This is a circuit I designed to generate a PWM signal for an RC servo given an analog voltage or high frequency PWM.  It has no microcontroller and can be built with one quad op-amp and passive components.

The op-amp I used in my implementation is a NJU7034D-ND, available from digikey, but any rail-to-rail quad op-amp will do.

Since the input stage has a low pass filter built in this circuit will accept a high frequency (>~5kHz) PWM as an input in addition to analog voltages.

PARTS:

• 1 Quad  Op-amp (I used NJU7034D-ND)
• 1 10K resistor
• 1 50K resistor (or 2 100K resistors in parallel)
• 7 100K resistors
• 1 0.47 uF capacitor
• 1 0.047 uF capacitor
• 1 10K potentiometer (can be subbed with 100K pot, makes calibration trickier)
• 1 100K potentiometer

HOW IT WORKS:

Linear Ramp Generator:

This is the beating heart of the system, literally.  This is an integrator and a hysteretic comparator hooked into a feedback loop that produces an oscillator with a linear ramp as output.

Low Pass Filter and Buffer:

This is simply a stiff RC low pass and an op-amp follower.  The RC low pass removes high frequency noise from the input.  The follower drives current to the calibration stage, which requires current that the RC low pass can’t deliver.

Calibration:

Every servo is a little different.  These two potentiometers map the full swing of the input voltage to a small range of voltages feeding into the comparator stage.  These have to be set properly for the servo to swing through its full range of motion.  For calibration instructions, see the CALIBRATION INSTRUCTIONS section.

Comparator:

This generates the PWM that is sent to the servo by holding its output high when the input from the calibration stage is higher than the linear ramp from the oscillator and vice versa.  The width of the pulse is directly related to the voltage coming in from the calibration stage.

CALIBRATION INSTRUCTIONS:

1. Connect power and ground.  Connect the servo you want to use to the output.  Connect the input of the circuit and set it to Vcc/2.  The servo should swing to a position, probably one of its physical stops.
2. Adjust the pot labeled ‘Amp Adjust’ all the way to one side so that its output is directly connected to the output of the ‘Bias Adjust’ pot.
3. Rotating the ‘Bias Adjust’ pot should rotate the servo.  Position the servo in the center of its range of motion.  The closer to the center the better.
4. Set the input of the circuit to 0V.  Now adjust the ‘Amp Adjust’ pot until the servo is hitting one of its hard stops.
5. Your circuit is calibrated.  The full range of input voltages from 0V-Vcc should now map linearly to the position of the servo from one hard stop to the other.

NOTES:

• If your PWM frequency to the servo is too low or too high, decrease or increase (respectively) the value of the capacitor labeled ‘freq adjust’.
• You can use 2 100K resistors in parallel as the 50K resistor, reducing the number of different resistor values you need from 3 to 2.
• You can use a 100K pot instead of a 10K pot, reducing the number of different pot values you need from 2 to 1.  This does make calibration trickier, just play with the pots until the range of motion is right.

Good luck!

~James

]]> http://www.charmcitynetworks.com/techblog/drive-an-rc-servo-from-analog-voltage/feed/ 0 http://www.charmcitynetworks.com/news/new-weekend-hours-11am-4pm/ http://www.charmcitynetworks.com/news/new-weekend-hours-11am-4pm/#comments Tue, 02 Mar 2010 16:45:07 +0000 Chris http://www.charmcitynetworks.com/?p=378 We are experimenting with weekend hours at our 1414 Key Highway shop, to provide better service to our residential customers.

We’ll be open this weekend from 11am to 4pm Saturday and Sunday.  These hours are subject to change as determine customer response.

Let us take a look at your computer!  Most repairs are just $99.

The Hard Disk password is stored not only on the logic controller, but also on the disk itself, and make the drive unusable without entering the password.

Under normal circumstances, people will password protect their windows logon, but their data is easily obtainable just by removing the hard drive and accessing it from another computer.  If you set the Hard Disk Password, this is not the case.

This customer happened to know their Hard Disk password, though they had inadvertently set it while going through tech support with someone else (I don’t know how it came to this, but we don’t ask questions, we just fix what people want us to fix.

The hard disk password was just an extra step to go through at startup… it wasn’t really a problem, just a nuisance.  She asked if we could remove it, which is usually an easy process by entering the BIOS.  To our delight, when we attempted to enter the BIOS, there was a password on that as well.

Again, the system started up just fine, so being locked out of the bios only becomes a problem when you need to get into the BIOS.  Most people never do, but it’s a fairly common thing around a computer shop.  The customer didn’t know her BIOS password, or that it had even been set.

