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Saturday, September 13, 2014

I mapped out the circuit today. I discovered lots of free online circuit drawers, which was cool. I used the digikey one this time.

Stock Circuit. Note: there are two crossovers that look like 4-ways.
The only 4-ways that are actually 4-ways have dots.
For comparison. I have L1 and L2 flipped on my circuit diagram.
I attempted to lay it out in a similar manner to how the components are on the board (single layer PCB). I also attempted to understand what was going on. I am no EE though, so please correct me if I got it wrong. Starting from the red "+AC" line (left), there is a 150Ohm resistor, followed by a 250V film cap in parallel with a 1MOhm resistor (hidden under cap), followed by a shottky diode (forward) and a zener diode (reversed). These diodes are copied on the -AC line side. I think those components all make up the AC to DC converter, guessing 120VAC to approx. 5VDC. The DC side of the shottky diodes are linked and the DC side of the zener diodes are linked, giving the +5V and 0V references respectively. Following the +5V reference through a 33Ohm resistor (now about 3V), we get to the LED that makes the ball glow (the one labeled L1 on the board and L2 in my diagram). In series with that is a 1Ohm resistor and the projector LED, which then terminates into the 0V reference.

Now for the control part: Photoresistor resistance decreases with light and increases in dark. So if the room is lit up, power flows from the 5v reference node to through the low-resistance photoresistor, into the base of the transistor, which means it is on. I'm guessing the 20KOhm resistor in series to 0V with the photoresistor (with the transistor base tapped off between them) acts as a voltage divider, but I'm unsure. With the transistor on, the path that branches off between the 33Ohm resistor and the L2 (schematic) diode is now a low resistance path to ground, so all of the power (~0.25W?) goes that way instead of through the LEDs. If it is dark, the photoresistor resistance is high, causing the transistor to be off, preventing power to flow through it, allowing the LEDs to be powered. It's probably a fairly efficient circuit when it is on, but when it is off it is constantly bleeding some power (guessing about 0.25W) through the 33Ohm resistor and the transistor. Probably a good idea to unplug these when not in use.

Modification plans: I want to add an option to power this using a Joule Thief circuit.


The "JT" voltage source represents the Joule Thief circuit. The switch will allow me to switch between AC and DC mode. It should be fairly safe this way. I'll probably put a diode in the JT circuit to prevent power from flowing across it in case I plug it in with the switch on. The good thing about this setup is that the photoresistor should still function normally in DC mode. However, the power drain when the transistor is on is concerning...it may kill the already partially dead AA's I'll be using for the JT circuit in a single day. We'll see. If that ends up being a problem, I could probably cut a trace somewhere and install another switch.

Challenges moving forward:

  • I wasn't able to get exact values for many of the components, so I will have to use a scope/nice multimeter to figure out what voltages are where in the circuit. I'm not sure how I'll power it since everything will be exposed...I need some sort of low current 120VAC source.
  • Knowing the voltage difference across the LED's will give me the JT design voltage.
  • I want to figure out how much power the stock unit draws when plugged in (off and on).
  • Designing the JT circuit (lots of internet tutorials).
The goal of this project is to modify a "Projectables" brand plug-in projection night light with a Joule thief so that partially dead AA batteries can be used to light it up, while maintaining plug-in functionality. Basically, I wanted an excuse to make a Joule thief and to use up a bunch of partially-dead AA's. You can buy these from Amazon, Home Depot, directly from the manufacturer, and other places. I have the solar system one that I picked up from Home Depot for $10.




You can see the light sensor (photoresistor) here
They work by shining an LED through a image and a lens, all of which is located in the globe, which can be rotated 360 degrees thanks to springy-metal contacts. The following pictures detail the take-apart process.

It's held together with three TA20 (triangle) right-hand threaded screws. These puzzled me for a bit because I figured they'd be standard threaded, but nope...they have reverse threads.

