Another retro computer website…

August 17, 2011 | Link of the Day | By: Mark VandeWettering

While researching some information on the old CP/M operating system (don’t ask) I found Herb Johnson’s excellent retrotechnology.{com,net} website. It’s chock full of good information on old Apple Macintosh systems, S-100 systems, and cool virtual exhibit on the PDP-11. Bookmarked for later consumption: tons of good information.

http://www.retrotechnology.com, .net/.

Addendum: It’s amazing how often I find people with odd interests have other odd interests that coincide with mine. Herb has also ground and polished his own telescope mirror. Awesome.

Astronaut Photographs Perseid Meteor

August 15, 2011 | Space | By: Mark VandeWettering

NASA astronaut Ron Garan snapped this awesome picture of a Perseid meteor from his vantage point on the ISS:

Astronaut Photographs Perseid Meteor… From Space : Discovery News

No additional commentary: it speaks for itself.

ARISSAT-1’s battery appears to be failing quicker than expected…

August 14, 2011 | Amateur Radio, Amateur Satellite | By: Mark VandeWettering

I didn’t get a chance to record any more ARISSAT-1 data this weekend, but I did catch up on some reading. Apparently, it’s batteries are giving out quicker than expected: the voltage is dropping low enough to cause a reset when the satellite goes into eclipse each orbit. If you were thinking of grabbing some telemetry/SSTV from the satellite, or even attempting some QSOS, perhaps sooner would be better than later.

You can read up on the Power System of the satellite here, which includes a link to this AMSAT Journal article which gave many details. Apparently the silver-zinc batteries which were used aboard the satellite (largely for safety, a dead short of these batteries does not cause a fire) have many features which were seemingly not that well understood. These batteries are typically used in a deep-discharge situation, and only recharged five times. Aboard ARISSAT-1, they are recharged in a shallower cycle, more times. There also seems to be some issues regarding temperature performance. I skimmed the article, but will reread it and rethink it some more soon.

Arduino PWM Laser Transmitter

August 14, 2011 | Amateur Radio, electronics, My Projects | By: Mark VandeWettering

A couple of weeks ago, I programmed an Arduino to take digitized sound stored in its rom, and send it out via PWM of an LED. A couple days after that, I used the same code to send voice using a small 5mw laser module. Ever since then, I thought it would be good to use the Arduino’s analog to digital converters to sample the sound from a small microphone, and then use that as the signal to send over PWM. Here’s my first test:



There is lots of noise remaining in the signal. Some further experimentation showed that the voltage regulation wasn’t very good: I did tap 5v from the Arduino, but the load must have been near its limit, because the overall voltage levels were varying by nearly 100 millivolts, which is only slightly less than the signal amplitude. I also have many leads which are too long, poor layout, and unoptimized values for AC blocking caps. But it does seem to work. I’ll be revising this over the next couple of weeks.

Addendum: I did measure the current through the op amp preamp, and the laser diode module. The op amp circuit only needs to drive the Arduino input, which is very high impedance, so it draws less than four milliamps. The laser diode averages 20ma, but has a peak power of twice that.

AMSAT SSTV gallery

August 10, 2011 | Amateur Radio, Amateur Satellite | By: Mark VandeWettering

I haven’t had the chance to do any ARISSAT-1 SSTV lately, but I thought I’d snoop over to their gallery to have a peek. Some good pictures are being received, but it appears that some earlier pictures are simply being removed: two pictures that I submitted to them earlier are no longer available. Their comment:

We may not be able to display every image because of quality or duplication but it is important for you to submit them for engineering analysis purposes. The rest will be archived onto a subsidiary page.

There is no hint of where this subsidiary page can be found. Honestly, AMSAT: if you are going to act as a centralized repository for SSTV images, why not make all received images available? If they are of interest to the principals, they are probably of interest to everyone else too.

Still, surf over and have a peek. But if you find a picture you like, archive it on your local system.

Welcome to the AMSAT SSTV gallery.

I before E except after Hellschreiber « SWHarden.com

August 8, 2011 | Amateur Radio, electronics | By: Mark VandeWettering

Scott Harden had a very cool idea: sending Hellschreiber, an old fax-like teleprinter code invented by Rudolf Hell, using just an Atmel ATMega48 and a canned oscillator. The idea is pretty simple: the canned oscillator will produce a square wave with the same amplitude as the input voltage. So, you simply power the oscillator with the PWM output (presumably with some low pass filtering) of the AVR, and you can produce a simple AM modulated signal. There are some issues with this, but the basic idea is pretty cool, and I have most of the parts I need to make this happen at home. I’ll try it out and let you know how it works out.

>>> I before E except after Hellschreiber « SWHarden.com.