If you start your googlehunt for BIOS password recovery, you will find everything from default backdoor passwords for various BIOS vendors, methods for resetting EEPROM memory by shorting out pins, removing the CMOS battery, and a hundred other “proven methods”, but not one thread that ends with “thanks, that worked!”

The bottom line is that the BIOS password is a security measure that is meant to protect your computer in the event that it is stolen.  Most never enable theirs at all, but if you do, make sure you put it somewhere secure and remember it.

In the case of our customer, after a few hundred tries we were able to guess the password, which was derived from the other passwords of hers that we already knew.  (That’s lesson number 2, don’t make your passwords to similar or people like us can guess them!)

-Chris

Some other reasons to do a clean install are:

• When transferring a computer from one owner to another.  This is to delete any personal data and make the computer “new again”.  This is a great option for parents who want to “hand-me-down” their computers to kids.
• When buying a new computer.  It’s a stretch, but some new computers are so bogged down with crapware from the manufacturer that customers just want a clean copy of Windows without popups from HP and Dell appearing every 5 minutes.  (It’s usually simple enough to disable or remove these programs individually, but some people just feel better with a clean install)
• When moving a computer to another user in an office.  Many employees like to make the computer “their own” and add every toolbar known to man, icon and sound tweaks, weatherbug, ding!, and a hundred other background apps.  Sometimes it’s easier and faster to do a clean install then to manually remove lots of programs.

The biggest causes for concern when doing a clean install are personal data and programs.  After the installation is complete, you’ll have a pristine copy of Windows with NO PROGRAMS, and NO DATA in the user’s My Documents, Desktop, and other personal folders.

Moving data from one installation of Windows to another is pretty straightforward.  When we do a clean install, we will generally backup the Documents and Settings folder in its entirety, storing it on one of our temporary file shares, and then restoring everything to the clean install.

The original inspiration for this blog entry was to emphasize that a clean install means you will lose ALL of your programs.  If all you use is Firefox, AIM, and BitTorrent, then your reinstallations are just a download away.  However, that expensive Microsoft Office Suite, Quickbooks, Games, and anything else that is not a part of Windows will NOT be there waiting for you after a clean install.  You will need the installation media and license keys that came with these applications, and you will need to run each installer separately.

We like to remind people that however their computer is running, it’s just a period between clean installs (like a warm period between ice ages).  You should always have installation media and get yourself int he mindset that you might have to rebuild your system tomorrow.

-Chris

This post will show you how to put a nearly dead laptop to good use: Make it a digital sign! You will need the following:

• A dying, old, or otherwise neglected laptop.
• A spare LCD display.
• Display cable, power cables, network cables.

First, we gut the laptop.  The screen was failing, the case was cracked, and it’s lugging around a dead battery, CD drive, and other parts we won’t need anymore.  We carefully remove the motherboard and discard the pile of screws, connectors, and plastic that’s leftover.

Next, re-seat the memory, the hard drive, and keyboard (normally we wouldn’t need a keyboard, but in this situation the power button is integrated into the keyboard).  Connect it to power and an external VGA display, and see if it boots.

Now we want to “wipe” the computer, or perform a clean installation of the operating system. This is a measure to make sure that the system doesn’t have any lingering junk from the previous user, and makes sure their data is GONE.

Once the new OS is up and running with all updates, assign the computer a static IP address, and make sure it can receive remote connections.  This computer has a licensed copy of Windows XP home, which does not support incoming remote desktop connections, so we went with RealVNC instead.

This Laptop also had wireless capabilities through a PC card, which eliminated the need for a network cable at the installation location.

After securing the LCD in our front window, we hooked everything up, powered on the computer, and VNC into it!  Now we can run a powerpoint slideshow, show a website , or cycle through images.

-Chris

Download the source here.

Download the windows executable here.

About:

One of the things I love about Gmail and Gchat is the logging. Every chat I make using Gchat is stored in my archives just like an email.

Unfortunately, though you can download and archive all your regular emails through Gmail’s POP interface using regular email backup tools, for some reason they don’t allow access to the chat logs this way.

Another thing I love about Gmail is the powerful search feature. Google writes search software well, ’nuff said. Unfortunately, you can’t really download emails through a standard POP-based program based on a Gmail search query (if there’s a tool that does this let me know!).

So I hacked together a small program that addresses all of these issues. GmailDownloader is a command line program through which you can:

• Log into your Gmail account
• Run a Gmail search
• Download the results (including chat logs!)