Lots of patents
Insides:

The LED under the globe is for making the globe glow.


Spring contacts allow for 360 degree rotation. Clever
 To disassemble it more, you have to take needle nose pliers and pull out the little gray piece of plastic holding the wall plug contacts in. Then you can push the wall plug contacts out.

Bottom: gray piece that holds in plug contacts. The clear piece is the photoresistor protector.


I'll map out the circuit later. It seems pretty simple. I want to be able to tie the Joule thief circuit into the two LED's. I'm hoping to be able to add a simple switch somewhere to allow me to switch between wall mode and battery mode.

Circuit
Popping open the globe (snaps together like one of those plastic Easter eggs) reveals the projector. It was a bit more complicated than I thought it would be and splits into a few pieces. Starting from right and moving left: lens, tube, solar system picture (the tabs keep the picture in place), lens, tube, LED.
The projector







Friday, September 12, 2014

I decided to take a mechatronics class at FIT because it would be different and very useful. The class is split into two portions: learning to program an Arduino pro-mini using C and Atmel AVR-Studio, and then learning Matlab/Simulink XPC. We have to use C because "the arduino IDE is too easy".

Project 1 was to make a stopwatch using a 4 digit 7-segment display and the arduino pro mini 328 5V/16MHz. It must count up in increments of 0.01s using interrupts/timing, and have a button for start/stop and reset. I used the FTDI/USB device to program the arduino (with bootloader) through AVR-Studio.

The breadboard circuit shown below was wired. The display has pins for each segment and a common cathode for each digit. There are 330Ohm resistors in series between the arduino and each anode. There is a NPN transistor for each cathode that act as a switch. When an arduino pin drives the base of the transistor high (through a current limiting 10kOhm resistor), they turn on, grounding the cathode, allowing individual digit control. The digits are cycled through rapidly to take advantage of persistence of vision. The button is hooked up to pin 13 on the arduino, which is the one with the onboard LED. This made using it as an input somewhat difficult because the onboard pullup resistor is 20kOhms, which results in a pin voltage of around 1.7V instead of 5V because the onboard LED is drawing current. So I had to drive the voltage on the pin high as the trigger instead of grounding the pin as I would have normally done.



As for code, there is a main loop that contains the button polling routines. There is also an interrupt service routine (ISR) that handles the digit counting and timing via a bunch of volatile variables and if trees. There is a "on" routine that takes care of turning on and off the correct segments for each number 0-9. I can't make the code available since the class is still running. Apparently the more common way to do it is to have the ISR act as a 0.01s timer that calls a separate function that handles the digit counting, which would probably avoid the use of volatile variables.

In addition to the component datasheets, the resources I used are below:


Sunday, September 7, 2014

L3 Rocket

Purpose: document the build and flight of my Tripoli Level 3 certification rocket

I've always liked the Phoenix AIM-54A missile. I had an Estes kit of it when I was little that I put a 24mm motor mount in it. I flew it probably close to 50 times before it died. I've seen a few larger scale Phoenix missiles, but I've never seen a 1/2 scale kit, so I decided to custom build one.

I got the specs from here. Using stock 7.5" ID, 7.75" OD phenolic tubing results in a 0.517 scale, which is close enough. Given that, the body tube length should be about 63", and the nose cone should be slightly less than 3 to 1. Though it's not ogive in real life, 3:1 7.5" ogive nosecones are commercially available, so I decided to use one and make the body tube slightly less than 63" to compensate for the slightly too long nose cone. I decided on a 3ft long, 98mm motor mount and 1/4" thick 12ply wood fins. I'm building it super tough, so it should be able to go supersonic (large M or small N motors).

The rocket was designed in OpenRocket and Solidworks.
Solidworks model of stretch 1/2 scale Phoenix

OpenRocket model of Phoenix

OpenRocket model of Stretched Phoenix
OpenRocket is predicting ~7000ft for the stretched (dual deploy) Phoenix on a medium M. It predicts ~2000ft on a medium K for the non-stretched Phoenix. Both configurations will use about 4lbs of nose weight. This is due to the stubby and spread-out nature of the fin area, causing the CP to be further forward than usual for a rocket this size.

