Archive for category Electronics Workbench
Sorting two lots of 50 transistors by gain kept me out of trouble for a while:
Those are MPSA14 NPN and MPSA64 PNP transistors, with DC gains ranging from around the spec’s minimum 10 k spec all the way up to well over 100 k.
The yellow trace shows the booster output voltage is 9 VDC, as set by the twiddlepot, and doesn’t vary much under load. It has 200 mV ripple at 220 kHz, the booster’s switching frequency, which doesn’t induce any meaningful noise on the scope’s display, because it’s well outside the display bandwidth and well inside the voltage spec.
The current traces are 100 mA/div from Tek Hall effect probes. The green trace is battery current to the booster, varying from 200 to 300 mA, averaging 250 mA. The cyan trace is DSO150 current from the booster, 75 mA min, 200 mA max, averaging 100 mA.
The battery current is 2.5 × the scope current, the battery voltage is 1/2.5 × the scope voltage, and all is right with the world.
Which went to a 100 Ω dummy load drawing about the same current as the DSO150:
Both seem to work OK, albeit with plenty of spiky noise:
Much to my surprise, there’s no visible noise on the DSO150 display, surely because the scope’s bandwidth is nowhere near wide enough to grow that kind of grass.
A power supply like that would convert the DSO150 into a bench instrument suitable for low frequency circuitry.
It lights up just like it should:
The blue LED works, too, but I didn’t catch any of those blinks.
The spider should be done in black PETG, just like the battery holder, but I didn’t realize which filament was running until too late. Even the blue LED lights up the orange spider just fine!
The circuitry behind (well, below) the RGB LED Radome consists of three copies of the original multivibrator, with mirror image layouts to match the wire struts:
The solder joints adhere to exactly none of the usual good practices:
The simulation matches the actual blink times reasonably well:
It’s unpleasantly frenetic in real life. The next version must have much much longer time constants.
Unfortunately, the simulation also confirms my suspicion that I’ve been abusing the electrolytic capacitors with reverse-polarity waveforms. I suspect it doesn’t really matter too much, as the maximum voltage in either direction remains under a volt at very low currents, but it’s the principle of the thing.
Soooo, lengthening the time constants by increasing the capacitances seems like a Bad Idea.
Alas, increasing the resistors by an order of magnitude won’t work, either, because (despite appearances) the whole thing sits right on the hairy edge of not working. As the battery discharges toward its 2.5 V cutoff level, the currents drop and the circuitry becomes increasingly sensitive to touch. After a day or two, one of the LEDs will jam solidly on, while the others continue to blink merrily away. Removing and reinstalling the battery will sometimes resume proper operation, but it’s definitely not stable enough for production use.
Which makes a MOSFET astable multivibrator seem like a Good Idea.
One could achieve the same visible result with a few cents of microcontroller and a dab of software, but most of the charm comes from its analog nature and all those visible components.
You can find anything on eBay (clicky for more dots):
The key information:
For that price, I’d expect in-person hand delivery.
Stipulated: ZVNL110A MOSFETs aren’t in production and we’re buying from diminishing inventory, but (as of late December 2018) they’re still available for under a buck apiece in small quantities.
Well, a spider with half the proper leg count:
One could argue the LED spider has an unusually large abdomen, but I’m not going there.
The solid model looks the same way:
And, yes, those are eye protection caps over the four wire struts, most useful during construction while maneuvering the radome into position.
For reasons unknown to me, they’re called “Pirhana” LEDs:
I trimmed off half of each pin, soldered on 28 AWG color-coded silicone wires, threaded wires through openings, then rammed the LED package into the recess so it sits just below the radome’s curve. The dent matching the ball comes from the chord equation, as always, and looks pretty good.
The radome is, of course, a one-star ping pong ball from the usual big box retailer’s sporting goods section. The stamped logo sits at a random position with respect to the ball’s interior structure (visible when lit, as in the top picture), so I erased it with a fine-grit sanding sponge. Hollow plastic golf balls might work just as well, with an even more interesting surface texture.
The source code includes a cutaway look at the printed parts to verify their innards:
The OpenSCAD source code as a GitHub Gist:
The original doodles give useful dimensions, plus some details not withstanding the test of time:
The actual center-to-center distances for the wire posts come from the battery dimensions, rounded up or down as appropriate, to the nearest multiple of 5 mm, so those are just serving suggestions.
One cannot (or, perhaps, should not attempt to) solder parts to 14 AWG wires seated in a 3D printed battery holder base, so I cleaned up the edges of two polycarbonate scraps:
Then drilled holes to match the strut positions:
The holes fit snippets of the original wire insulation, because, after all, polycarbonate is a thermoplastic, too.
Stretch some copper wire to straighten and work-harden it, add insulation snippets, then maneuver everything in place:
I definitely need a third (and maybe a fourth) hand to hold each part, the solder, and the iron, but at least the wires won’t walk away in the middle of the process.
A new pair of shoes arrived with extravagantly long laces requiring shortening. Years ago, I found heatshrink tubing completely unequal to the task, so I deployed Real Metal:
The ferrules come from a kit of such things, minus their plastic strain relief:
That’s a fancy hexagonal crimper for round-ish results. If you have a square terminal block, you should use the square crimper that comes with the kit.
Worked perfectly and produced immediate customer satisfaction.