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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.

Author: Ed

eBay “49E” Hall Effect Sensors: Parameter Spread
A bag of 50 cheap Hall effect sensors arrived from the usual eBay vendor, who was different from all previous eBay vendors (if in name only). Passing 124 mA through the armored FT50 toroid with 25 turns of 26 AWG wire, we find this distribution of bias points, measured as the offset from the actual V_CC/2:

eBay 49E Hall Effect Sensor Bias Histogram

The bias point is actually referenced to the negative terminal (usually ground) with a ±0.25 V variation around the nominal. SS49 sensors run about 0.5 V below V_CC/2 (2.25 V with a 5 V supply), SS49E sensors at 2.5 V with a tighter V_CC limit that suggests you better stay pretty close to 5.0 V.

Allowing for the fact that I really don’t have good control over the actual magnetic field, the gain distribution seems tight:

eBay 49E Hall Effect Sensor Sensitivity Histogram

You’ll recall the Genuine Honeywell sensor specs:
- SS49 – nominal 0.9 mV/G, limits 0.6 to 1.25 mV/G
- SS49E – nominal 1.4 mV/G, limits 1.0 to 1.75 mV/G
The gain is roughly half that of the previous “49E” sensors, confirmed by sticking one of them this field. I don’t know which is more accurate, but these have a much prettier distribution.

So this lot resembles 49E sensors in both bias and gain.

Given the bias variation, though, it’s obvious that a DC application must measure the zero-field output and apply an analog offset to the amplifier, because a twiddlepot setting won’t suffice. Most likely, you’d want to update the offset every now and again to compensate for temperature variation, too.

Tossing the outliers gives an average gain of 1.17, which would give results within 10% over the lot. Given that you don’t care about the actual magnetic field, you could calibrate the output voltage for a known input current and get really nice results.

If you were doing position sensing from a known magnet, you’d want better control of the magnetic field gradient.
2013-08-14

Series-Parallel LED Current

The collection of LEDs that I’ve been abusing with 100 mA pulses at 20% duty cycle got lined up in parallel, with three LEDs in series, driven from a bench power supply set to limit the current to about 180 mA:

I sweetened the mix by adding a few other LEDs that had served their time in hell, then took some data by clipping the Tek Hall effect current probe around each of the white wires in turn:

Color	LED1	LED2	LED3	Divisions	Current – mA	Total voltage
Red	1.98	1.98	1.94	4.7	23.5	5.90
Red	1.95	1.95	2.00	3.4	17.0	5.90
Red	1.97	1.97	1.97	5.1	25.5	5.91
Yellow	1.97	1.97	1.96	4.5	22.5	5.90
Yellow	1.97	1.96	1.97	4.6	23.0	5.90
Red	1.98	1.98	1.94	4.6	23.0	5.90
Red	1.95	1.95	2.00	3.3	16.5	5.90
Red (new!)	1.99	1.96	1.94	6.4	32.0	5.89

Despite the decimals, don’t trust anything beyond the first two digits.

The LEDs started out with 6.03 V across them at that current, then settled down to 5.92 V after a few minutes of warming up.

The “new” Red string replaces a trio of old LEDs incinerated by a DVM probe fumble. They have a much higher current at the same voltage; the older LEDs have been abused enough to pass a lower current.

As an experiment, I swapped the LED with the 2.00 V drop in the string with the lowest current (line 7) and the LED with the 1.94 V drop in a string with higher current (line 6), only to find that the current followed the LEDs. Evidently, those LEDs were the limiting factor, even though their forward drops weren’t the same in their new strings.

So it seems binning based on forward drop doesn’t help much. Perhaps just line up a bunch of three-LED strings (forcing all of them to see the same forward drop), measure their current, and reject the highest and lowest strings to get a decent match among the remainder?

