
per hour: 38.4J x 60 = 230.4J
2 Blue LEDs in series:
Voltage: 3.2 x 2 = 6.4 V
Resistor for 1 Blue LED = (8V - 3.2V) / 0.02A = 240 ohm Resistor
Resistor for for 2 Blue LEDs = 80 ohm Resistor