It runs on the Gmail web interface, so it bypasses the restrictions of POP access.  There are limitations to this program:

• If you download too many messages in too short a time, Gmail could detect this as unusual activity and lock you out of your account. I’ve successfully downloaded 1100 messages in one query without being locked out.  I’ve been locked out several times though when trying to download much more all at once.  The lock was released several hours after filling out the unlock captcha each time.
• Since the Gmail web interface is not a standard like POP, it could change at any time and make this program stop working.  Let me know if that happens and I’ll try to update it.

This program uses python, libgmail, a hack to libgmail from this awesome guy, mechanize, ClientForm and ClientCookie.  It is based on the libgmail example program ‘archive.py’.  The Windows executable was generated using py2exe.

Howto:

1. Download and unzip the executable (or the source code… it should run out of box if you satisfy its dependencies)
2. Run the executable.
   
   
   
3. Enter your login credentials.
   
   
   
4. Enter the search term for which you want to download the results.  You may want to experiment with searches in the Gmail web interface to get your search right, searches with many results take longer through the program take longer because the program runs through every page of the results before displaying anything.
5. The program will tell you the number of hits the search term brought and show you the first few results.  Confirm that this is what you want and hit ‘y’ and enter.  If it’s not what you want hit ‘n’ and enter and return to step 4.
   
   
   
6. Sit back and watch the emails roll in.  They will be dumped in a timestamped file in the same directory as the executable.
   
   
   

Enjoy!

Our colleague thought we might be in a great position to set up a maintenance contract to deal with his server issues given our close proximity to DataPoint.

It was kind of surreal to walk through what resembled a library but was 100% composed of 6 foot tall data racks, packed to the brim with servers, network cables, fiber, power distribution, and every brand of server, computer, firewall, switch, PBX, or other network gizmo you could imagine. One rack even had an old beige analog phone sitting in it! Every server in the center has one or more fans; when added to the ambient noise of the HVAC system, it’s a WHIRRR like you’ve never heard before.

The beauty of cloud computing is that these (presumably) thousands of companies don’t need to worry about their network connections going down or their buildings catching fire, power outages, et cetera… they can be on the other side of the planet and their servers are whirring away, safe and sound in Baltimore.

-Chris

From what we’ve seen Targus dominates the universal power adapter market with this. From what we’ve seen the adapters themselves work alright (though there are reports of overheating around the internet). The main problem we’ve seen is that there are a wide variety of tips available, and several tips may work to power one laptop model even if they don’t fit properly.

It usually goes like this: the user buys a new universal power adapter, which comes with 7-10 tips for various laptop models. He sits down with his laptop and tries to jam each of the tips into the laptop power jack one by one until he finds one that goes in. Then he stops looking and starts using this tip. The problem is even if it goes in, it may not be the right tip, and an ill-fitting tip will damage the laptop power jack over time as it bends the contacts out of place.

To find the tip that fits, use Targus’s Tip Configurator. Simply enter the make and model of your laptop and it will tell you the correct tip to use. Keep in mind that many tips are not included with the universal adapter and must be ordered separately. They’re around $10 each, but if you already have the universal adapter it’s probably worth it, as it may save you an expensive laptop power jack repair necessitated by prolonged use of an ill-fitting tip.

Another option is to just buy the power adapter made for your laptop by the manufacturer and not bother with Targus’s crap. This is what I would do.

Opening the old files was simple enough. There is a free program out there called Lotus KeyView that allows you to open (but not edit) .lwp files. We were easily able to copy and paste the text that Kevview presented to us, but it was certainly not encoded properly. After installing Russian Language Packs on Windows and an exhaustive search on how to let Microsoft Word know that this was indeed Russian text and not English, we discovered a neat little trick.

In the language bar on Windows, you can easy select between installed keyboard languages.  By copying the encoded text, switching to Russian keyboard layout, and pasting into MS Word, we were able to restore the Russian text without a flaw.

We were able to restore the 30-page Russian Dissertation written using Lotus WordPro to working, editable, order in Microsoft Office.

This video was awesome at explaining the suction cup method for opening and iPhone, which works like a charm. You remove the two screws at the bottom, apply a 2″ suction cup to the face, and pull gently back revealing… iPhone innards and dog puke residue.

I let the components dry a little bit more, removing the logic board and battery… look it all apart and reassembled it a bit, but no dice.

I was able to get ahold of a working iphone with a cracked screen, and hoped that the even if my iPhone was dead, the screen might still be intact. The swap was performed tonight, and all appears to be running smoothly. iTunes is restoring my apps, and I’ll soon be on my way.

-Chris