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   Worked on several projects today while partly watching some historical documentaries.  I have 5 picture frames that require prints, 4 of which were completed but never had a print and one that broke the glass during the move and scratched the print.  While I thought the process of selecting prints and ordering them was going to be quick I was mistaken.  One of the frames in particular ended up with very strange dimensions.  I had to figure out how to offset a print such that I could get it to fit in the strange size.  In addition, I had designed a couple of frames for print sizes I can only get from one print house.  But the prices are much higher than my usual place.  It is cheaper to order a larger print and trim the print down.  Wanting to get it right I spent much longer than I probably should have needed verifying the math.  However, prints have been ordered.  More projects are now in the queue, but none should take too long.  I think I'm going to have to find more work to do.  It is fall though, and the daylight hours should be more occupied with photography.
   So the hole in the basement door is too small to accommodate the new handle I bought.  The problem is I don't have a good way to re-drill the hole because there is no way to center the drill bit.  My solution is to fill the hole with a plug and re-drilling it.  Filling the hole isn't easy.  I do have a hole saw, but nothing of the exact size the hole needs.  Hole saws are usually designed to give you a specific outer diameter, but the inner diameter is naturally smaller.  After thinking about it, I thought I could do this project by cutting a larger plug and then working the plug down to size on a lathe.  Problem is, I don't have a lathe.  I do have a drill press though, and it was time to see if I could improvise.
   The first task was to cut a plug slightly larger than the hole I wanted to fill.  After doing this I set the speed of the drill press to it's highest setting.  I placed a bolt and nut on the wood plug so it could be spun from the bolt.  This bolt I put into the chuck of the drill press.  This spun the plug rapidly.  I raised the table of the drill press very close to the spinning plug, and clamped on a guide to brace my cutting tool.  I used a fairly sharp chisel, making sure to keep it flat against the table of the drill press.  While it was a slow process, I verified with my micrometer I was able to work the plug down to the desired size.  Then a quick pass with sand paper and the plug fit nicely into the hole.  A generous helping of wood glue and this hole was plugged.  This will allow me to drill a new whole slightly larger.  While the drill press isn't a great lathe, it does make a functional lathe.
   It is fall, color is upon us, and it is time for me to start concentrating on photography.  Today I biked into work.  I usually do this on Friday, but I needed to take my car (Eve) to the shop for a bad bearing.  She's over 200,000 miles old and hardly ever has any issues, so I don't feel bad about this.  On return ride I stopped off at a few locations to capture some of the early fall color.  I think it's going to be a pretty year this year.

   The results of the last couple of days.  This is the cabinet with mirror in this bathroom.  The sink has very little shelve space which is needed for things like contact lens cases when installing/removing contacts.  I wanted a small shelve above the sink to assist in tasks like this.  In this room I can't modify the walls at all.  They are painted with a strange texture paint that would be visibly marred if I were to install brackets and remove them when I left.  To avoid this I decided to build a shelve that wrapped around the cabinet.  This would avoid needing to screw anything into the existing structure.  I've been meaning to do this project since we moved in last year, but now it's complete.
   Building only took a few minutes.  We had some 1x4" scrap wood, and a decent board for a shelve.  A few cuts and some screws and the shelve was complete.  I had a can of white spray paint I used to paint the visible sides of the fixture.  It was then my plan to varnish the setup.  However, my varnish had partly dried out in the can and just made an awful brown mess when applied to the shelve.  I had to sand the thing down and try again.  This time I used lacquer and my spray gun.  That worked much better and after about 3 coats the shelve was complete.  I let it try over night and hung it this afternoon.  Looks quite natural I think.
   This weekend I picked up some track lighting to see how well it would work for illuminating the edge of my bench.  I wanted a light directly overhead with a shallow angle.  I currently have one 15 watt LED spot light bulb that works quite well for this, and bunch of them I was sure would do the job.  I've always wanted track lighting so now was the time to test it out.  My first strip had 3 lamps on a 3' section of track, but my initial tests show it was going to work fine.  So today I picked up an 8' section of track, and a 90 degree elbow.  The new track was black while the only color I found for the initial track I found was white.  So out came the can of spray paint.  Soon I had the track and the black lamp housing all installed and functional.
   I forgot to take a picture so I'll include this rather artistic shot of my test leads.

September 22, 2014

Fixing old supplies

Problem found.

Problem found.

