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1  General Category / General Discussion / How does the Shine do Wireless Sync?? on: November 27, 2012, 01:47:57 pm
So has everyone heard about the Shine already?
http://www.technationnews.com/2012/11/15/misfit-shine-activity-tracker-beautiful-design-weird-name/

Seems like a neat device, my style than substance really, but the wireless sync seems intriguing. Does anyone care to fathom a guess with me as to how it works?

Considering it would need to work with an iOS5 device it could work via Bluetooth and Wifi but the video says they use a proprietary communication technology. I thought it first it could be something to the Electric Imp which utilizes a light sensor and synchronized flashes as a pseudo-1-wire interface but the video seems to point at something that is bi-directional (it knew the location of the device on the screen). Even ultra-sonic sounds in the same vein could not do that.

Unless that was a total fabrication made just for the video how can it determine the position of such a device (encased in metal) with just the few sensors an iPhone would have. Perhaps it can read the current pixel it is over and send that back to the iPhone, but how, via Bluetooth? Considering the size of this thing and that it's battery is supposed to last 6 months that seems highly unlikely.

Thoughts?
2  Using Arduino / Motors, Mechanics, and Power / Re: Powering 3.3v from 5v booster on: September 04, 2012, 02:42:14 pm
IC is ATTiny24 @ 5v to 3.3v EM-406 GPS module.

After some additional research it seems that while it is the most expensive option, the buck converter would result in highest battery efficiency. The reason being as opposed to a linear regulator or diode that convert the extra energy to heat the buck converter switches the energy on and off and stores it in a magnetic field (hence the inductor). This fast switching results in the energy being stepped down with less waste.

I plan to verify this with some qualitative tests and reply back here if I remember.
3  Using Arduino / Motors, Mechanics, and Power / Powering 3.3v from 5v booster on: August 29, 2012, 10:39:55 am
I have two rechargeable batteries that make around 2.5v. I use a boost converter to get this to 5v to power an mcu and other things that rely on 5v. Now, I have an ic that requires 3.3v and I'm curious what the best way to handle this would be. Here's my brain dump but I'm sure I'm missing a better more obvious way (which is why I'm asking you smart people).

1) I could (naively) use a low dropout 3.3v voltage regulator to step the 5v down to 3.3v. This is very inefficent however since a lot of power will be lost to heat and undesirable since I'm on limited battery power.
2) Use a diode to drop the 5v to around 3.3. Since the switching boost ic is keeping the supply constant I need not worry about major power fluctuations (this assumptuion is wrong of course until I test it with an o-scope). Same problem as above as far as efficiency.
3) Same as above but with a voltage divider? I'm not sure what the + or - of this would be.
4) Use a _second_ switching regulator to boost a parallel 2.5v line from the battery to 3.3v. This adds a lot of parts and cost but seems like the best way (yes, please prove me wrong).
5) Opposite of above -- use a buck converter to drop the 5v to 3.3v but regulated. I honestly have no idea what the efficiency curve for this would look like. Seems like black magic or crossing the streams.
6) Run the new ic at 2.5v with a couple of caps to smooth and pray. Yeah, I'm not doing this.

Thoughts?
4  Using Arduino / General Electronics / Multiple LEDs with 1 resistor on: August 10, 2012, 01:53:50 pm
So I'm revisiting a circuit to see how I could cut down on cost, board space and assembly time and I've started doubting myself in one of my improvements.

Basically I have a two-sided board and an SMD RGB LED on each side. The setup is identical on both sides -- 5v to a ~330ohm resistor to each color channel. This works well but I think I can do without those 3 additional resistors on the bottom side.

So the solution seems simple -- decrease the resistor values to allow more current for each LED channel and connect the LEDs in parallel to the resistors from 5v. I've implemented the idea in a simulator and it seems to work but I'm worried I'm missing something. The current is divided correctly but I'm not sure each LED will reach the forward voltage drop to turn on.

The simulator tells me the voltage drops by about half to around 2.5v but even with the LED forward voltage drop around 2.9v it thinks the LED will turn on. I'm not sure if this is a bug in the simulator or I am missing something. If I remember my theory correctly the entire line stays 5v except after the LED at which point it drops so they _should_ light up despite being split from the same power source.