Addendum: I experimented with this basic idea using my LED/Laser PWM code/circuit that I built on my Arduino. I’m uploading the youtube video now, but I realize that now that I didn’t do what Scott did, and what he did was somewhat more likely to work better. If you watch my latest YouTube video (uploading now) you can anticipate that I’ll be apologizing for my stupidity. My experiment did work, after a fashion, but his idea is somewhat better. Stay tuned.

Addendum2: Here’s the video that I shot tonight. It’s still horribly wrong, but in the interest of full disclosure, I thought I’d toss it out there. I need to work at understanding the filters and the like, maybe add a buffer, and consider the biasing. What can I say, my eyes were dialated, my brain foggy. The aliased copies I hear at the end are probably not going to be present if I actually did what Scott did. Stay tuned, I’ll redo this after some thought.


More on Dino’s simple common emitter amplifier…

August 8, 2011 | electronics | By: Mark VandeWettering

I hadn’t worked through the biasing calculations for collector feedback common emitter amplifiers before, so I thought I’d do that for the simple one transistor NPN preamp that Dino built as his weeklyhack. But even before I got to it, I was confused by something. Check out the schematic (cribbed from Dino’s post):

I was confused by R1. It’s only purpose seemed to be to provide a DC offset, which was immediately removed by the action of C1. So, since I had built the circuit in LTSpice, I tried varying its value from 1 ohm to 100MEG ohms, and indeed, it had no effect on the overall gain of the amplifier. So, I removed it entirely. And the circuit continued to work just fine.

So, what’s the deal? I suspect that R1 was part of a circuit that powered an electret microphone: if the Input on his schematic were an electret microphone, you’d need to provide some voltage into it to power the built in FET preamp (like you see in the diagrams on the Wikipedia page for electrets). If that power is not needed, then all that happens is the current is dissipated by the input as heat. If you cascade two of these in the naive way, you end up a resistor feeding between voltage between two DC blocking transistors, which doesn’t waste a lot of power (no current flows at DC), but you end up with an extra cap and resistor.

Amplifier Tutorial

August 7, 2011 | electronics | By: Mark VandeWettering

I was looking for some amplifier references to brush up on amplifier design. I found this one to be pretty good:

Amplifier Tutorial.

Gutenberg Gem: Francis Galton’s Finger Prints

August 7, 2011 | Gutenberg Gems | By: Mark VandeWettering

I’ve always had an interest in forensics in general, and fingerprints in specific. Previously, I had blogged about the FBI publication The Science of Fingerprints. Today, I noticed that another major work on the subject had been released by Project Gutenberg: Francis Galton’s Finger Prints.

Francis Galton was a prolific scientist in a number of fields. Half-cousin of Charles Darwin, he’s perhaps most famous for pioneering the field of eugenics, but he also was the first to create a system for the classification of fingerprints, and argued based upon statistics that fingerprints were unique.

Nice to see this online.

Preamps and Microphones…

August 6, 2011 | diy, electronics, My Projects | By: Mark VandeWettering

A few weeks ago I did an experiment that stored an 8 bit PCM file in the flash memory of an Arduino, and then used PWM to flash a laser at a high rate so I could transmit that audio over a long distance to a solar cell based receiver. A few days ago, I assembled a nifty little gadget called the bliplace, which interfaced a small electret microphone to an AVR ATtiny processor. For the last couple of days, I considered merging the two: the idea was to build a small interface circuit to condition the signal from an electret microphone to feed into the A/D converter of an Arduino. Then, it would be simple enough to use the Arduino to modulate the PWM output.

I’m a bit confused about the bliplace, it doesn’t seem to employ any kind of preamp to increase the output. My understanding is that the output from these kind of microphones is typically quite low, so a preamp would seem to be necessary to boost the overall voltage swing to something that the Arduino can measure with some precision.

My idea was to use a simple op amp circuit to boost the voltage from the microphone, and center it around 2.5v. It’s not hard. You create a resistor divider to halve a 5v supply, and put it into the non-inverting input of the op amp. You then feed the input signal through a blocking cap (I used 0.1uF) and a 1K resistor into the inverting input. Then, feedback from the output of the amp is fed through a 100K resistor back into the inverting input. Here’s the schematic in LTSpice:

I put it together today on a breadboard, and it seems to work. I couldn’t dig up a suitable microphone (I know I have some somewhere), but I’ll get to it in the next day or two and get it all soldered up so I can hook it to my Arduino. Since the LM324s are a quad operational amplifier, I could even use the spare op amps to do some additional filtering. I’m still working on that part.

