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Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Electronics Workbench

Electrical & Electronic gadgets

Siglent SDS2304X Screen Shot File

Poking the Print button on the front of the Siglent SDS2304X scope saves the screen to a BMP file (in the /BMP directory) on a USB flash drive plugged into its front-panel port:

Which produces files like these:

ll --block-size=1 /path-to-USB-stick/BMP/
total 2318336
drwxr-xr-x 2 ed ed    4096 May 23 13:13 ./
drwxr-xr-x 4 ed ed    4096 Dec 31  1969 ../
-rw-r--r-- 1 ed ed 1152054 May 23 13:13 SDS00001.BMP
-rw-r--r-- 1 ed ed 1152054 May 23 13:13 SDS00002.BMP

The files are 1152054 bytes long, as specified by the BMP header inside the file:

hexdump -C /path-to-USB-stick/BMP/SDS00001.BMP | head
00000000  42 4d 36 94 11 00 00 00  00 00 36 00 00 00 28 00  |BM6.......6...(.|
00000010  00 00 20 03 00 00 e0 01  00 00 01 00 18 00 00 00  |.. .............|
00000020  00 00 00 94 11 00 00 00  00 00 00 00 00 00 00 00  |................|
00000030  00 00 00 00 00 00 01 01  01 01 01 01 01 01 01 01  |................|
00000040  01 01 01 01 01 01 01 01  01 01 01 01 01 01 01 01  |................|
*
00000880  01 01 01 01 01 01 01 01  01 01 01 01 01 01 1e 1e  |................|
00000890  1e 1e 1e 1e 1e 1e 1e 1e  1e 1e 1e 1e 1e 1e 1e 1e  |................|
*
00000990  1e 1e 1e 1e 1e 1e 01 01  01 01 01 01 01 01 01 01  |................|

The first 14 bytes contain the Bitmap file header, with the file size in Little-Endian order in the four bytes at offset +0x02: 0x00119436 = 1152054.

The four bytes at offset +0x0A give the offset of the pixel data: +0x36. That’s the series of 0x01 bytes in the fourth row. Unlike most images, BMP pixel arrays start at the lower left corner of the image and proceed rightward / upward to the last pixel at the upper right corner.

The data between the Bitmap file header and the start of the pixel data contains at least a Device Independent Bitmap header, identified by its length in the first four bytes at offset +0x0E. In this case, the length of 0x28 = 40 bytes makes it a Windows (no surprise) header.

The two bytes at +1C give the bits-per-pixel value: 0x18 = 24 = 3 bytes/pixel, so parse the pixels in RGB order.

The four bytes at +0x12 give the bitmap width in pixels: 0x320 = 800. Each pixel row must be a multiple of 4 bytes long, which works out fine at 2400 bytes.

The tail end of the file shows one dark pixel at the upper right:

hexdump -C /path-to-USB-stick/BMP/SDS00001.BMP | tail
00118330  00 cc 00 00 cc 00 00 cc  00 00 cc 00 00 cc 00 00  |................|
00118340  cc 00 00 cc 00 00 cc 00  00 cc 00 00 cc 00 00 cc  |................|
00118350  00 00 cc 00 00 cc 00 00  cc 0f 0f 75 1e 1e 1e 1e  |...........u....|
00118360  1e 1e 1e 1e 1e 1e 1e 1e  1e 1e 1e 1e 1e 1e 1e 1e  |................|
*
00118ad0  1e 1e 1e 01 01 01 1e 1e  1e 1e 1e 1e 1e 1e 1e 1e  |................|
00118ae0  1e 1e 1e 1e 1e 1e 1e 1e  1e 1e 1e 1e 1e 1e 1e 1e  |................|
*
00119430  1e 1e 1e 01 01 01                                 |......|

Which looks like this, expanded by a factor of eight (clicky for more dots to reveal the situation):

Screenshot - upper right corner - 8x expansion — Screenshot – upper right corner – 8x expansion

The scope can also transfer a screenshot over the network:

lxi screenshot -a 192.168.1.42 /tmp/lxi-shot.bmp 
Loaded siglent-sds screenshot plugin
Saved screenshot image to /tmp/lxi-shot.bmp

Which has the same header:

hexdump -C /tmp/lxi.bmp | head
00000000  42 4d 36 94 11 00 00 00  00 00 36 00 00 00 28 00  |BM6.......6...(.|
00000010  00 00 20 03 00 00 e0 01  00 00 01 00 18 00 00 00  |.. .............|
00000020  00 00 00 94 11 00 00 00  00 00 00 00 00 00 00 00  |................|
00000030  00 00 00 00 00 00 01 01  01 01 01 01 01 01 01 01  |................|
00000040  01 01 01 01 01 01 01 01  01 01 01 01 01 01 01 01  |................|
*
00000880  01 01 01 01 01 01 01 01  01 01 01 01 01 01 1e 1e  |................|
00000890  1e 1e 1e 1e 1e 1e 1e 1e  1e 1e 1e 1e 1e 1e 1e 1e  |................|
*
00000990  1e 1e 1e 1e 1e 1e 01 01  01 01 01 01 01 01 01 01  |................|

But the resulting file is three bytes = one pixel (!) too large:

ll --block-size=1 /tmp/lxi.bmp
-rw-rw-r-- 1 ed ed 1152057 May 23 19:09 /tmp/lxi.bmp

The tail end of the file:

hexdump -C /tmp/lxi.bmp | tail
00118330  00 cc 00 00 cc 00 00 cc  00 00 cc 00 00 cc 00 00  |................|
00118340  cc 00 00 cc 00 00 cc 00  00 cc 00 00 cc 00 00 cc  |................|
00118350  00 00 cc 00 00 cc 00 00  cc 0f 0f 75 1e 1e 1e 1e  |...........u....|
00118360  1e 1e 1e 1e 1e 1e 1e 1e  1e 1e 1e 1e 1e 1e 1e 1e  |................|
*
00118ad0  1e 1e 1e 01 01 01 1e 1e  1e 1e 1e 1e 1e 1e 1e 1e  |................|
00118ae0  1e 1e 1e 1e 1e 1e 1e 1e  1e 1e 1e 1e 1e 1e 1e 1e  |................|
*
00119430  1e 1e 1e 01 01 01 01 01  0a                       |.........|

Because the file header doesn’t include those three bytes, they don’t go into the image and the resulting screenshot is visually the same.

Which looks like a picket-fence error, doesn’t it? I’d lay long odds the erroneous loop runs from 0 to NUMPIXELS, rather than 0 to NUMPIXELS-1. Raise your hand if you’ve ever made that exact mistake.

I have no practical way to determine whether the error is inside the scope or the LXI network code, but given Siglent’s overall attention to software fit-and-finish, I suspect the former.

One can convert BMP files to the much more compact PNG format:

convert /tmp/lxi.bmp /tmp/lxi.png
convert: length and filesize do not match `/tmp/lxi.bmp' @ warning/bmp.c/ReadBMPImage/829.

Yes. Yes, there is a mismatch.

The space savings is impressive, particularly in light of PNG being a lossless format:

ll /tmp/lxi.*
-rw-rw-r-- 1 ed ed 1.1M May 23 19:09 /tmp/lxi.bmp
-rw-rw-r-- 1 ed ed  14K May 23 19:17 /tmp/lxi.png

You can eliminate the nag by truncating the file:

truncate --size=1152054 /tmp/lxi.bmp

One could wrap it all up in a script:

#!/bin/bash
lxi screenshot -a 192.168.1.42 /tmp/"$1".bmp
truncate --size=1152054 /tmp/"$1".bmp
convert /tmp/"$1".bmp "$1".png
echo Screenshot: "$1".png

And then It Just Works:

getsds2304x.sh "Test Shot Starfish"
Loaded siglent-sds screenshot plugin
Saved screenshot image to /tmp/Test Shot Starfish.bmp
Screenshot: Test Shot Starfish.png

SpaceX uses Test Shot Starfish tracks for pre-launch background music; the actual test shot was spectacular.

2018-05-30

Subaru Forester Fuse Boxes

Speaking of automotive fuses, our 2015 Subaru Forester has a pair of fuse boxes, hereby documented in case of need.

One under the hood:

2015 Subaru Forester – engine compartment fuse box

Note the white fuse puller near the top.

The layout chart doesn’t say what “SBF” might be (per the comment: Slow Blow Fuse), but we have a lot of whatever it is:

2015 Subaru Forester – engine compartment fuse ID

The spare fuses line up along the lower edge of the cover.

Another under the dashboard:

2015 Subaru Forester – dashboard fuse box

And their functions:

2015 Subaru Forester – dashboard fuse ID

The string of fuses down the right side of the main block looks like a line of spares, but they’re not. What they might be isn’t documented anywhere, which seems to be very deliberate.

Memo to Self: Having never replaced an automotive fuse, I shouldn’t start worrying now.

2018-05-28
SJCAM M20 Date/Time vs. Battery Change

A recent ride reminded me to do something about this:

M20 – Date-Time Reset – detail – 2018-05-18

So I wrote up a support ticket on the SJCAM site:

The time-of-day clock in my M20 often resets when I change the battery in the middle of a bike ride.

I turn the camera off, wait for the status light to go out, remove the battery, install the new battery, turn it on, and the time-of-day displayed on the screen has reset to 2016-01-01 00:00:00.

I’m using firmware 1.3.1 (the latest), genuine and fully charged SJCAM batteries, and swap the batteries as fast as I can. Sometimes it works, but maybe half of my bike rides end years before they start! [grin]

It seems my turned-off M20 is extremely sensitive to the power fluctuations occurring during a battery change.

What do you recommend?

Thanks …

Their reply:

The capacity of internal memory battery on main board is very small due to hardware limitation so it can save date and setting for about 10 seconds after pulling out battery.

Would you please check it again ?

I’d call that a design screwup, not a “hardware limitation”. Perhaps I don’t understand how putting a slightly larger capacitor on the PCB, in place of the one that’s already there, would pose a problem.

They also recommend checking with my “re-seller”, but, seeing as how I bought it directly from their nominally official Amazon store, so:

In case they are not able to offer help, SJCAM Technical Department offers a maintenance service. The steps of such service are:

1. You ship the camera directly to our Technical Department address at your own cost (it is located in Shenzhen, China).

2. We check and repair the camera. The repair process usually takes about 3-5 working days.

3. We ship the camera back to you.

Note: The whole process usually takes about 20-30 days, and if your camera doesn’t have damage on the main-board, screen or lens, the maintenance will be free, but we charge 15$ as return shipping cost.

As usual, round trip shipping to Shenzen costs half the price of the M20 camera package, a fact I’m sure they’re well aware of. I did a warranty return to Australia with the Cycliq Fly6, before replacing the battery myself, and (re)learned valuable lessons about warranties and batteries.

I turned SJCam’s offer down, which prompted a curious proposal:

You can send back your camera to SJCAM factory and then we can replace internal memory battery for you.

So the “hardware limitation” has morphed into a (presumably inadequate) internal battery that, when replaced, will resolve the problem. Huh.

Note: you can’t use the M20’s “Car mode” with the timestamp function, because you must remove the battery to let the camera start when the USB power goes on. Unlike basically all other cameras-with-clocks, the M20 wasn’t designed to run its internal clock without a battery.

Improving my battery change speed definitely has the best ROI. Alas, my dexterity has a definite upper limit …

AFAICT, the M2 has a glued-together / assemble-only shell, so cracking the case and hacking a cap isn’t happening.

2018-05-25
Littelfuse Automotive Fuse Tester

Fuses came up during the Second Squidwrench Electronics Workshop, so I’ll bring a handful along to the next session. Conveniently, the doomed Sienna’s rear storage compartment disgorged a new-old-stock box of fuses, along with a fuse puller/tester containing a pair of ~~feeble~~ vintage 164 / LR60 / AG1 alkaline cells:

Littelfuse Mini Auto Fuse Puller-tester – as opened

Somewhat to my surprise, the other side of the PCB has components:

Littelfuse Mini Auto Fuse Puller-tester – circuitry

It looks like a transistor switch to minimize the current through the fuse and protect the LED from over / reverse voltage, should you apply it to a live circuit.

A pair of new cells had it working just fine, not that I expect to need it in real life.

2018-05-24
Kenmore 158: First Needle LED Failure

The first white LED fixture built to illuminate one of Mary’s Kenmore 158 sewing machines has been in regular use for the last four years:

Kenmore 158 Sewing Machine – mixed LED lighting

We never found a good time to rip-and-replace the “prototype” with brighter SMD LEDs and one of the LEDs finally gave up.

They’re 10 mm white LEDs with five chips wired in parallel, which is obvious when you look into the remaining LED running at 1 mA:

10 mm white LED – chips

The center chip is just dimmer than the others, which means their QC doesn’t tightly control the forward voltage spec.

The wire bonds on the anode terminal of the failed LED look a bit sketchy:

10 mm white LED – wire bonds

Fortunately, I hadn’t removed the 120 VAC wiring for the original bulb and I have two OEM bulbs from other machines, so I just removed my LED gimcrackery, installed a good old incandescent bulb, and she’s back to sewing with a pleasantly warm machine.

The fixture holding the LEDs broke apart as I extracted it, but it’ll never be used again:

10 mm white LED – fixture

The LEDs are rated at 3.5 V and 200 mA (!), but were reasonably bright in series from a 6 V unregulated supply. Perhaps a power glitch killed the poor thing? We’ll never know.

LEDs are reputed to have lifetimes in the multiple tens of thousands of hours, but I’ve seen plenty of failed automotive LEDs and fancy new LED streetlights out there, not to mention many dead and dying traffic signals. Seeing as how they’re in (presumably) well-engineered fixtures with good power supplies and are at most only a few years old, there shouldn’t be any failures yet.

2018-05-23
Silicone Insulated Wire

Quite unlike the non-standard ribbon cable in the WWVB loop antenna, these 10 foot hanks of 24 AWG silicone insulated wire were exactly as described:

24 AWG Silicone Wire

The probes report a resistance of 270 mΩ (net of the 50 mΩ probe-to-probe resistance), as close as one could ask to the nominal 26 Ω/1000 ft spec for 24 AWG wire at the current Basement Laboratory temperature.

They are exceedingly limp and flexy, due to many teensy conductors; not at all like PVC hookup wire.

If you’re willing to buy 500 feet of each color, the cost-per-foot from a reputable supplier gets downright competitive, but I’m not in that market.

2018-05-22
QRPme Pocket Pal II

A QRPme Pocket Pal II could be a suitable project for a Squidwrench “advanced soldering” class:

QRPme Pocket Pal II – front

Yes, it comes with a tin case:

QRPme Pocket Pal II – tin case

You must fit your own insulating sheet under the PCB; polypropylene snipped from a retail package works fine.

It’s intended as a “mint tin sized tester for all kinds of hamfest goodies”, but it seems like a nice source of small currents, voltages, and signals suitable for stimulating all manner of circuitry one might encounter in later sessions of a beginning electronics class.

Before using it, of course, one must solder a handful of small through-hole parts into the PCB, a skill none of us were born with.

For completeness, the back side, hot from the soldering iron:

QRPme Pocket Pal II – rear

The kits (always buy two of anything like this) arrived minus a few parts, which I suspect was due to an avalanche of orders brought on by a favorable QST review. Fortunately, I (still) have a sufficient Heap o’ Parts to finish it off without resupply, although a hank of 9 V battery snaps will arrive in short order.

2018-05-21