HP Clock

Sunday, December 6, 2020


Pi Arduino Development Station

Having picked up a folding Plexiglas prototyping assembly for a few dollars at the amateur radio swap meet I decided to use it to build a portable Raspberry Pi Arduino prototyping station. I could use the Raspberry Pi 4 to do hardware development directly and with the Raspian Linux operating system I would be able to run the Arduino IDE. The completed system consists of a Raspberry Pi 4, Arduino Uno, ten-inch HDMI display, and a USB receptacle outlet for power.

Fig. 1 Combination Raspberry Pi Arduino Dev Station

Here you can see where I added a prototyping board that includes a GPIO breakout connector for the Raspberry Pi. As with most of my prototyping setups I like to add an aluminum angle bracket to hold some BNC connectors for easy IO and banana jacks with ¾” spacing for connecting external power. I can also add potentiometers as needed.

Fig. 2 Closeup of Prototyping Section

Here is a view of the banana jacks used for connecting external power supplies. You can also see how easy it is to either probe or inject signals with the BNC connectors.

Fig. 3 Banana Jack Power Connections

The USB receptacle outlet provides plenty of power for the Raspberry Pi, Arduino and HDMI display. The exposed connector on the top is neutral, however it has since been covered as well.

Fig. 4 Modified USB receptacle

Here is where the completed project comes together where the Arduino is driving and reading an arrangement of gates and counters in a simple ‘Hello World” test.

Fig. 5 Development Assembly Running

Here is a picture of the development assembly folded up and ready for travel.

Fig. 6 Folded up for Travel

Resource Links

Wednesday, November 25, 2020


A Simple ESR Meter

I was always envious of my friends analog ESR meter that allowed him to check and find bad capacitors while they were still in circuit. The particular meter he had is no longer available, which meant doing some research to understand how they worked and to come up with my own version. First, what is ESR? ESR stands for the equivalent series resistance of a capacitor. ESR is frequency-dependent, temperature-dependent, and changes as components age. It’s typically important for ‘Wet’ aluminum electrolytic capacitors used in power supplies to have a low ESR.

The typical method used for measuring ESR is to supply the capacitor with a known AC current (Icap) at some frequency where the capacitive reactance of the capacitor is very low so that the ESR dominates. By measuring the resulting AC voltage developed across the capacitor’s terminals (Vcap) the ESR can be determined with Ohm’s law:

ESR = Vcap/Icap

Most of the designs I found worked along the same lines as this block diagram:

Fig. 1 Typical ESR Meter Elements

Going from left to right there is an oscillator that supplies AC voltage to be applied to the capacitor. Next the AC signal is fed into an impedance converter and detector. The detected signal is then rectified and buffered so that it can drive the meter on the right of the diagram. Since the ESR meter is to be battery operated the power supply circuit supplies split rails for the operational amplifiers that will be used in the ESR meter. The oscillator in most of the examples I looked at operated at 100 kHz to 150 kHz. The driver used to reduce the impedance of the AC signal could be anything from a transistor current boost, transformer, or paralleled logic gates. The detector was usually back to back diodes. The detected AC signal is then rectified, amplified and fed into a DC meter.

In the circuit I decided to build I used several design elements from the DIY examples I found on the Internet.

Fig. 2 Schematic of the ESR Meter

For the oscillator and impedance converter I used a single 74HC14 that provides 6 inverters with hysteresis. One of the inverters acts as a relaxation oscillator and the remaining 5 inverters operate as the impedance converter. This part of the circuit came from Lawrence P. Glaister VE7IT. The detector portion of the ESR meter is the same as the detection circuit in the commercial ESR meter built by Creative Electronics. Sadly, these meters are no longer made. Diodes D1 and D2 clip the top and bottom of the 100 kHz AC to one silicon junction drop. This allows capacitors to be tested in circuit because any other silicon junctions will not be forward biased by the relatively low AC signal. The low-level AC signal is DC decoupled with C3 and amplified by two operational amplifiers, each with a voltage gain of 10. The Absolute value circuit was taken from the Burr - Brown Application Bulletin “Precision Absolute Value Circuits”.  The absolute value circuit had plenty of drive for the 100 microamp meter I used.