Through some strange coincidence, a couple of other people I know have been playing with amplifiers too. Whisker over at tymkrs has been working on a preamp for a microphone, and MakerDino’s weekly hack at HackAWeek was to build a single transistor preamp. It’s a simple little class A amplifier, and he did a great video:

I played around with the circuit in LTSpice a bit, and it seems to work pretty well. It provides about 36db of gain, and seems reasonably linear. You can get distortion of the input signal is too high, but other than that, it looks pretty good. The gain is enough to convert a 10mV signal to a 600mA signal. Current draw is about 700 microamps with a 3v supply. Not bad! I could use similar circuit for electret preamp. I’ll play with both circuits some more. Thanks Dino!

Morning pass of ARISSAT-1

August 5, 2011 | Amateur Radio, Amateur Satellite | By: Mark VandeWettering

Success! I managed to record the pass of ARISSAT-1 from my front yard this morning.

I woke up around 7:00AM, and tried quickly to get ready. I pulled on some clothes, then I dug out my handy Arrow antenna (a hand held dual band Yagi antenna for 2m and 70cm) and my tiny Yaesu VX-3 that I had remembered to charge, a patch cable, my MacBook, and headed to the front yard. Yikes! The sprinklers were on. I turned them off, then dug a camping chair out of the back of my SUV so I had a dry place to sit. I also got a tripod with my little Kodak HD video recorder mounted on it, and aimed it roughly at the chair, thinking I’d shoot some video.

Here’s the prediction that my home grown library did for the pass:

RADIOSCAF-B will be visible from grid CM87ux starting in 00:13:35 at 14:28:04
  14:28:04  +0.1° 312.2° ?  49.1°N 144.6°W AOS 381.52
  14:29:00  +3.9° 313.7° ?  47.8°N 139.7°W     382.01
  14:30:00  +9.2° 316.1° ?  46.1°N 134.7°W     382.55
  14:31:00 +17.0° 320.3° ?  44.3°N 130.1°W     383.11
  14:32:00 +30.9° 330.2° ?  42.3°N 125.8°W     383.68
  14:33:00 +55.2°  10.5° ?  40.0°N 121.7°W     384.27
  14:33:14 +58.2°  32.9° ?  39.5°N 120.9°W MAX 384.41
  14:34:00 +43.6°  90.6° ?  37.7°N 118.0°W     384.86
  14:35:00 +23.4° 110.2° ?  35.2°N 114.5°W     385.46
  14:36:00 +13.0° 116.6° ?  32.6°N 111.2°W     386.07
  14:37:00  +6.5° 119.8° ?  29.9°N 108.2°W     386.67
  14:38:00  +1.9° 121.7° ?  27.2°N 105.3°W     387.27
  14:38:28  +0.1° 122.3° ?  25.9°N 104.0°W LOS 387.55

The satellite was supposed to start to the north, then pass to the east, reaching maximum altitude at about 14:33. I figured I wouldn’t be able to hear the satellite at all until it was above 20 degrees or so, maybe at 14:31UTC, because I have a rather high horizon to the north. But at 14:29 I tentatively turned on my radio, tuned to 145.950 and aimed it roughly north, and it was clear that there was some signal coming in. I started Audacity recording. After a minute or so, the signal was very strong, full quieting between deep fades that seemed to occur at around six second intervals.

Here’s the .mp3 file. It’s fine for listening to, and may (or may not) be adequate for SSTV decoding. If you really want to try SSTV, mail me a request and I’ll try to arrange a way to get you the 33M uncompressed audio file. I beeped out the “secret word” that appears in the recording, so as not to spoil it for others.

ARISSAT-1 Recording, Aug 5, 2011, 14:30UTC, 3.3M MP3 File

After I was done, I fired up MMSSTV on my windows box and did some SSTV decodes, here are the images as they appeared in the recording.

Not too bad. It’s pretty clear from the recording and from the pictures that the satellite is tumbling: periodic deep fades appear to happen about every six seconds or so. Still, the imagery is a pretty neat bonus. As the satellite’s spin slows, we should be able to easily get good SSTV imagery, even with equipment as primitive as mine.

According to the ARISSAT-1 website, the satellite was 20 seconds ahead of the ISS, and maybe a kilometer lower. It’s projected lifetime is short: maybe just sixty days, so don’t delay. Tune it in, collect your certificates, and have some fun.

Schematic for tanjent’s “bliplace”

August 4, 2011 | electronics | By: Mark VandeWettering

Yesterday, I wrote about soldering together my bliplace kit. Today, I thought I’d have a peek at the code and the schematic. Here’s the schematic:

Not much there, but a couple of interesting things. The microphone is an electret, which needs some power to be functional (supplied through the 10K RU resistor). To me, the capacitor CC and the RD resistor look like a high pass filter, with the supplied values the cutoff frequency is nominally about 160Hz. Not 100% sure I understand how the capacitor CN or what the overall input impedance is for the ADC input pins on the Atmel chip, so I am not 100% sure I understand the subtlety.

