On its own, the servo takes power, ground, and a position input and moves the shaft within a range of rotation to match the angle requested on its input wire. It has one or more stops in its gearbox to prevent it from rotating past the end of its range; these need to be removed. It also has a potentiometer as part of its positioning system, which — for the usual modification — needs to be tricked into thinking it’s always centered. The modified servo then runs at full speed forward or backward trying to reach a requested position and thinking it has never succeeded in doing so; and the controller requires extra, external rotation sensing if you want to detect what the servo has actually done so far.

I wanted something a little different — PWM H-bridge control of the servo’s motor for variable speed forward and backward and access to the potentiometer to detect position (crudely and at low speed) and count wheel rotations (acceptably and at high speed). This is actually an easier modification — but, though I’m surely not the first to do it, I’ve not run across it before. I started last night.

The cute little SG90 has four screws holding its case together.

After removing them, the base can be pulled off, revealing the back of the motor and the back of the control board.

With the control board pulled out, it’s easy to see that the motor and potentiometer are wired exactly as would be expected; and nicely color-coded, making it easy to see how to rewire the servo.

The top of the case also pops off to reveal the gearbox. The output shaft is in the lower right and the two nubs on the back side of the output gear are the stop that prevents continuous rotation. I snipped them off in place using a very sharp diagonal cutter.

Next steps: Select wiring harnesses and rewire the motor and potentiometer.

The smaller the battery, the more important it is not to forget that the desulfator / dewhiskerer is on. This was, as I recall, only about a fifteen-minute overdose. The magic smoke, I assure you, got out.

The left two are open — you can see the broken wire at the bottom of the left coil and the top of the middle coil. (As always, click for full-sized image.) The right one is shorted and I haven’t found where.

I find it interesting that these are wound with round wire and the replacements are wound with flat-cross-sectioned copper “ribbon,” to get more current capacity in the same vertical space.

I tend to assume that batteries I happen upon will not be charged. Also that lead-acid batteries I happen upon will be low on water, even so-called sealed lead-acid batteries.

I wanted to start charging the batteries from my “new” scooter while working on other aspects of the project and the scooter didn’t come with a charger — I’ll deal with that later. Not knowing much about the wiring circuit yet, I didn’t want to connect an external charger to the batteries while they were still in-circuit and chance damaging the speed controller, so I needed to disconnect and remove them.

Wiggle wiggle wiggle <grunt> wiggle pull <grunt> WIGGLE WIGGLE BEND <grunt>

Wha?

Who does this??? When they said solder the quick-disconnect terminals, they meant the wire side.

Fine. My uncle’s iron and a pair of pliers solved that problem.

Two batteries, extricated and not yet cleaned.

In spite of being “sealed,” you can pry off the cover (preferably after cleaning, which I did first with Goo-Gone and then with dish soap and water)

and get to the cell caps, each with a little absorbent pad in case the cell venting carries too much moisture.

I could see no water in any cell of either battery. I borrowed a jug of distilled water from my folks (I don’t know why Mom always has some, but she does) and started filling them up … after taking measurements.

I filled each cell, waited for air bubbles to trickle to the top, refilled, waited, refilled, waited … I’m guessing between the initial fill and while charging, I ended up putting at least 10 ml of water into each cell.

Then put battery 1 on my variable power supply with the voltage set to 13.8 V and the current limited initially to 0.1 A, raising the current limit to 0.3 A as it was clear that nothing horrible was happening. I checked on it every half-hour to hour, frequently refilling at least one cell in which I could no longer see any water.

After about four hours, it was up to 13.5 V. The water level in the cells had risen to overflow the opening and fill each reservoir. If I watched long enough, I could see the water in a couple of cells <pop>, indicating they were just starting to gas and it was time for me to stop this method of charging for the day. (More on that on a subsequent day, hopefully tomorrow.)

While battery 1 was charging, I was also checking water levels in battery 2 and refilling low cells, just sitting on the counter.