   This is an old power supply in which the variable side quite working some years ago.  I use to use this supply back in the early 2000s when I worked from home for DigiCon (no longer in business).  I don't recall how it broke, but I never looked into it.  The fixed voltages worked, and I still had 5, 12, and -12 VDC.  But without the variable side this power supply's uses were limited.  Today I decided to dive in and investigate.  I probed around for awhile and didn't see anything too obvious.  Voltages were routed to all the locations and looked correct, and nothing on the board looked damaged.  After searching around online for a bit I was able to find a schematic.  The company, Elenco Precision, includes schematics in their manuals.  My supply was still sold, but a new version and the manual wasn't much help as the layout was clearly different.  However, after some more searching I did find a manual for my old model.  What I noticed were two voltage control chips, LM723CN.  This is a linear voltage regulator chip, and one is used on the 5 VDC, and the other on the variable supply.  I decided to try an experiment.  Since the 5 VDC supply worked fine I thought I'd swap the chip from the 5 volt side with the one from the variable side.  Just like that, the variable side was functional.  The best part, a LM723CN costs about $0.69.  I have an order I need to do for a couple parts, and will be including this chip among them. 
   Does this mean I bought a variable power supply for nothing?  No.  My new lab power supply is much nicer and has two channels.  However, having this older supply working will be nice since it has a variable supply, 5, 12, -12 and the two 12 volt supplies together give me 24 VDC.  Now to work it into the setup.
   With Operation Lux back to being operation it is time to take care of some other little projects I had around the house.  I've started on a shelve for the first floor bathroom, and some additional lighting for my work bench.  As I started I noticed the LED reel light illuminating my tool shelve was falling.  I am not surprised as the double sided tape that holds this up was already used.  I had noticed after I installed it I had several LEDs that didn't light.  Several LEDs had fallen been broken off the strip during a move, and this takes out three LEDs when this happens.  So I decided to try and transport some from an other piece of LED reel.  This gave me the ability to use my inspection station and soldering iron, and I want to get better at both of them.  I found it fairly easy to remove an LED, but replacing the missing LED was more difficult.  When they broke off they took the pad, meaning I had nothing to solder onto.  Not deterred I used a razor blade to scrap off the coating around the trace and soldered some 30 gauge wire to the trace leading to the missing pad area.  There I soldered the new LED.  While the connection was functional, it didn't work.  Not sure why, but no luck.
   I decided to try an other approach.  The LEDs can strips can be cut ever 3 LEDs which is just a couple of inches.  I decided to cut out the bad sections and splice in good sections.  At first I was using 30 gauge wire to bridge the two pieces together, but discovered something that should have been obvious.  The sections can be soldered from the front and the back.  This allows one to tin both, simply press them together, touch it with a soldering iron, and done.
Operation Lux's New Housing

Operation Lux's New Housing

   Last night I left the enclosure for ππ under water to check for leaks.  This morning when I opened the case up I detected just a bit of leakage around the glass on the top, but nothing serious.  I deemed the seal was good enough for outdoors and it was time to load ππ into the housing and return the setup to the roof where it can continue Operation Lux.
   The last part of the setup was to seal the incoming Ethernet cable.  I had had success using an old contact lens case as water tight connector.  To run wire through this I used a piece of cork with a hole drilled in the center, and a slit down the side (picture).  The hole in the center allowed the Ethernet cable to pass through, and the slip allowed the Ethernet cable to be placed into the hole.  This fights snugly in the area sealed by the contact lens case and cap.  A little silicon caulk the seal the end and the setup was ready. 
   On the roof the setup booted without issue and now runs completely from Power over Ethernet.  A little editing to the log file for lux data to remove all the empty entries from bench testing and the experiment is again logging data.  The sun was already setting by the time everything powered on, but there was about an hour with above zero lux readings.  Tomorrow will be the first full day of sunlight.  It is forecast to rain in the morning and gradually become sunny during the day.  This will be a good first day run for ππ.  I am interested to know how much light we lost in the 20 days of downtime.
   Pictured is the experiment on the roof.  Like the previous incarnation it is mounted to a piece of plywood using some drywall screws.  Four screws in the corners of the plywood stick out just a big to allow the plywood to grip the shingles.  A brick sits on on the other side of the board to add weight to the experiment doesn't move.  This has sat on the roof without issues since July.  Now, however, there is only one cord—the Ethernet cord.  I could remove the extension cord from the experiment and Power over Ethernet runs the setup.  The setup uses a 15 VDC supply to feed a DC/DC converter that supplies the Raspberry Pi.  The Ethernet cables is run across the roof using Christmass light hooks and is painted white to blend in.  Running this setup should prove out the power supply, cable, and cable hooks before the Odroid moves onto the roof.

