Jay's Technical Talk

Robot Pool Skimmer Propulsion Test

Jay — Wed, 16 May 2012 00:45:12 +0000

One of my long term projects is to build a robot pool skimmer. It will probably turn out to look even more ghetto than this pool net taped to the front of an RC boat:

The point of this exercise was to test the thrust of the motors on one of the boats I purchased as a donor hull. It is able to push a normal sized leaf net around the pool (slowly).

This particular boat has dual motors/propellers and steers via “tank control”. It turns in relatively large circles even when a leaf net is not taped to the front. Luckily, I bought more than one…my plan is to built a dual hull boat (catamaran) with the leaf net (and a larger 12v sealed lead acid battery, and perhaps a solar cell) between the two hulls. That way I’ll have two props on each side of the boat, and should be able to turn much faster because the thrust will have a larger moment arm. Also, I’ll have twice the thrust (and hopefully less than twice the drag?).

I don’t know if the motors in the boats can be ran in reverse. They can’t be reversed using the provided remote control, but once I wire them up to a standard H-Bridge I suspect I may be able to travel backwards. That will be key for keeping from getting stuck directly up against a wall.

I’ll need to work out a nice roller arrangement to make it easy to skim along the sides of the pool (where a lot of leaves stick) while still being able to scrape and escape corners. My current plan is to built the mechanical platform making use of the provided remotes for testing purposes (the boats can be selected between 3 different channels, so I’ll be able to control two at once) and only work on the robotic control (sensing, h-bridge, autonomous motion) after the mechanical system is working well. At a minimum, I want it to be able to pilot around all 4 walls, and perhaps just do a random walk through the rest of the water. I also want it to be able to detect the pool skimmer opening, and be able to drive the debris net up against it (and wait, or reverse) to deposit the debris it has picked up.

Here are some commercially built similar items:

Solar powered robot pool skimmer – $550
Remote controlled pool skimmer – no longer available
Hose powered surface skimmer

Using Cloudflare to speed up webpage loading speeds

Jay — Sun, 13 May 2012 18:22:38 +0000

My web host (Dreamhost) has an agreement with Cloudflare, a content delivery network, that allows you to enable the Cloudflare basic service for any of their domains. In mid-April I enabled the Cloudflare service for one of the domains I host with Dreamhost. The chart below from Google Labs’ “page load speed” report clearly indicates that the Cloudflare network helped the speed of loading the web pages. Of course, if you look at the far left of the graph, you can see that Dreamhost on their own was able to provide the same page load speeds without Cloudflare back in January, and the page load-time has been growing slowly since then. It is possible that Dreamhost is using the availability of Cloudflare to offset a higher load on their servers. But all in all, the website is close to the top 20% (which Google labels “fast”), which isn’t bad for a personal web hosting account.

(click the image for a full sized version)

Modifying LED christmas lights to run off of 120 volt DC

Jay — Thu, 10 May 2012 18:32:03 +0000

Normal Christmas lights will run off of a 120 volt DC source just as well as an AC source because they are a resistive load.

LED Christmas lights will also run off of a 120 volt DC power supply (such as the traction pack on my electric pickup), but because they are diodes the polarity of the supply voltage must be correct. With the string I bought, half of the string was oriented one way, and the other half of the string was oriented the opposite way. This results in each LED only being lit 1/2 of the time when powered from an AC line, but if you plug the string of LED’s into a DC source only 1/2 of the string lights up. By reversing the connections at the middle of the string I was able to get the whole string to light up.

Of course, it could be that the LED Christmas tree lights are designed to only operate at a 50% duty cycle, so I may be over-heating them, but I figure that since they are mounted on the front of my truck they should get plenty of air cooling. So far they haven’t died!

Lensmount STL file for Opteka Fisheye Lens & Microvision show-wx laser projector

Jay — Tue, 08 May 2012 16:59:10 +0000

Nirav Patel has made a 3D model of a lens/projector/globe mounting system for a Microvision show-WX laser projector, a Opteka fisheye lens, and a frosted glass globe for a spherical display system first described here using OpenSCAD. The OpenSCAD file is here. I went through the trouble of installing OpenSCAD and exporting the file as an STL file so that I could print it. If you want to avoid that trouble, you can download it from me here: lens_mount.zip.