Recalling that it had an initial 10 V charge to battery 1′s 11.7 V, noting that they had been connected in series, and knowing that the worst cell in a battery generally has a cascading failure, I expected a different charging experience from battery 2, and I was quite right.

I connected it to the power supply and it immediately showed 13.8 V at a 0.1-A current draw. Now, about an hour later, it’s at 13.5 V and 0.3 A and most of the cells have overflowed. It’s nearly done charging but I haven’t put nearly as much energy into it as I was able to put into battery 1 — that is to say, it’s not “taking” as much of a charge.

Tomorrow, schedule willing, a load test and an attempt at desulfation.

I was given this high-quality, hefty, well-balanced, well-performing LED flashlight several years ago with batteries already in it. I keep it in the glovebox; and recently when I needed it, it didn’t power on.

In my decades of using consumer electronics, there’s only one brand of alkaline cells that has never leaked in my equipment — even things I’ve forgotten about and found again years later — and I drive miles out of my way to buy them.

Doubtless someone will pop up with their own horror story, but I’ll still make the claim: You will never, ever, need this.

Cleaning the Flashlight

In order to get out the crud, I disassembled the flashlight completely and cleaned it with a wire brush, a wire wheel on the Dremel, a wire brush on the Dremel, and a toothbrush and dishwashing detergent. I went a little overboard to make sure I had got it all, but at every stage I was getting out more crud.

I’ve long been impressed with the heft and solidity of the flashlight; now that I’ve seen the inside, I’m equally impressed with its design and with the ease of (dis)assembly.

Loosely clockwise from the left, the reflector isn’t adjustable for beam focus but screws off anyway. The heatsinked LED has a separate plastic housing with beveled forward edge that centers the LED against the back side of its reflector cone. The LED housing’s retaining ring doubles as one of the LED terminals.

The aluminum head holds the LED housing and the separate LED driver housing, dropped in from the tail end, and screws tightly to the reflector. The plastic housing for the tiny power switch and LED driver board is made of two identical, completely reversible parts, holding in the inner power switch pushbutton on the one side and leaving a window to check board orientation for assembly on the other.

The rubber outer power switch pushbutton installs after the LED driver is dropped into the head and does not seal against the housing, suggesting that the o-rings on the aluminum housing are for a great feel during assembly and battery replacement rather than for water resistance.

The battery cage holds three AAA cells and assembles easily. The stiles are marked with the cell orientation; the filled cage looks a bit like a C cell and drops into the flashlight handle, with the spring-loaded button and the metal ring contacting corresponding surfaces on the bottom of the LED board.

The tail cap doesn’t make electrical contact with the battery’s negative terminal and the flashlight body doesn’t conduct current, as it does on many less expensive flashlights (not that I think I care).

Cleaned, reassembled, and with AAA cells reinstalled, it once again works perfectly. I look forward to many more years of service … and to not having to clean it out again.

The bus conversion project is languishing but not forgotten, and I’ve been wanting to put one of these chargers into the bus for the same reasons as I had for the van. The wiring situation is a little different, though — the bus has no cigarette lighter / power port, I’m intending to wire 12VDC throughout the bus with Anderson power pole connectors, and I might like to have multiple solar trickle chargers (even before I install larger solar panels on the roof).

The issue with multiple panels, and even with a single panel connected to a battery that will also be charged by the alternator, and even with a single panel that may still be connected to the battery at night, is that photovoltaic cells don’t like to have reverse voltage applied. The photovoltaic effect happens in a semiconductor junction, and although I can no longer find the reference I was reading the other day, I still know the cell doesn’t do well with a reverse voltage and should really be diode-protected.

Because I wasn’t sure how much (if any) circuitry was in the panel and how much (if any) was in the automotive power plug connector, I had to take both apart to (A) make sure the panel would be diode-protected even after I chopped off the power plug and (B) see whether either held any relevant / useful circuitry.

Panel — nope, nothing there.

Power plug — the LED that flashes while light is falling on the panel, a rectifier diode, and some passives.

Oh, and an integrated circuit under a glob of epoxy.