This ESR meter operates of a single 9 volt battery. The plus, minus and ground voltages needed for the operational amplifier are derived using this opamp voltage follower with current boost.

Fig. 3 Power Supply Circuit

To test out the circuit I built up a conventional bread board. BNC connectors allow for easy monitoring of the various waveforms. Initially for testing I simply looked at the signals with an oscilloscope intead of the 100 micro amp meter.

Fig. 4 ESR Bread Board

In this picture you can see the output the oscillator as well as the output of the ESR meter while testing a 50-microfarad electrolytic capacitor.

Fig. 5 Oscilloscope Test

In this waveform dump from the oscilloscope you can see the difference between a capacitor with a high ESR in blue and a capacitor with a low ESR in orange.

Fig. 6 Waveform Data Showing High & Low ESR

Even before I was able to package the meter into a case I used it to trouble shoot our air conditioner by locating a bad motor capacitor.

Fig. 7 Checking Motor Capacitor

The completed ESR meter is sufficiently portable so that it can be used remotely and away from the work bench. Simple Post-It® Note calibration was accomplished with a handful of 2-ohm resistors. The ESR meter works like an ohm meter. Before measuring a capacitor, the leads are shorted together and the knob is adjusted for a full-scale reading (Zero ESR). Rotating the knob fully counter clockwise turns the meter off with a switch.

Fig. 8 Completed ESR Meter

The circuit is wired on two prototyping Perfboards. The smaller board on the left is the circuit used to derive the plus, minus and ground references for the op-amps. The larger perfboard is the ESR meter circuit and it is held in place by the nuts on the meter.

Fig. 9 Inside the ESR Meter

Resource Links

Tuesday, April 28, 2020

Home Made UVC Sterilizer

This is a series of pictures describing a simple UV-C Sterilizer I built for mail, face masks, and small items. Much of it was built out of leftover parts from other projects. The germicidal lamps and ballasts were purchased on eBay. The timer and some electrical parts came from Amazon. Be warned! UV-C is very nasty, and at no time should you look directly at the bulbs or allow your skin to be exposed to the radiation. Any time that I energized the bulbs outside the enclosure or with the enclosure open I wore welder's goggles and did not look directly at the lamps.

Fig. 1 Bread board of the lamp and ballast

This is the schematic of the UV sterilizer. Each lamp requires a separate magnetic ballast. A timer allows a user to select the amount of sterilization in minutes. Typically, 5 to 10 minutes should be sufficient for most objects.

Fig. 2 Schematic

The box I built to house the sterilizer was fabricated from spare Ikea shelves and other press board material I had in the shop. I glued aluminum foil to the interior wall of each piece of box. I then followed up with aluminum tape around the edges. The aluminum tape is the kind used for air conditioning duct repairs and can be found at Home Depot or Loews.

Fig. 3 Press board covered with foil and aluminum tape

After the individual pieces are covered with foil they can be assembled into a box. I used dry wall screws to attach the sides together. After the box is assembled I followed up with aluminum trim on the outside edges for extra strength.

Fig. 4 Assembly of the press board box

Next I wired the junction box for the hot and neutral buss that feeds each of the lamps. I used a typical barrier strip and plastic project box. Rubber grommets were added to the box to protect the wires entering the box. The ballasts and G23 sockets use solid wire which works well with the screw terminals on the barrier strip.

Fig. 5 Close up of junction box

With the box completed I attached the junction box to the back of the unit.

Fig. 6 Junction box attached to the sterilizer

After the junction box was installed I attached the individual ballasts to the back of the sterilizer.

Fig. 7 Philips LPL-5-9 ballasts

Next I screwed down and wired the sockets for the germicidal bulbs. These particular bulbs required a G23 base. These can be found on Amazon for a few dollars. After the bulb bases were installed I drilled small holes for the wire from the ballasts and the neutral buss.