2013-08-13

Measuring Objects for 3D Modeling: USB Video Camera
A brace of “Fashion” USB video cameras arrived from halfway around the planet. According to the eBay description and the legend around the lens, they’re “5.0 Megapixel”:

Fashion USB camera – case front

The reality, of course, is that for five bucks delivered you get 640×480 VGA resolution at the hardware level and their Windows driver interpolates the other 4.7 megapixels. VGA resolution will be good enough for my simple needs, particularly because the lens has a mechanical focus adjustment; the double-headed arrow symbolizes the focus action.

But the case seemed entirely too bulky and awkward. A few minutes with a #0 Philips screwdriver extracted the actual camera hardware, which turns out to be a double-sided PCB with a lens assembly on the front:

Fashion USB video – case vs camera

The PCB has asymmetric tabs that ensure correct orientation in the case:

Fashion USB camera – wired PCB rear

In order to build an OpenSCAD model for a more compact case, we need the dimensions of that PCB and those tabs…

Start with a picture of the back of the PCB against white paper, taken from a few feet to flatten the perspective:

img_3300 – Camera PCB on white paper

Load it into The GIMP, zoom in, and pull a horizontal guide line down to about the middle of the image:

Camera PCB – horizontal guide – scaled

Rotate to align the two screws horizontally (they need not be centered on the guide, just lined up horizontally):

Camera PCB – rotated to match horizontal guide – scaled

Use the Magic Scissors to select the PCB border (it’s the nearly invisible ragged dotted outline):

Camera PCB – scissors selection – scaled

Flip to Quick Mask mode and clean up the selection as needed:

Camera PCB – quick mask cleanup – scaled

Flip back to normal view, invert the selection (to select the background, not the PCB), and delete the background to isolate the PCB:

Camera PCB – isolated – scaled

Tight-crop the PCB and flatten the image to get a white background:

Camera PCB – isolated – scaled

Fetch some digital graph paper from your favorite online source. The Multi-color (Light Blue / Light Blue / Light Grey) Multi-weight (1.0×0.6×0.3 pt) grid (1 / 2 / 10) works best for me, but do what you like. Get a full Letter / A4 size sheet, because it’ll come in handy for other projects.

Open it up (converting at 300 dpi), turn it into a layer atop the PCB image, use the color-select tool to select the white background between the grid lines, then delete the selection to leave just the grid with transparency:

Camera PCB with grid overlay – unscaled

We want one minor grid square to be 1×1 mm on the PCB image, sooo…
- Accurately measure a large feature on the real physical object: 27.2 mm across the tabs
- Drag the grid to align a major line with one edge of the PCB
- Count the number of minor square across to the other side of the image: 29.5
- Scale the grid overlay layer by image/physical size: 1.085 = 29.5/27.2
- Drag the grid so it’s neatly centered on the object (or has a major grid intersection somewhere useful)
That produces a calibrated overlay:

Camera PCB with grid overlay

Then it’s just a matter of reading off the coordinates, with each minor grid square representing 1.0 mm in the real world, and writing some OpenSCAD code…
2013-08-12
Jerusalem Cricket vs. Dust Bunny

Sometimes crickets make their way into the basement. This one, a model that I’ve always known as a Jerusalem Cricket(*) evidently lost a pitched battle with one of the Dust Bunnies guarding the Basement Laboratory:

Jerusalem Cricket vs Dust Bunny – top view

A rear view:

Jerusalem Cricket vs Dust Bunny – rear view

From the front:

Jerusalem Cricket vs Dust Bunny – front view

I deported it to the flower garden outside the basement door, where I hope it can brush off the Bunny’s entrapments…

It may not be a Jerusalem Cricket, because they’re more common out west, but that’s the best match in our bug books and that’s what we’ve always called them.

[Update: (*) It’s most likely a Cave Cricket. See the comments for details.]