September 19, 2014

Testing Holding Capacitor

8 volt to 12 volt switch

8 volt to 12 volt switch

I wrote before about selecting a hold capacitor for power source switching.  The principle is simple: should the battery voltage from the solar panel setup become too low, the solar powered web server will be able to switch to a Power over Ethernet (PoE) power source using a relay.  The power during switching must be seamless so the server doesn't reset during the transition from the battery source to the PoE source.  To acomplish this a capacitor is placed on the power supply before the DC/DC converter that allows the stored energy from the capacitor to be used while the relay transition is taking place.  The voltage in the capacitor will slowly be depleted until the relay has engaged the alternative power source.  Because this capacitor is placed before the DC/DC converter, the 5 volt supply going to the server will never sag as long as the voltage to the DC/DC converter doesn't drop too low. 

Now I have parts and I wanted to verify this was going to work. I placed my relay on the breadboard, which wasn't easy because the pin-out was rather strange. But with a little finessing (i.e. pushing hard to bend the pins) I was able to install it on the breadboard (picture). I wired the inputs of the relay to my bench power supply, one running around 8 VDC and the other 12 VDC. Both these voltages are enough to power the DC/DC converter. On the output side of the relay I placed the capacitor—a 220 µF 35 V electrolytic that was more than double what I calculated I would need.

I drove the relay with the fixed 5 VDC power from my bench supply (the first time I have used all the voltages from my supply at once) and watched the voltage of the power in the capacitor on my oscilloscope. Switching the relay I saw the voltage transition, but really no observable sag in power during the transition. I used the setup to run ππ and could see the computer did not notice the power transactions.

According to my oscilloscope, the voltage drop during the relay switch over was 1.32 volts taking just under 2 milliseconds. I would likely not wait for the battery voltage to sag to 8 volts before switching, but even if I did I can live with the observed numbers.

Pictured is the trace of a switch from 8 VDC to 12 VDC.  The voltage starts off a steady 8 VDC, but then begins to drop as the relay disengages from the 8 VDC supply and begins to travel to the 12 VDC supply.  This lasts roughly 2 ms and the voltage sags from 8.24 volts to 6.92 volts as shown by the horizontal cursors.  Then the relay makes contact with the 12 VDC supply and the capacitor begins to charge up.  About half way into the charge is an other tiny reversal of voltage.  This is the metal contact of the relay bouncing off the 12 VDC side briefly.  The charge transitions to the 12 VDC and the switch over is complete.  Xiphos found observing the physical side effects of a relay transition evident from an oscilloscope interesting, and on reflection so do I.  Having used an oscilloscope regularly in my work for 17 years I sometimes forget just how cool the insights they bring are.

September 18, 2014

Testing Current Measurement

The parts for the monitor circuitry having arrived, it is now time to put the parts to work. First on the list was the current sense circuit. I attempted this once before with poor results. The op amp circuit I was trying to build had a ridiculous amount of gain to work with the 0.01 Ω current sense resister I had. When that failed to function it was time to admit I am suck at designing analog circuits. However, I am pretty good at utilizing my resources, and since I work with several electrical engineers I had help. I was directed to an Analog Devices current sense part, designed to do exactly what it was I wanted to do. This part has a 60x gain meaning I could use a 0.1 Ω sense resister and still have a full voltage range going into the analog to digital (A/D) converter.

There was only one problem with this part: it was only available as a surface mount part—an 8 lead MSOP. These are quite small and don't work on through-hole breadboards. Again EEs to the rescue. I was recommended a small board that converts surface mount parts to through-hole. The trick is you have to be able to attach the surface mount part to the converter board. Today was the day to give it a shot.

My new soldering iron has all the bells and whistles, and with my inspection microscope I felt I had a pretty good chance of attaching this tiny part to the converter board. I initially tried using the hot air rework wand. I could see this was melting the solder, but the solder was not joining to the pins on the part. Thinking I need more solder on the converter board I tried switching to my smallest soldering tip (which is impressively small) and quite easily added more solder. Now I had so much solder I couldn't get the chip to sit on the pads. Since I had pretty good luck with the tinny soldering tip I tried using that which worked great. I quick touch to each pin and the device was cleanly soldered to the converter board. My inspection station allowed me to do all of this work with ease and turning the board to the side allowed me to verify the solder joins were all secure.

With the current sense chip soldered it was time to build a test circuit. I used my breadboard to wire a circuit was a 0.1 Ω sense resister, some 100 Ω load resisters, and the current shunt amplifier. I powered the current shunt amplifier with 5 VDC from my fixed power supply, and used one of the variable sides to run power through the shunt resister and load resisters. I placed my oscilloscope across the shunt resister and on the output of the shunt amplifier. The results were exactly what I expected and the circuit works perfectly.

I will need to make voltage dividers for the higher currents because the gain of the shunt amplifier is so high, but that isn't a problem. This portion of the circuit—the most tricky (if any part is tricky)—is functional.