kmod's blog


New parts!

I just got my orders SparkFun and DigiKey, and now I have a bunch of new toys to play with:

And a bunch more.  To add to the fun, a friend lent me their $600 Metcal soldering station since they weren't using it; I don't have enough soldering experience to really evaluate the differences, but I can only assume that this is a big step up from my $11 RadioShack kit.


First up was to replace my discrete-MOSFET inverter with the dedicated inverter part.  This part is quite nice actually: it can take any supply voltage between 2V and 6V, so I don't have to worry about stepping it up/down for use in 3.3V or 5V circuits.  I set it up in a simple configuration, with a red LED for logic high/low, and a yellow one for an edge detector, and used a potentiometer to generate an adjustable voltage [the edge-detector is missing in this photo]:

2013-08-01 15.07.18

Everything pretty much worked as expected: if I turn up the input voltage, the first (level-detector) led turns on, and if I turn it down, the first one turns on and the second one (edge-detector) briefly flashes.  For fun, I decided to check out the inverter's transition from low-to-high: it turns out that it does this reliably across a very narrow voltage band, much narrower than 1.35-3.15V range that they allow themselves in the datasheet.  Interestingly, I could pick a voltage that was right at the edge, which is what's going on in the above photograph.

As you might expect, in this case the indicator led is at an intermediate brightness.  The surprising thing, at least to me, is that the edge-detecting led is also on [sorry for not having the photo for this]!  This made me doubt my design, but after looking at it again I don't think there's any way for a DC current to sneak through there in a way I didn't anticipate.  The only alternative I could think of is that there must be some oscillating going on.  Unfortunately, I didn't have the tools to directly measure this, but luckily I had just received:

DIY Oscilloscope

The only tool I know of to debug this kind of issue, and presumably the most common, is the oscilloscope.  Looking on amazon, it looks like decent oscilloscopes start around $300 and up, which is a bit out of my budget.  Instead, I went with this kit from SparkFun (originally made by, since it's only $60, and also I wanted the soldering practice -- they list it as "on the higher end of difficulty as far as through-hole kits go", so I figured it'd be a good way to benchmark my soldering skills.  Getting this kit working is another story, but suffice it to say that after a few hours of soldering and re-checking my joints, I eventually had it up and running.

Unfortunately my phone couldn't take a good picture of the results so there aren't pictures for this section, but I immediately hooked the completed scope up to the inverter circuit above to see what was going on.  Well, actually first I had to jury-rig my setup a bit, since I only have one 9V wall wart, and the oscilloscope takes one for power, so I had to run everything else off the power from my USB breakout board.

Anyway, once I got it working, the result was pretty obvious: the circuit had very noticeable oscillations with period of about 16ms, for a frequency of about 60Hz, which nicely explains what's going on: the oscillations I'm seeing are from power supply ripples from the mains 60Hz frequency.  Unfortunately, I won't be able to able to debug my own power management circuit until I have two separate wall warts, since I can't run both my power supply and the oscilloscope at the same time, but hooking the oscilloscope leads to the USB power lines showed that the USB power was actually pretty good, less than +-100mV (the scopes finest resolution).  Unfortunately, I don't think the scope has very good power isolation, since if I don't connect the leads to anything, the display shows me a 60Hz wave, which I assume must be from coupling internal to the scope.  So to really test this, I'd either need a oscilloscope that I trust to not have power supply ripples, or somehow connect the scope to a different power supply, such as a battery.

So anyway, nothing truly revolutionary, but it's cool how much you can discover yourself with only some modest equipment.

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