2013-08-11
Too Many Deer: Aftermath

We spotted this crumpled front end at a local repair shop:

Deer crash damage – overview

A closer look at the bumper tells the tale:

Deer crash damage – hair detail

Pop Quiz: estimate the total cost of that collision, including the overhead of having to deal with the insurance company and arrange alternate transportation for a week or two.

Essay: explain why it’s possible for someone to insist that both deer and humans are better off under these conditions.

In this area, vehicles serve as the top predator for deer…

2013-08-10
SMD-to-DIP Adapter: Pin Soldering “Fixture”

I needed a rail-to-rail op amp in a DIP package to breadboard the amplifier for those toroids and came up dry, so I sawed out a protoboard square, glued a MAX4330 in SOT23-5 atop it in classic dead-bug style, rearranged the leads into the standard DIP pinout, and moved on:

MAX4330 dead-bug style on DIP adapter

The trick to getting the header pins aligned is to stick ’em in a pile of perfboards, which instantly makes them stand up straight and parallel:

img_3430 – SMD-to-DIP adapter – PCB soldering fixture

Stick the square adapter atop the headers, solder all eight pins, glue down the SOT23, and solder it with wire-wrap snippets. Ugly, but workable…

2013-08-09

Air-gapped Ferrite Toroid Data

For an upcoming Circuit Cellar column on Hall effect current sensing, I slit another pair of toroids:

Then wound them with grossly excessive amounts of wire (the up-armored core on the right appeared earlier):

The smaller toroid is an FT37-43 that barely covers the active area of an SS49-style Hall effect sensor, but experience with the FT50 toroid suggests that’ll be entirely enough:

slit FT37 toroid trial fit to SS48-style Hall effect sensor

Data on the uncut toroids:

Property	FT50-61	FT37-43
Outer diameter (OD) – inch	0.50	0.375
Inner diameter (ID) – inch	0.281	0.187
Length – inch	0.188	0.125
Cross section area – cm²	0.133	0.133
Mean path length (MPL) – cm	3.02	2.15
Volume – cm³	0.401	0.163
Relative Permeability (μ_r )	125	850
Saturation flux G @ 10 Oe	2350	2750
Inductance factor (A_L) – nH/turn²	68.0	420

Those overstuffed windings improved the sensitivity, but increased the winding resistance far beyond what’s reasonable.

Data on the slit toroids:

Toroid ID	FT50-61	FT37-43	FT50-61
Measured air gap – cm	0.15	0.15	0.17

Winding data
Turns	120	80	25
Wire gauge – AWG	28	32	26
Winding resistance – mΩ	530	920	100
Predicted B field – G/A	872	660	163

Hall effect sensor @ 1.9 mV/G
Predicted output – mV/mA	1.7	1.3	0.31
Actual output – mV/mA	1.9	1.9	0.37
Actual/predicted ratio – %	+12	+46	+19

The last few lines in that table show the transimpedance (transresistance, really, but …) based on the winding current to Hall sensor output voltage ratio (in either mV/mA or V/A, both dimensionally equivalent to ohms), which is why the toroid’s internal magnetic flux doesn’t matter as long as it’s well below saturation.

Gnawing the 80 turn winding off the FT37-43 toroid and rewinding it with 15 turns of 24 AWG wire dropped the winding resistance to 23 mΩ and the transimpedance to 0.36 mV/mA:

FT37-43 with 15 turns 24 AWG - Hall sensor — FT37-43 with 15 turns 24 AWG – Hall sensor

However, applying a voltage gain of about 28 (after removing the sensor’s V_CC/2 bias) will produce a 0-to-5 V output from 500 mA input, which seems reasonable.

2013-08-08

Author: Ed

eBay “49E” Hall Effect Sensors: Parameter Spread

Series-Parallel LED Current

Measuring Objects for 3D Modeling: USB Video Camera

Jerusalem Cricket vs. Dust Bunny

Too Many Deer: Aftermath

SMD-to-DIP Adapter: Pin Soldering “Fixture”

Air-gapped Ferrite Toroid Data