I've supplied the schematic from the circuit simulator as an attachment. Each of those circuits should provide an equivalent level of brightness for each LED (I think it was ~8ma per LED).

Apologies if this sounds like an ignorant question and thanks ahead of time for any constructive input!
5  General Category / General Discussion / Re: For Sale: Sparkfun Geiger Counter on: May 24, 2012, 10:00:55 am
Indeed, but the plan was to make a fully embedded unit not reliant on a PC for logging, serial communication etc... Display was to be through an LCD and controls would allow you to configure the unit. Logging would be dumped via USB or ideally, wifi. A few other things too -- I'm sure you can imagine the possibilities.

I'll throw in a 2x16 LCD (normally $10+ depending on where you buy one) and take $120 if anyone were interested.
6  General Category / General Discussion / For Sale: Sparkfun Geiger Counter on: May 15, 2012, 02:12:02 pm
http://www.sparkfun.com/products/9848

Basically brand new -- used it once for testing and indeed -- it can detect radioactivity. I've kept it in a moisture controlled container in it's anti-static pouch so it's in perfect shape.

I was planning on building a full scale Geiger Counter but I don't have the time. Sparkfun has a revision that includes auto-reset and a switch but same functionality otherwise.

$130 and I'll pay for shipping (if in continental U.S. or Canada). I'll even include some Uranium Glass Marbles for you to test it with.

Thanks!
7  Using Arduino / Project Guidance / Re: LT1302 for constant current on: May 10, 2012, 04:45:06 pm
Indeed, those properties are inversely proportional, however, the DC/DC booster is rated to supply the necessary current I need.

I did some more research and wikipedia had some interesting insights: http://en.wikipedia.org/wiki/LED_circuit

If I understand what the author is saying correctly I am able to use a constant-VOLTAGE device successfully (as opposed to a constant-CURRENT device) but more care will need to be taken to ensure the current is stabilized, specifically with resistors.

In the end I would prefer not to add the cost of a LM317 (which I can't use anyways because of the high dropout) but I may opt for something like the LM2941 which allows me to remove the transistor I'm using to enable/disable the light.
8  Using Arduino / Project Guidance / LT1302 for constant current on: May 10, 2012, 01:09:11 pm
I'm designing a circuit that powers a 1W 'Luxeon' style high-power LED from an LT1302 that boosts a small voltage to 5v (this is required for other reasons I won't go into). My plan originally was to use 4 parallel 1/4 watt resistors to bring the current down to about 200ma (maybe 250). There are four of them mainly to give me some breathing room with the watts/heat dissipated (since I could swap out the 1/4 watt for 1/2 watt and so on). To turn the LED on I'm using a BD681(NPN darlington power transistor).

From everything I've read most people here recommend using a constant current device (like an LM314 in constant current mode) to drive the LED but I'm curious if I could forgo this since I'm using the LT1302. The datasheet doesn't specifically say that it guarantees a constant current though it hints at load balancing which should give me a steady enough current.

I'm also curious if someone could explain to me why the constant current device is so critical. If I just hooked up the LED in a similar configuration but instead of the LT1302-5 I used an LM7805 with power from a wall adapter, what issues could I run into?

In that light, when is the constant current device overkill?
9  Using Arduino / Project Guidance / Re: Best way to choose between DC Jack and USB power input on: February 02, 2012, 06:19:04 pm
I think you misunderstand -- I'm not asking how I should power the device. What I'm asking is if the end-user plugs in the device with USB AND with a DC Jack, what circuitry could I use to take power from just one of those sources (and thus not frying the device by supplying current from multiple sources).

The idea of just using a switch the user can toggle is reasonable but I would prefer to have an automatic way.
10  Using Arduino / Project Guidance / Best way to choose between DC Jack and USB power input on: February 02, 2012, 03:50:32 pm
I would like to power a device from either a DC Jack, JST socket or a USB Socket and I'm curious, what's the best way to do this?