Hm, will that be some sort of power conversion or regulation circuit, or just the LED flasher?

The ground wire comes in from the panel, is soldered to the board, and connects to a ground wire running to the plug’s sleeve terminal. The positive wire comes in from the panel, is soldered to the board, and connects to the rectifier’s anode; the cathode connects to a positive wire running to the plug tip. The epoxy blob is therefore just the LED flasher.

The flashing LED is kind of a nice indication that the panel is delivering power to the battery; but I’m going to dispose of the plug; it’s not worth transplanting the PCB to the panel enclosure; and I can tell it’s making power ’cause I can see there’s sunlight falling on it. Also, something in this unit is flaky and the LED sometimes doesn’t flash even when the unit appears to be working.

So I need to chop off the whole power plug and wire in Anderson power poles, and somewhere in between the panel and the connector I need a protection diode. Might as well use the one from the circuit since it’s free, and it turns out to be a 1N5817 Schottky. The Schottky’s fast turnaround time isn’t important for this application — it would have been selected for its low forward voltage drop of .45V @ 1A and about .3V @ the 120mA for which the panel is rated.

Put the diode inline in the power cord or move it inside the panel? Since the diode is there to protect the panel from damage, I’m of a mind that it should be integral to the panel assembly — hard to separate and hard to bypass. There’s lots of room inside the case, so I moved the diode there and enclosed it in heatshrink.

Anderson power poles (the red and black connector at the center) are genderless connectors with generous metal-to-metal contact suitable for fairly high-current applications and popular in the amateur radio community. The individual shells have notches in the sides to slide together into whatever configuration of multi-pin connector you want.

And because they’re genderless (rotate the connector 180° and it would plug together with a connector sitting in its original position), they’re great for batteries — put the same connector on the battery, the battery charger, and your load, and you can pick any two devices to plug together.

This suits my plan for the bus: run appropriate-gauge DC wiring throughout and anywhere there’s a drop, I could plug in an AC or solar charger, a 12VDC light, an automotive USB charger, etc. Even before I do all the wiring, it still pleases me to have a cable coming from the battery into which I can easily plug the solar trickle charger, a small power inverter, or a USB charger.

My one concern is that everywhere I’ve seen power poles, red/black has been used for 12V connections; but the Anderson color code chart lists that red signifies 24V and yellow should be used for 12V. Yellow is a lot harder to find, not only in use but also to purchase. Although it seems important to conform to Anderson’s de jure standard of yellow/<what?>, I think using anything other than the de facto standard of red/black risks confusing anyone else familiar with power poles.

I was over at the aviation department last week and happened upon the installation of a new Stratasys rapid-prototyping machine.

It has a much larger build chamber than NIAR’s previous ABS machine — this one is something like 14″ x 14″ x 18″.

The case was open and I was intrigued by the thick blanket of insulation around the build chamber. I asked the installer if the whole chamber was heated and he said yes, to 80°C. Interesting point of reference, as RepRap / CupCake owners seem to have settled on 60°C as the standard temperature for heated build platforms.

It was fairly dark inside the build chamber and I couldn’t get a great shot with my cell phone camera, but you can see the extrusion head with two nozzles for support and build material. I found it interesting how extremely broad and shallow the white nozzle cones are — maybe it helps prevent snags?

With the lab manager’s blessing, I fished two filament strands out of the trash. The upper, black filament is ABS; the lower, translucent brown filament is a dissolvable support material that apparently washes out in an agitated hot water and detergent bath. Wish I knew exactly what it was!

I measure the diameter at .070″ ± .001″ ≈ 1.778mm ≈ 1.75mm ≈ .069″, so it looks like they’re using 1.75mm filament. The stretched section on the end is recognizable as having been in the hot end and then backed out.

Note the toothmarks all the length of each filament (about 3m), suggesting that either something is pushing the filament from that far back or (more likely) the hot end has a quick-release for cleaning and this filament was run through the machine after removing the hot end.