Missed Schedule for wordpress posts due to .htaccess password protection blocking wp-cron.php

Jay — Fri, 27 Apr 2012 01:00:13 +0000

I recently set up a private blog that is fully password protected using an .htaccess file. (Users have to enter a username and password to view anything on the blog.) Everything was working well until we tried to set up posts scheduled to publish in the future. They would not post. The dashboard would show “Missed Schedule” and we would have to publish them manually.

After lots of head scratching, I finally figured out what was causing this. Apparently wordpress will call the wp-cron.php file every time a user views a page. HOWEVER, because the web server is doing this, and not the user’s browser, if the wp-cron.php file is protected by a .htaccess user authentication system it will not load (because wordpress doesn’t have the correct username/password!). So none of your scheduled actions will take place.

The solution is to allow (only) that file to be loaded without the normal username/password check in your .htaccess file like this:
#Require username and password for all files! AuthName "Enter your username and password:" AuthType Basic AuthUserFile /home/yourdirectory/.htpasswd Require valid-user

#Exclude the wp-cron.php file! Order Allow,Deny Allow from all Satisfy any

Of course, if your blog is password protected, the wp-cron.php file won’t get hit until somebody logs in to view the blog…but at that point any tasks that are pending will activate.

Spare One Phone – Dead on Arrival – Two Red LEDs

Jay — Sun, 25 Mar 2012 13:35:51 +0000

UPDATE: I called the company twice (once on Monday and again on Tuesday) and on Tuesday after asking a few questions they shipped me a new phone (arrived on Saturday and worked correctly!) and a label to ship back the defective unit. (I also emailed them on Sunday, and received an email reply 8 days later, after already receiving the replacement phone…so I would recommend persistent phone calls (it took a few tries to get to a human instead of voice-mail) over email.

Original post:
I purchased a SpareOne phone, which is a very simple GSM handset that is powered by a single AA battery. It has even less functionality than the Motorola MotoFone F3, as it has no display other than two indicator LEDs, so you can’t send or receive SMS messages. It comes with an Energizer Ultimate Lithium battery which has a 15 year shelf life (if you keep the battery disconnected from the phone with the provided plastic pull strip) and offers “up to” 10 hours of talk time on a single battery. My invoice number was 393, so it looks like I was within the first few hundred orders.

Unfortunately, when I received the phone and pulled the battery guard out, instead of turning on it just lit up both LED’s solid red (which I assume is some type of error code, although it is not mentioned in the small instruction sheet that came with the phone. I have tried a new battery, veracious press combinations of the ON button, and the keylock button, etc, but have gotten none of the feedback beeps demonstrated in the “how to” videos.

I will be calling their support line on Monday to see if they have any advice for how to re-set it, or return it for a replacement.

Dark wood table on a sewing machine base

Jay — Sun, 11 Mar 2012 18:17:37 +0000

This cast iron sewing machine base is a “Standard Manufacturing Co” and has four small wooden drawers that came with it. Valerie chose a dark wood stain to get a close match to the drawers. This table is going to be a computer station, and we used a flexible arm to mount the monitor.

Book Recommendation: Midnight Never Come by Marie Brennan

Jay — Sat, 10 Mar 2012 14:12:09 +0000

Midnight Never Come by Marie Brennan is a work of historical fiction set in the court of Elizabethan England, and the corresponding faerie court below London. If you like historical fiction and fantasy, Marie Brennan has done a good job at both. Her historical research makes the world very detailed, but it’s the characters and plot that make this book truly captivating.

Singer Sewing Machine Base gets a new table top

Jay — Sun, 04 Mar 2012 03:08:38 +0000

My wife purchased a few singer foot petal operated sewing machine bases and we finally finished one of them. The table top is made out of a piece of 18″ x 36″ wood purchased from Lowes that I edged with a piece of rope trim. After lots of sanding to get the edges of the trim and the wood to line up (the trim started off just slightly wider), my wife took over and stained and sealed the table top. After I screwed it to the base she had cleaned up and spray pained, here is the finished result.

Using refrigerator door bins and vegetable crisper drawers for garage organization

Jay — Mon, 23 Jan 2012 01:53:38 +0000

Cardboard boxes are fine, but refrigerator door bins and vegetable crisper drawers are nicer for storing things. The good ones I used here are made from clear plastic so you can see inside them easily and they don’t block light.

Of course, they are terribly expensive to purchase for this use, but with the (lack of) quality of Kenmore /Amana refrigerators manufactured after 2004 you can find free donor components relatively frequently. I also believe that with the addition of standard closet shelving support brackets the glass shelves from these fridges can be re-purposed into standard wall shelves.