My dad gave me two 48" long, 7.5" phenolic tubes, two 12" couplers, and 48" of 98mm phenolic motor mount tube. He also has (had) tons of fiberglass, so he let me use it. This wasn't my first time making composite rocket tubes, but I didn't have all the supplies I was used to (MIT Rocket Team, DBF had vacuum bagging supplies we used to make tubes with...I was spoiled) at dad's house, so I had to do a basic wet wrap. I measured and cut two wraps of ~10oz 0-90 E-glass and two wraps of ~4oz 0-90 S-glass for each tube. I used medium cure time West Systems epoxy and wet out the FG as I was laying/stretching it around the tubes.

Setup for layup. Katie's L3 kit is in the background



Lots of fiberglass

Finished layup

Heated cure
Each tube went from 3lb pre-glass to ~5lb 12oz post-glass. Final OD was ~7.75". The surface finish was terrible (no peel ply or release, no vac bag, etc. ), so I had to do a ton of sanding and filling (two coats of automotive spray primer and filler). You can see the result in some of the pics below. Total time spent on the tubes was probably 20-30 hours. 

If I had left the couplers stock, I ran the risk of coupler failure (breaking in half), which is common in these large rockets during high accelerations. So I fiberglassed the insides with 2-3 layers of 10oz glass. Balloons were used to hold the glass against the insides, then popped after curing.


 
I wanted to do 1/8" G10 fins, but I do not have access to a CNC router or large enough laser cutter here. Custom fins would have been ~$400, which is crazy, so I bought a $50 sheet of 12ply plywood from aircraftspruce. I penciled the lines using a cardboard stencil and checked them all with a ruler. 

I then cut them out with an oscillating saw. I tried to sand them with a palm sander and sanding block, but quickly realized it really needs to be done with a belt sander, particularly the beveling. I'll finish them later. I'm going to build a jig for the FIT shops's belt sander and do the beveling there.

I slotted the tube next using a Dremel and reinforced cut-off wheel. I made two slits lengthwise per slot, then broke the material in the slot out using needle nose pliers. Fiberglass is brittle, and the thin/small amount on the ends of each slot made breaking it out easy. I then finished the ends of the slots with a 1/4" sanding drum attachment.




All tubes done
All tube ends were sanded smooth, treated with thin CA, then sanded smooth again to prevent phenolic wear.

Left to do:

  • Buy plywood centering rings, bulk head, nose cone, altimeters
  • Cut motor mount tube
  • Finish sanding fins
  • Make motor mount
  • Install fins
  • Filleting 
  • Build altimeter bay
  • Minor stuff
  • Final primer coat, sanding, then painting

The fins will have internal and external fillets. The rear two centering rings will not be installed until the set of fins they contact are completely done so that the volume they enclose can be filled with expanding foam. The shock-cord mount will be a pair of U-bolts in the forward centering ring. Altimeter bay will be classic rods + sled construction. I haven't picked the altimeters yet. The goal is to be finished by December.

Next time I build a big rocket, I will probably order pre-glassed tubes unless I can figure out a way to get a much nicer surface finish. I'll get them pre-slotted, too, unless I build a nice router tube slotting machine/jig. I want a CNC router and laser cutter in my home shop eventually, so that will make fins significantly easier. 

Thursday, August 21, 2014

Random Verizon Rant

I hope someone out there finds this remotely useful. I never imagined how big of a pain it would be to switch from a post-paid account to a pre-paid account on verizon.

Let's say you have a postpaid account with multiple lines and the contract end date has come. Now let's say you want to get a new phone for one of those lines, but you want to get that phone on a prepaid account, but you also want to keep your phone number. That's what I wanted. I got on verizon.com to shop for some prepaid smartphones and ended up buying two, one for each of the lines on my account, and 1 month of prepaid "allset" accounts with new numbers/sim cards to go with them...all under the guidance of a online verizon sales rep. She said that I would just call and they could activate the phones with my old number. Simple right? WRONG.

I was on the phone with verizon for no less than 4 hours being passed around like a hot potato. Almost no one knew how to handle this.

Tips and lessons learned to avoid this headache:
1. Do not buy more than one prepaid phones with prepaid accounts at the same time online. Make sure they are in separate orders. Shipping is free, so not a big deal.
2. Make sure your billing address is your shipping address. Their online ordering system can't handle separate billing and shipping addresses. Otherwise, you will have to call customer service for an hour to get them to re-enter the order with your billing zip code to get it to go through.
3. If you want to use your new prepaid accounts that you paid for, do NOT follow any online instructions on activating your new phones, i.e. do NOT switch the SIM cards. The only way to activate a prepaid phone with your old number is to call them.
4. Do NOT call late at night. They will go home and leave you on hold until the line dies.
5. Start with verizon wireless customer support and say "agent" in the menu. That will transfer you to a person. Explain to that person that you want to activate a new prepaid account and phone you purchased from verizon.com with a number you currently have in a postpaid account.
6. WARNING: IF you ported a number from another carrier, do not let them deactivate your old line, or the number will revert back to the original carrier. There is another process they have to follow that involves higher-ups if your number was originally from another carrier.
7. Verizon stores (the legit ones) can't help you with this problem.
8. Ask lots of questions and be skeptical

The process is as follows: First to have customer service deactivate your old line and reserve the number. Then they have to toss you to the prepaid department. I think if you follow the above tips and have an original verizon number, the prepaid department can simply activate your new phone and prepaid account with your old number.

My experience: I did the opposite of those tips (at the suggestion of various verizon representatives who were all wrong). Thus, my experience was painful. Once I finally got to people who could help me...First they deactivated the line I wanted to switch. Then I was tossed to another department. They tried to activate my new phone but it wouldn't work. Then the line I was on hold on died around midnight. The I called again in the morning.  They tried activating the phones but found that the problem might be that I had purchased two prepaid phones on same order and the system got confused. Then they realized the old number was from another carrier and had to reactivate it in the original postpaid account to save it. Then they tried splitting the order so that the new phone could be activated. This took me to hour 4. Eventually they tossed the problem to "systems", which I'm guessing means tech guys. I got a few calls back, but after another 5 or so hours, it was finally resolved.

If the mistakes under lessons learned had been my doing, ok, I wouldn't be upset. But everything I did was under the guidance of verizon representatives!! At least all of the verizon reps were nice and friendly, and the higher-ups I talked to were helpful.





Monday, June 30, 2014

Over 2 years...

Wow time flies. It's been 2 years since my last post. Life... Graduate work is sucking most of my time currently. I need to start my thesis (in the field of cyrogenic fluid slosh dynamics) soon, which won't make doing side projects any easier.

Projects update:
-EHB and LITE are suspended until further notice. I'm currently in a small apartment with very little access to machine tools, which makes making things difficult. They will eventually be built and will be followed by some other similar projects I have in mind, probably a scooter.
-I'm working on my Level 3 certification rocket. It's going to be a half scale Phoenix missile.
-Also designing a huge rocket with some MIT Rocket Team cruft. More details on that closer to launch day (BALLS 2015)

I'll post a build log of my L3 rocket similar to my L2 rocket when it's complete.


Sunday, May 13, 2012

3 months....more ouch

Yeah...I haven't done anything on my boards. I got swept away by MIT's work load again. All my free time was spent leading the team that built this:

MIT DBF 2012 Competition Plane
It's our plane from the DBF competition this year. I am an aerospace engineer after all...gotta build some things that fly :P .

About DBF:
DBF (Design/Build/Fly) is an international RC aircraft competition sponsored by Raytheon, Cessna, and the AIAA. Every year in September, a new set of rules/missions are released, and 60-80 teams design, build, and fly a plane for those rules. The contest site location alternates between Tuscon, AZ and Wichita, KS. 1 plane is allowed, and the missions are usually very different, making it a complicated design/optimization problem. Also, weight is usually a HUGE scoring factor (in some years it's been the only one that really matters). Also, lithium batteries are not allowed...It's like Flintstones meet the Jetsons in that regard (NiMH battery tech meets top-of-the-line composite technology).
The missions this year were to fly as many laps as you can in 4 minutes, carry 3.75lbs of aluminum block for 3 laps, and carry 2L of water up to 100m as fast as possible and then dump it.

About MIT DBF:
We're a small team (~12) undergraduates, usually with one or two graduate advisers who help out here and there. No profs at MIT have the time to devote to helping the team out, so we're really undergraduate run.

About our plane:
Unfortunately, I can't tell you very much about our plane because DBF is SUPER competitive, and a lot of how/what MIT DBF does is secret. I can tell you what you see in the pictures though. This plane is the product of a semesters worth of design/analysis, and a semesters worth of building. (for a reference length: it's wing span is about 1.4m). It has composite reinforced (various amounts of carbon fiber/fiberglass/Kevlar...I won't reveal weights or locations) foam wing and tails. The airfoils are very precise thanks to the CNC foam cutters the Aero/Astro department has. The fuselage is made out of a honeycomb/fiberglass composite; we CNC milled our molds in house for this fuselage, and developed our own manufacturing processes. The propulsion system is: NiMH battery pack, Castle Creations ESCs with Axi Gold brushless motors, and APC propellers (different props for different missions).

More pics:


Filling with water.

The team.
How the competition went:
Not well. Fucking tornadoes. I'll start from the beginning though: The plane made it to Wichita via freight no problem. It was SUPER windy every single day there. First day, we successfully completed Mission 1 (the speed mission) in the morning with 6 laps, at the end of which, an ESC fried (it was a bad one...we weren't anywhere near the amp limit). Then we decided to call it a day and go do Mission 2 flight testing at a small RC airfield nearby. The next morning we successfully completed Mission 2 (aluminum block passenger mission) in 20+mph winds. We were sitting in the top 5 at that point (top 3 if you didn't count the heavy planes that had already finished all 3 missions, but were way too heavy to place in the final top 5). Then we were waiting around for our next turn (they cycle through everyone for flight attempts) to do Mission 3 when they closed the flight line due to some rain and high (30ish) winds. It's unfortunate...we designed our plane to be very fast, so it could have handled the wind. Oh well, we decided to go back to the hotel and wait for tomorrow, the last competition day. That night, 90 tornadoes ripped through Kansas, out of those, 10 hit Wichita, and 1 happened to go right through the competition site, wrecking everything. They had to cancel the competition; no reschedule, nothing. We were super bummed: 1000's of man-hours and 1000's of dollars went into that plane. But we went back to the small airfield and completed Mission 3 (2L water dump mission) in ~40s in 35mph+ winds. We did the math...if we had completed Mission 3 in anywhere close to that time at the competition, we would have won. It sucks...a lot. The judges decided to get rid of the 3rd cycle of flights in the competition (the one we did M2 on), so we ended up in like 12th (because we only had one mission score...), or something. The places are total bullshit this year; 1/2 the teams never even flew, and the ones who placed high would have placed much lower had half the competition not been canceled.
I'm just glad Tuscon doesn't have tornadoes.


I'd like to thank the MIT Edgerton Center, the MIT AIAA Chapter, the MIT GEL Program, Lincoln Laboratories, the Department of Aeronautics and Astronautics, Aurora Flight Sciences, and Lockheed Martin for their support this year, and hopefully in many years to come!

-Jed Storey
President, MIT DBF