He had in mind to continue a series of his sculptures based on the form of a fishing lure but wanted to enhance this sculpture with one or more LEDs, preferably that would come on only at night. We discussed a wide variety of options that we hope to develop for another installation in the future; but in the end, in the interest of time for this project, Steve found control modules that flash up to five LEDs at random and installed them behind a set of cones protruding from a recessed panel.

He asked how to make the LEDs turn on at night and also wondered whether he could power them for a year from a primary battery or whether he should use rechargeables.

About seven years ago, I had come into possession of some discarded solar yard lights, and out of curiosity had reverse-engineered their charging and control circuits. Since yard lights accomplish both functions — charging and switching — I figured the circuit would be perfect for the sculpture. I was able to find one and instruct Steve how to modify it for his needs.

The circuit is very simple and I find it rather elegant. During the day, the solar panel assembly (left — for want of a proper schematic symbol, I just drew another battery) charges two AA cells through a diode that prevents the battery from damaging the panel with reverse voltage at night. Additionally, through the R₁ – R₂ voltage divider, the solar panel pulls up the base of Q₁, switching it off and allowing R₃ to pull up the base of Q₂, switching it off and switching off the load LED₁.

At night, the panel’s output approaches 0V and R₂ pulls down Q₁‘s base, causing Q₁ to conduct and pull down Q₂‘s base (in a Darlington-like arrangement — I don’t know whether it’s still considered a proper Darlington with R₃ pulling up the Q₁ emitter – Q₂ base connection), switching on Q₂ and LED₁. In fact, depending on the panel’s exact voltage, the load may switch on even before full darkness, and R₁ – R₂ can be tweaked to tune the turn-on point.

Steve removed the LED from the control board and replaced it and the fly wires for the solar panel and battery with screw-terminal connectors for ease of installation inside the sculpture. He bought a new solar panel with a higher output voltage to charge the higher-voltage battery for the white LEDs he wanted to use (the yellow LEDs in my yard lights didn’t require as high a forward voltage) and milled a Lexan cover for it to protect the panel from hail, with an O-ring groove to protect it from rain as well.

With higher battery and solar panel voltages, Steve indicated the load was turning on before the ambient light got as dark as he wanted, so I told him how to locate R₁ and replace it with fly wires to a 100K pot. After the swap, he said he was able to tune it perfectly and he was delighted.

I’ve not had a chance to visit the sculpture garden and probably won’t while Lure 22 is installed. If anyone’s in the area, I’d love to hear from you how well it’s working and how well the electronics hold up over the course of a year outdoors.

Eight sealed lead-acid batteries are bolted down and connected through a circuit breaker / switch to the two input/output jacks in parallel.

The cable to daisy-chain the battery expansion cabinets to the UPS is … substantial.

Installed in the basement server rack (bottom) and connected to the UPS. Sure wish I had a bezel for the battery cage.

Note that I don’t blame the vendor whose logo happens to be on it — I’m sure they didn’t manufacture it.

After driving two and a half hours a week ago starting with a half charge on my BlackBerry, plugging it in midway through the trip, and arriving to have the BlackBerry finally shut off its radio due to depleted charge; and due to being in the presence of Cort; I decided it was time to see why the adapter couldn’t provide enough charge for the BlackBerry.

Inside the Power Adapter

After peering and squinting at the control IC, I deciphered the part number RT34063APS and found a datasheet for the DC-DC converter. It contained no theory of operation but did provide one sample circuit for step-down conversion. The IC maintains 1.25V across R₁, so R₂ and R₁ (on this diagram — different part numbers in the device at hand) program the output voltage by dividing V_OUT. The values of 3.6KΩ and 1.1KΩ should program it for 5V operation.

R_SC is supposed to program a current limit (SC is “sense current,” perhaps?), with the IC maintaining 330mV across V_CC to V_IPK. I’m not clear on how that limits the current, but I’ll take their word for it.

Having found the IC datasheet, the next order of business was creating a schematic of my power adapter. I examined the PCB and positioned all the components in EAGLE, substituting a couple of four-pin connectors for the IC that doesn’t exist in my library. After verifying I had captured all the connections, I rearranged the components on the schematic from their positions on the PCB to this more logical placement.

