O’scope Fun

My first FDIM was memorable for a variety of reasons, but the most exciting part for me was participation in the the Buildathon. The Buildthon, hosted by Rex Harper W1REX of QRPme unleashes 40 or so builders with soldering irons in a hotel ballroom. From a home brew perspective, that’s about as exciting as it gets. QRPme is the source for a large number of ingenious kits, and of course the famous MePads which facilitate Manhattan style scratch building.

Builders at the Buildathon purchase a small board kit, hopefully small enough that it can be assembled in a few hours in the trenches. Ususally this is something like a keyer, or a small CW audio oscillator, but this year the project was a bit different.

The focus this year was one of the pocket-sized Chinese portable oscilloscopes that are proliferating on the net these days. These scopes are single channel and typically have bandwidth of about 200 KHz, and cost between $20 and $40 dollars. You can find them in kit form, or assembled, with alligator clip leads or with inexpensive 1x-10x bnc scope leads. Wow, an o’scope for $20! – but what can you do with it?

That is the question that is addressed by this year’s Buildathon.

This year’s project was an departure from the norm for Buildathons. Instead of just a simple board kit, it combined an inexpensive piece of test equipment with a “trainer” board which supplied interesting signals to examine.

QRPMe O’sope Fun Board with DSO 150 scope

The kit from QRPme, called the O’scope Fun Board, contains several circuits which produce interesting signals you can look at with your $20 scope. The board provides a number of separate modules on a single board, along with standard pin headers that allow their interconnection. The list of functions, in order of increasing complexity:

  • a small piezo speaker
  • audio frequency oscillator at around 750 Hz
  • a microphone and preamp
  • an audio amplifier with volume control
  • a configurable 555 timer circuit
  • a single chip microprocessor, which outputs ASCII characters in a loop at 2400 baud

and some support functions:

  • a regulated 5v power supply (for the microprocessor chip)
  • a section of built-in MePads available for adding your own components/circuits

Ample opportunity to have fun with a small oscilloscope.

I had attended one of Rex’s Buildathons in the past, held at the 2012 LobsterCon, and have fond memories of rummaging on picnic tables for kit parts and soldering using battery powered soldering irons by the light of Coleman laterns at 2:00AM. In short, it was a blast.

So as soon as the announcement for this year’s Buildathon was made, I signed up. Then I thought: I’m a pretty handy builder, maybe it would be fun to assist at the event. I emailed Rex, and in short order was added to a list of eight “beta-builders” and general event helpers. Our kit orders would be fulfilled before FDIM and shipped to us directly, so that we could assemble the kit, make suggestions on the build and instructions, and generally become “experts” in advance so as to be most helpful during the event.

Along with seven other helpers, I received a beta version of the kit a couple of weeks before FDIM. We all quickly got to work building the boards, and proofing the construction directions. Since the board is intended as a training tool, we also tried out all of the circuits and experimented with the scopes to determine what worked well.

The boards we got were very “beta”, and had manual re-work done by Rex before sending them out to us. My build of the preview board went smoothly, and took about 90 minutes. I am a slow, methodical builder, doing a lot of checks along the way, so I think most folks could finish the kit in less time. Everything worked, and the only change I suggested was to increase the value of resistor in series with the white LED on the board, which was blindingly bright using the 1k resistor supplied.

After completing my board, I decided to add a few components to the MePad section to allow its use as a morse code practice oscilator, by routing the output of the audio oscillator to a 1/8th inch stereo jack, and then to the input of the audio amplifier. I added additional header pins to allow the connections to be made with jumper wires.

O’scope Fun Board implementation of CW practice oscillator

This worked fine, and allowed me to demonstrate a cool feature of the the DSO 150: you can turn off the trigger function altogether, and stream the signal to the display. By setting the timebase to about a second, I could see my CW wave forms go by as I keyed the circuit, as you can see here:

Moving on to checking out the small scope, the first thing I did was to make a list of suggested starting settings for each of the test circuits, as it was fairly easy to hook up the scope and see nothing because the vertical amplitude, horizontal time base, or the triggering was not set correctly.

Next up was a request from Rex to produce better documentation for the scope, since the instruction sheet included with the device could be cryptic to someone not familiar with oscilloscopes to begin with. One of the other helpers found an excellent online resource for the scope (more on this below), but I put together a quick “cheat sheet” listing all of the keys and corresponding functions. We wound up not using this, but I’ve uploaded it as a link on this site for anyone who is curious.

The scope we got is a Jyetech DSO 150 Shell scope. These can be found on eBay, Amazon, Alibaba and a bunch of other places. The units we got are from Circuit Specialists, and come assembled.

This little scope seems handy enough to me that I decided to take it to the next step, and add a rechargeable L-ion battery too it, eliminating the need to deal with the dangling 9v battery on a pigtail (which has the habit of dropping off just when you get the probe attached where you need it.)

You can find a number of YouTube videos on adding a rechargeable battery to the DSO 150, but the best solution I found was a kit produced by gianlucaarenadesign. You can find these on eBay. It is an ingenious solution: he provides a nicely 3D printed replacement back for the DSO 150, with room for a battery and requisite circuitry.

Replacement back for the DSO 150 scope with L-ion battery, charge controller and buck converter

The kit comes with the back, a charge controller for the L-ion battery, and a DC-DC buck converter to raise the 3.7v battery to the 9 volts needed by the scope. Also included is a sliding on/off switch (needed because the buck converted will eventually drain the battery if left connected, even if the scope is not turned on), and a cute transparent plastic “light pipe” to make the charging LEDs visible on the side of the case.

You have to provide the L-ion battery, some wire, and in my case a JST two wire connector to run from the buck converter to the main board. (Caution: the connectors available on Amazon have varying color coding, check the polarity you need on the board, and ignore the wire color code, which has a 50/50 chance of being wrong.) Optionally you can add a small diode at the output of the converter, I had one so I put it in.

All is explained in the excellent single sheet instruction which come with the kit. It’s an easy upgrade, and cost about $14 shipped from Italy. Yes I know, sounds like an expensive upgrade to a $20 scope, but if you really intend on using this thing portable I think it’s worth it.

So are these little scopes just a toy, or are they useful?

Certainly as a teaching tool for introducing someone to the uses of an oscilloscope you cannot beat the pricetag. Beyond that though, I think these instruments could be very useful additions to your toolkit. Certainly for troubleshooting audio circuits you are all set, plenty of bandwidth. Also, if you do any building with Arduino or Rasberry Pi projects, this scope can be very handy looking a states of GPIO pins and PWM output.

The fact that it is the size and weight of a small DMM means it can go in the tool bag, where bench scope stays at home on, well, the bench… So it probably won’t replace a good bench scope, but all in all a useful, and fun tool.
73
de N2HTT

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