I also thought that current limiting resistors were interesting. The center LED always seems to glow brighter, and this seems to be because R2 is only 100 ohms, compared to the 1000 ohms that were used for the other two. I also wondered about the LEDs: this gadget is powered by a single 3V cell. The ATTiny25/V can run down to 1.8 volts, but I thought that white LEDs had fairly high forward voltage requirements. For instance, this page on Wikipedia says that the typical voltage drop for white LEDs is around 3.5 volts. I don’t have a lot of white LEDs lying around, but it does suggest some experimentation.

Antenna? Who needs an antenna?

August 4, 2011 | Amateur Radio, Amateur Satellite | By: Mark VandeWettering

Drew, KO4MA, didn’t let a little thing like the missing antenna on ARISSAT-1 keep him down. He aimed his antennas at the satellite, and recorded the following:



What’s goin on here? Drew has a pretty good antenna setup, which includes an M2 CP42 for the uplink. That provides about 16.8db of gain. Since the ARISSAT-1 receiver was supposed to be reasonably sensitive to be workable with portable equipment, that gain is enough to put a reasonable signal into the transponder. I don’t think I am going to be working the bird with my setup, but i suspect that there are quite a few amateurs with similar setups that could still work ARISSAT-1 even with its lack of antenna.

Very cool.

Addendum: I was rereading my posts from yesterday, when the frustration of deploying ARISSAT-1 with a missing antenna was fresh in my mind. And I do frankly admit to a fair amount of frustration with AMSAT. But I was reading some even more negative traffic on the amsat-bb list, and that put it in a bit of perspective for me. ARISSAT-1 wasn’t a waste, nor is it useless, nor was it a bad thing for AMSAT to invest in. In terms of its primary, largely educational mission, I suspect it will be quite successful. Several times during the NASA briefing yesterday, they mentioned that this was the first of a series of educational birds to be launched from the ISS. These launch opportunities are incredibly valuable to the amateur community, and we should make sure that our frustration with problems doesn’t boil over into lashing out at our allies who are helping us achieve our goals in amateur radio and space.

Building tanjent’s “bliplace” kit…

August 4, 2011 | electronics, Music, My Projects | By: Mark VandeWettering

I was in the mood to melt some solder, but didn’t really have a lot of time and/or brainpower last night, so I turned to my box of little electronics kits that seems to have been growing over the last few years. I located a small plastic bag which contained tanjent’s “bliplace”, a tiny kit that he generously was handing out at a conference we both attended. It’s simple: an Atmel ATTINY 8 pin controller, three caps, five resistors, an electret microphone and a battery is all it takes to get it to run. I figured it would take me about 20 minutes to assemble. It took about 35, mostly because I wasn’t paying attention and soldered the first two resistors in the wrong place (the board has a very symmetric layout, and I got turned around). That got me some practice in using my solder sucker, and had to solder in two new 1K resistors from my junkbox. But in the end, it worked!

What is bliplace? In his words:

Bliplace is a wearable, hackable, sound-activated blinky light toy. It uses a small microcontroller and a mix of hardware and software feedback to automatically synchronize with and adapt to the sounds around it – it should pulse along with the ambient noises around you no matter if you’re in a quiet park or a thunderously loud concert.

Here’s the video of the thing working. Pardon the sound levels when I turn the radio up, I shot and edited this on my iPhone.



It’s a clever little gadget by itself. But what’s especially cool is that it is open source. You can get the board and schematic files, Gerber files, and the source code for the firmware. I think it would be interesting to make an “amped up” version of this thing, which would drive some big power transistors to switch some truly high power CREE LEDs. The code is released under the MIT license, so modifications should be easy and redistributable. It should be trivial to get this to run on the Arduino platform as well, which will make experiments easier for the wider audience.

I’ll be staring at the code a bit harder over the next few days. Stay tuned.

ARISSAT-1 Update…

August 4, 2011 | Amateur Radio, Amateur Satellite | By: Mark VandeWettering

Okay folks, I’m sorry, I promised that I’d try to stay up late enough to record some of the first passes of ARISSAT-1, but physical need for sleep outpaced my natural enthusiasm and curiosity, and the first good pass found me sound asleep. Through some perverse quirk of fate, all the best passes of the ISS are occurring in the early morning hours. Running my own homebrew satellite prediction library, I find that my best opportunity for a pass where I am awake and the satellite is illuninated, and therefore in high power mode, will likely be tomorrow starting around 7:28AM PST. I’ll try to get myself all setup for that, and will hopefully have some audio and/or video tomorrow.

In the meantime, it has been heard.

VK2BRB recorded this YouTube video:



SW1OBT heard the voice beacon, and the start of the SSTV transmission: