Archive for category Electronics Workbench
With a terminal voltage falling from barely 3 V, the LED drew about 3 mA (1 mA/div), tops, without a ballast resistor:
Hacking in a charged NP-BX1 secondary lithium cell boosted the supply to 4 V:
Which, diodes being the way they are, raised the LED current to nearly 400 mA (100 mA/div):
Somewhat to my surprise, a few weeks of abuse didn’t do any obvious damage to the LED, but I added a resistor while I was soldering up another holder:
There’s not quite enough room for a 1/8 W axial resistor, so why not blob in a surface-mount resistor?
Which cuts the current down to a mere 15 mA (10 mA/div) from a lithium battery at 4 V:
It’s still blindingly bright, but now I don’t feel bad about it.
Adapting the NP-BX1 battery holder to use SMT pogo pins worked well:
The next step is to add sockets for those 14 AWG wires:
Start by reaming / hand-drilling all the holes to their nominal size and cleaning out the pogo pin pocket.
Solder wires to the pogo pins and thread them through the holder and lid:
That’s nice, floppy silicone-insulated 24 AWG wire, which may be a bit too thick for this purpose.
The pogo pins will, ideally, seat with the end of the body flush at the holder wall. Make it so:
Dress the wires neatly into their pocket:
Butter the bottom of the lid with epoxy, clamp in place, set it up for curing, then fill the recess:
While it’s curing, make a soldering fixture for the 14 AWG wires:
The holes are on 5 mm centers, in the expectation other battery holders will need different spacing.
Solder it up and stick the wires into the base:
Jam a battery in and It Just Works™:
- Green = supply current at 20 mA/div
- Yellow = LED driver transistor base voltage
- Purple = other transistor collector voltage
- White = base – collector voltage = capacitor voltage
The measurement setup was a bit of a hairball:
For completeness, here’s the schematic-and-layout diagram behind the circuitry:
I love it when a plan comes together!
The OpenSCAD source code as a GitHub Gist:
As part of converting the halogen desk lamp to LEDs, I replaced the hulking iron transformer with a flatter counterweight:
Under normal circumstances, you’d use something like steel or lead sheets, but Tiny Bandsaw™ can’t cut any appreciable thickness of steel and I gave away my entire lead stockpile, so I sawed disks from a pile of non-stick pancake griddles and drilled suitable mounting holes:
Another disk (from a formal aluminum sheet!) goes into the lamp head, with a trio of 3W COB LEDs epoxied in place:
The other side of the disk sports a heatsink harvested from a PC, also epoxied in place:
Realizing the head required only a little filing to accommodate the heatsink sealed both their fates.
A test firing showed the heatsink needed more airflow, which didn’t come as much of a surprise, so I milled slots in the lamp head:
Deburring the holes, blackening the sides with a Sharpie, and tucking a bit of black window screen behind the opening made the vents look entirely professional.
The small dome in the base originally cleared the transformer and now holds the entire 10 W LED driver, along with all the wiring, atop the counterweight sheets:
A cork pad covers the base for a bit of non-skid action:
I couldn’t convince myself filling in those sectors would improve anything, so I didn’t.
And then It Just Worked:
All without a trace of solid modeling or G-Code …
The DSO150 oscilloscope’s specs give a 200 kHz bandwidth, so a 50 kHz sine wave looks pretty good:
A 100 kHz sine wave looks chunky, with maybe 25 samples per cycle:
The DSO150 tops out at 10 µs/div, so you can’t expand the waveform more than you see; 25 samples in 10 µs seems to be 2.5 Msample/s, exceeding the nominal 1 Msample/s spec. I have no explanation.
A 10 kHz square wave shows a blip just before each transition that isn’t on the actual signal:
At 50 kHz, there’s not much square left in the wave:
And, just for completeness, a 200 kHz square wave completely loses its starch:
A 10% (-ish) duty cycle pulse at 25 kHz has frequency components well beyond the scope’s limits, so it’s more of a blip than a pulse:
The pulse repetition frequency beats with the scope sampling and sweep speeds to produce weird effects:
Tuning the pulse frequency for maximum weirdness:
None of this is unique to the DSO150, of course, as all digital scopes (heck, all sampled-data systems) have the same issues. The DSO150’s slow sampling rate just makes them more obvious at lower frequencies.
Key takeaway: use the DSO150 for analog signals in the audio range, up through maybe 50 kHz, and it’ll produce reasonable results.
Using it for digital signals, even at audio frequencies, isn’t appropriate, because the DSO150’s low bandwidth will produce baffling displays.
The only scope mod consists of embedding a JST-ish connector in the back panel:
Then soldering it to the battery pads and applying generous hot-melt glue blobs:
Add a scrap 18650 Li-Ion cell, a regulated boost converter, and a switch:
The switch is directly below the DSO150 BNC connector to get a little protection for its handle, which would otherwise stick out in harm’s way. This being an afterthought, I drilled the switch hole, rather than modify the solid model.
Some testing with a bench supply showed that the DSO150 will not operate correctly from the voltages produced by a pair of lithium cells, despite what you’d think from looking at the case. Below 8 V, the internally generated negative supply becomes larger than the positive supply, so the 0 V point isn’t properly centered and the scope loses headroom for large signals; monitoring the internal 3.3 V test signal makes the problem painfully obvious.
More color commentary from my summary email:
- Combining a case from Thingiverse with a Li-Ion cell and a regulated boost converter produces a portable scope.
- The PCB has provision for battery input, so I drilled / filed a square hole for a teeny JST-ish connector on the back panel, secured it with a blob of hot melt glue, and globbed the wires onto the PCB battery pads.
- The boost converter draws about 400 mA from the cell, so a 2500-ish mA·h cell should last Long Enough™. This is a scrap cell from the recycle box and gave out after maybe four hours.
- It idles at 8 mA, so I drilled a hole in the back of the case for a toggle switch disconnecting the battery; you’d want the hole in the solid model. Perhaps a better converter would have lower idle current; you’d never be able to tell from the eBay descriptions.
- Aaaaand it switches around 200 kHz under load, just barely beyond the scope bandwidth. It doesn’t add much noise to the signal, at least with a 50 Ω terminator jammed in the BNC, but the square-wave “cal” output looks awful at 50 mV/div; a real scope shows even more noise. I assume the noise comes directly from the logic supply; with luck, the DSO150’s analog circuitry has Good Enough™ filtering.
- Which might not matter for logic-level and moderate analog signals, of course, which is the whole point of the DSO150.
- Conspicuous by their absence: a Li-Ion cell protection PCB and any way to recharge the poor thing …
I’ve occasionally wanted a portable scope and now I have one!
I did a quick build of a JYE Tech DSO150 oscilloscope to see how it’d work in a proposed Squidwrench advanced soldering class / kit build session.
The main board requires adding only a few switches and headers, then removing a 0 Ω jumper resistor:
The analog board requires a handful of 1/8 W resistors, various capacitors, switches, and the BNC connector:
Some (lightly edited) color commentary from my summary email:
- Just finished assembling the kit, which required two hours; I’m admittedly fussy. The one joint I missed on the input coupling switch required a complete disassembly, but all the rest worked fine.
- The UI is much better than the DSO138.
- Soldering the BNC connector requires lots of heat. My ordinary Hakko iron had inadequate grunt, so I deployed the hulking Radio Shack 150 W gun and did the job in seconds.
- The resistors require a meter to measure them during installation, because they’re 1% 1/8 W jobbies with many teeny color strips in Chinese tints you’ve never seen before. I could not sort them visually, even with a lighted headband magnifier, and I know what I’m looking for.
- The caps are marked, but using a meter builds confidence.
- And, yes, the kit had all the right parts and they all worked. The instructions call for powering up the main board before starting assembly, then again after removing a 0 Ω jumper resistor, but that’s the extent of the “testing” required.
- They recommend a flush cutter and I’d say it’s pretty much required. An ordinary diagonal cutter won’t get close enough to the PCB.
- I needed an angle-tip tweezer to lay the PCB screws in place.
- Don’t install the knob until the very last step and maybe wait until you’ve verified all the functions. You have been warned.
- The minimum power supply voltage really is 8.0 V, not the 7.4 V from a not-quite-fully-charged pair of lithium cells. A 9 V alkaline battery will last a few minutes. A noisy boost converter / crappy 9 V wall wart translates directly into noise on the display, particularly on the internal calibration signal.
- The “0.1 V” calibration signal turned out to be 150 mV, as measured on a real scope, at 1 kHz. The 3.3 V signal is closer to reality. Both are noisy from a noisy supply.
- All in all, it’s a pretty good scope for thirty bucks!
- Newbies will find it a challenging three hour build, for sure.
The next step involves adding a case and battery power:
Hitching a charged, albeit worn, NP-BX1 lithium battery to the astable multivibrator produces a blinding flash:
The current pulse shows the wearable LED really takes a beating:
The current trace is at 100 mA/div: the pulse starts at 400 mA, which seems excessive even to me, and tapers down to 200 mA. It’s still an order of magnitude too high at the end of the pulse.
On the other paw, maybe a 14% duty cycle helps:
The top trace shows the base drive voltage dropping slightly, although I suspect the poor little transistor can’t take the strain.
The LED really does need a ballast resistor …