While I was working on that, Cort had hooked up the adapter to his bench power supply and fed it 12-13V. He measured the output and got about 4.64V with no load around the same time I was finding that R₂ = 3.0KΩ and R₃ = 1.00KΩ, programming the circuit for 5.0V operation.

The issue is that the series diode D2 drops the output voltage by somewhere in the .6V – .8V range (from the 1N4007 datasheet, in the no-load to 200mA load range); and Cort measured 5.29V – 5.30V at D2′s anode.

Grrrrreat! We’re calling this a 5V output, but due to the protection diode, feeding the output scarcely over 4.6V, and thinking that devices expecting 5V will charge on it!

Works Fine with No Output Protection

The first thing we did was pull D2 and replace it with a jumper wire. No D2, no voltage drop. 5.30V output. Great!

But then my conscience got the better of me. D2 is obviously there to protect the DC-DC converter from a higher voltage where it’s expecting a load; and although I don’t intend to connect such a thing, who knows what may happen.

Change the Voltage Divider?

	Original	Change R3	Change R2	Change Both
R2	1000	1000	850	1200	935	1134
R3	3000	3528	3000	4234	3300	4000
V(REG)	5	5.66	5.66	5.66	5.66	5.66
V(OUT)	4.34	5	5	5	5	5
V(D1)	0.66

My next thought was to change the resistor values to regulate the voltage to 5.66V so the post-diode output would be 5V. I made a spreadsheet and played around with substitutions for R₂ and R₃ but didn’t come up with any combination I loved using standard values.

Diode in voltage regulation circuit

Finally I had a better idea, the one we stuck with. We inserted a diode (a small-signal diode ’cause it was easier to fit in place) between the regulated voltage and the top of the voltage divider. It drops .6V – .7V from the regulated voltage before it gets sampled for the feedback circuit, so the voltage is regulated to about 5.6V – 5.7V, then D2 takes its .6V – .7V, and we get about 5.0V – 5.1V at the output like we oughtta.

We were curious about the accuracy of its regulation, so Cort grabbed a resistor, hooked it up to the outputs with gator wires, and measured the output voltage. It was a bit high but the load was very light, so we tried a lower-valued resistor to increase the load. Then two, then more, and more, and more; and by the time we got to the 8Ω sand resistor we were giddy and cackling for no reason I can figure out in retrospect.

Looks like it has pretty good regulation up to about 500mA load, and we haven’t even monkeyed with R_SC yet. That should be plenty good to charge my phone.

The Trial

I drove two and a half hours home with my phone plugged in all the way. I deliberately didn’t fully charge it the night before, so I started the trip with half a charge and ended with no charge.

You gotta be kidding me.

USB Power Negotiation

I’m familiar with the formal power negotiation a device is supposed to do with a USB hub (using a maximum of 100mA unless it negotiates more), but I also know that most devices can charge from power sources I’m pretty sure don’t have full-fledged negotiation in them. And I started thinking about Adafruit’s great writeup of charging iPhones with a MintyBoost.

The bottom line is that iPhones want the USB data lines strapped to certain voltages to tell them “I’m not really USB but I’ll give you power.” Not only that, but in later iPhones, different voltages inform the device of different amounts of current it’s allowed to draw.

I was back at Cort’s house Wednesday night and mentioned that I wanted the data lines strapped to 3.3V. He asked whether I wanted him to fix my power adapter while I finished proofreading my conference presentation for Thursday, and we had a deal.

Looks like he used 10KΩ and 15KΩ on the 5Vish output to give me 3V on the data lines. We weren’t sure whether we needed a separate voltage divider on each data line like the MintyBoost uses, but when we plugged in my Blackberry the current draw (as registered by the power supply) jumped from 72mA to 290mA.

I’d forgotten my USB charging cable at home so didn’t get a chance to charge the Blackberry on the return trip, but the huge increased draw on the adapter’s supply side is a pretty good sign that I’m really finally charging.