Fig. 8 Basses for G23 bulbs

I used a couple of wire in-baskets to suspend anything being sterilized over the top of the bulbs. This helps to optimize UV-C coverage and reduces fire risk by keeping objects being sterilized away from the bulbs.

Fig. 9 Wire baskets installed

Wiring at the back can now be completed. This includes running power from the junction box to the 2S electrical that I attached on the side. It is also time to put the cover on the black junction box. The 2S box houses the timer that will turn the germicidal lamps off after a sterilization time is selected. The timer I installed is a simple spring wound timer that can be set from 1 to 15 minutes. I have ordered a digital timer that has pre-set buttons for 5, 10, and 15 minutes. Both of these were found on Amazon.

Fig. 10 Caption for picture 10

The G23 sockets are installed and wired to the top of the box. After they are wired the top can be attached to the top of the enclosure.

Fig. 11 G23 sockets installed to top

With everything wired it was time to test the bulbs. Again, I wore welding goggles during the very short test.

Fig. 12 Lamps energized

I created a custom warning label based on OSHA samples found with Google Images. The size is based on a spare piece of Plexiglas I had in the junk box. I will provide a link to an SVG file that should work with almost any drawing program.

Fig. 13 Custom OSHA warning label

The last step is to attach the warning label to the front door of the enclosure and then attach the hinges to the door and the door to the enclosure.

Thursday, September 5, 2013

Modeling Molecules

Most of us remember making the models of molecules by snapping together multicolored plastic spheres.

Inspired by this childhood experience I decided to attempt the virtual version using Python and Blender.  The secondary motive was to gain some proficiency in reading and parsing data from a text file using Python and then use Blender to do the rendering of the molecule.  The first thing I needed was some data to parse using Python.  I was happy to discover that the data I needed for the molecules was available at the Protein Data Bank.  Initial development and parsing of the text data was done using IDLE, Python's Integrated Development Environment.  After I was satisfied with my ability to read and parse text data I moved my script into Blender.  Here you can see Blender in the scripting mode.

You can see from the screen capture where Line 22 of the script points to the text file that contains the molecule data.  In this case I’m drawing a picture of an LSD molecule.   After running the script I wanted to create a nice 3D image of my molecule.  Using Blender in the Default mode I set up a stage for the molecule to be rendered on.  Here is how the image of the LSD molecule turned out.

If you want to try this on your own you can download the scripts and text file data from this web page.  In the example on this page I created a complete 3D model and animation of a short piece of DNA.  All of it can be downloaded from this URL.

This page also contains links to the Protein Data Bank, as well as information about the color and size of the individual atoms.  I’ll try to update the page adding CSV files of different molecules when I create them from the protein database files.  You can also find the code on GIT Hub using this link


Sunday, January 13, 2013

Cold War Artifact

When my friend Curtis visited me last week he surprised me with this cold war artifact possibly from the late 50s early 60s.  He picked it up at the museum in Los Alamos, New Mexico.  Initially, none of us recognized the device or knew what its purpose was.  I sent this picture to my daughter and she suggested this was worthy of blog post.

Opening up the box only deepened the mystery when I was greeted with a circuit board containing flash light bulbs, resistors, diodes, one transistor and a transformer.  There was also a place for a single D Cell battery. There was a tantalizing clue to the devices purpose when I discovered a sticker on the inside with a schematic.

Here is a closer view of the interior of the case and the schematic.

Using Google and the number CD V-750 that appeared on both the outside of the box and the interior sticker the manual for the device was located.

Using Photoshop I was able to create a better image of the schematic.

So what is it?

The operating and maintenance manual explained everything, including the purpose of the device, how it worked and how to fix it.  The yellow box is a Radiological Dosimeter Charger.  It’s used to charge, or ‘zero’ a quartz fiber dosimeter.  This style of dosimeter is essentially a small electroscope, and the quartz fiber is a delicate gold plated indicator.  This quartz fiber indicator is inside a small airtight ionization chamber.  The ends of this chamber are transparent so that the fiber can be viewed with a built in microscope, and compared to the built in reticule to determine the charge on the fiber.  To reset a dosimeter of this type requires 150 to 200 volts.