I examined the Arduino Uno R3 Schematic and the way they do it is somewhat involved (with more components than I would like at an SMD level). Is anyone aware of a TH IC that would just take multiple power inputs and handle the job for me?

http://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf
11  Using Arduino / General Electronics / "Correct" way to make an Eagle Schematic=>PCB on: January 31, 2012, 02:43:39 pm
I'm working on an Schematic in EAGLE right now which I intend to make a PCB for. The Schematic includes buttons, switches and an LCD so it was easy to just pick the right parts and drop them in.

When designing the PCB I decided to use connectors/interfaces to the buttons, switch and LCD (since I don't want buttons for instance on the PCB) which means I would have to change from the functional schematic/part to something like a connector that I would create with labeled pins matching the function. This is a problem to me because I feel the Schematic loses a bit of readability.

I considered making a custom part with the correct symbol and the connector part but that won't work since I use a connector for multiple buttons. Is there some canonical way in which I can clearly delineate that an input pin goes to a button but also a connector pin (with physical presence on the PCB).

The solution seems to be to sacrifice readability and use connectors that are clearly labeled.
12  Using Arduino / Motors, Mechanics, and Power / Motor causing Arduino to Brownout on: January 26, 2012, 05:46:57 pm
I have a 12v motor that peaks at about 500ma at start, the same occasionally when running and hovers around 100-300ma normally. The motor is supposed to run off battery but for this case I've been testing off a regulated power adapter (12v,1amp).

I'm using PWM to control motor speed switched by a heavy duty power mosfet with resistor on gate to pin, diode on +v and cap on v+->gnd. When starting the motor at full speed everything works pretty well but occasionally the Arduino would reset. To remedy this I added some big e-caps between motor +v supply and gnd and it helped to some extent except when starting the motor at a lower speed the Arduino would almost immediately reset. After I added some more logic caps it would corrupt the LCD first, stop working then reset.

Long story short I separated the power supplies so that the motor works off the main 12v supply and the Arduino off of a 9v battery and my problems were gone. What I'm curious then is how people are able to make their motor and processor/Arduino work off of the same power supply. I would think that a power adapter that can pull 1amp would not have issues.

I understand how an inrush occurs when first starting the motor which could result in what I am describing but why would the reset happen after normal operation? Shouldn't the power supply be able to keep up with the power requirements (unlike a battery which will have internal resistance and inductance). I could just keep adding bigger capacitors across the battery +v/-v but I have a feeling something else is happening here and I would really prefer not to require two different power supplies!

Thanks ahead of time for any insights!

13  Using Arduino / General Electronics / Re: Affordable prototype PCBs for surface mount? on: January 17, 2012, 06:02:48 pm
The dipmicro tqfp32 adapter is discontinued from their site but I've gotten that exact same one from Futurlec.com for a decent price.

They're pretty good but they sometimes forget about your order if something is on backorder and you have to harass them. Otherwise very cheap with good (tiered) shipping prices and reasonable delivery times from Asia to NA.
14  Using Arduino / General Electronics / Re: Transistor and Battery Life on: January 17, 2012, 05:55:53 pm
All great suggestions, thanks much for the feedback.

Udo, PWM is a great suggestion. I did already attempt this and liked the potentiometer better because I had less flickering at lower intensity/frequency (and 1 less pin used) but I may have to go the PWM route due to the heat issue you pointed out.

James; indeed, 3W at full power (which it won't be driven at) would kill a battery pack very quickly which is why I'm trying to find a reasonable solution. I just didn't want to compound the issue further by choosing the wrong transistor. You're absolutely right, though; in the grand scheme of things I'm probably worrying about the wrong thing.
15  Using Arduino / General Electronics / Transistor and Battery Life on: January 17, 2012, 03:24:13 pm
I'm trying to understand what effect the base current of a transistor has on battery life and I'm curious if anyone could shed some light on this for me. Specifically, I'm curious if a transistor which has a base current of 100mA will incur a concurrent drain on the battery that could lead to poor battery life.

As an example I'm looking at using this transistor to drive a 1w or 3w high power LED:
http://www.fairchildsemi.com/ds/BD/BD681.pdf

The biggest things I am concerned about are power dissipation and battery consumption (brightness will be controlled by a potentiometer so I just use the MC to turn on/off the transistor).
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