With the manual in hand I wondered if this charger was still functional.  The circuit is very straightforward and is actually a simple switching supply used to generate the high voltage from a D Cell battery.  Transistor Q1, capacitor C1, and transformer T1 primary windings create an oscillator.  The output of that oscillator is stepped up through the transformer where it is rectified by CR1 and filtered by C2 to create the high DC potential.  Potentiometer R2 and resistor R3 create a voltage divider.  The wiper of R2 creates an adjustable output voltage to reset the dosimeter.  In this picture you can see the printout of the manual showing the waveform at the anode of CR1, the D Cell, and the jumper used to bypass S1 to activate the circuit. 

What about the light bulbs?  The light bulb that is lit is used to read the dosimeter.  The other bulb is simply a spare held in a rubber grommet.

What could be better than to test the charger with a vintage instrument from the same era?  I used my Tektronix 535A tube oscilloscope to view the waveform at the anode of CR1.  You can see that the waveform is nearly identical to the waveform shown in figure three of the manual.  Each large division on the oscilloscope reticule is 10 microseconds. 
A more exact reading was taken with a digital scope.  The period of the transistor oscillator is 36.26 microseconds, or about 27.6 KHz.  The ringing between each major pulse had a period of 5.8 microseconds, or about 172.4 KHz.  So it does work!

One last observation...

Opening the cover let out the unique aroma of 50s Science and Science fiction movies.

Monday, January 7, 2013

More Junk Box Astronomy & the Transit of Venus

I thought it would be a good idea to do my intended post about the Transit of Venus before it happens again.  Of course that won’t happen in our lifetime since the next two transits of Venus will happen in December 10–11, 2117, and in December 2125.  My inspiration for this post was based on my reading of the book by Andrea Wulf “Chasing Venus”.

The book brings to life what was the first big international scientific collaboration.  Sir Edmund Halley, knowing that he would not be alive to see the results, postulated that by accurate measurement the physical size of the solar system could be determined.  He encouraged his younger contemporaries to undertake the adventure that would take them to the far ends of the earth to make their measurements.  Here you can see what was predicted for the transit of 1761.

For us, our adventure led us to the top of the parking structure at the office where I work.  Using the same set up that I had used just a few weeks earlier to view the partial eclipse of the Sun by the Moon we waited for Venus to appear in front of the Sun.  

In this projected image both Venus and Sun spots can be easily  seen.  The large fuzzy ring in the projection is an artifact of the folded optic system used by the telescope. only one person asked why Venus did not catch fire...

One of us brought the darkest of welding goggles.  This did work, but Venus was barley visible since there was no magnification with this method.

In this Green image the goggles were simply placed over the eye piece of the telescope and the picture was taken with a phone camera.  We had to work quickly since the focused rays of the Sun on the goggles heated them to very high temperatures within a minute of exposure.

Sunday, May 20, 2012

Eclipse Viewing with Junk Box Parts

Backyard astronomy is always fun, and it doesn't get much better than a solar eclipse.  I set up my small spotting scope today for neighborhood kids to safely view the eclipse.  One of the safest ways that I know of is to use the telescope to project an image of the eclipse on to a screen.  Here is the screen I built from aluminum from the hardware store and a small piece of foam core available from any office supply store.

I also used a simple clock drive I built using a wide assortment of junk box parts.  The design was mostly based on adapting the other parts to the found worm drive.

With a way to project the image on to a screen and a clock drive that would compensate for the earths motion I was ready to watch the eclipse.  Here you can see all the parts coming together.  The added lead dive weight is used to take out slop in the gears of the drive train.  This picture was taken just as the eclipse was starting.

This picture was taken right at the height of the solar eclipse event.  The air was noticeably cooler and ambient light was less.