Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
My venerable amateur radio HT APRS-voice interfaces have recently begun failing and, given poor APRS coverage in Poughkeepsie due to having two iGates shut down (due to the aging radio geek population), I decided it’s time to simplify the radio interface. Given that HTs are designed to run with an external electret mic and earbud, the “interface” becomes basically some wires between the radio’s jacks, a repurposed USB plug on the bike helmet, and the PTT switch on the handlebar.
I expected to add a resistive attenuator to the earbud, but it wasn’t clear whether the mic would need an amplifier similar to the one in the APRS interface, so I decided to start as simply as possible.
The general idea is to anchor all the cables to a plate on the back of the radio, interconnect as needed, then “protect” everything with tape. The pocket clip has M2.5 screws on 26 mm (not 25.4, honest) centers, so that’s how it started:
Baofeng headset wire plate – dimensions
The four holes beside the tabs will serve as starting points for rectangular notches holding cable ties lashing the wires to the plate:
Baofeng headset wire plate – drilled
Like this:
Baofeng headset wire plate – sawed
That’s hot and nasty, straight from the bandsaw.
After some edge cleanup, add obligatory Kapton tape to insulate stray wires from the aluminum:
Baofeng headset wire plate – installed
The alert reader will note beveled corners on one plate and square corners on the other; think “continuous product improvement”.
The big rectangular gap in the middle of the plate provides (barely enough) finger clearance to push the battery release latch.
Now, to wire it up …
The dimensions of the recess surrounding the jacks on the Baofeng UV-5, just to have them around:
Baofeng headset jack socket – dimension doodle
Which came from measurements of both the Wouxun and Baofeng radios:
Baofeng Wouxun headset jack sockets – measurements
A long time ago, a pair of white LED + red laser flashlights powered by an AA cell diverged: one flashlight worked fine, the other always had a dead battery. The latter ended up on my “one of these days” pile, from which it recently emerged and accompanied me to a Squidwrench Tuesday session:
Small Sun flashlight – original wiring
The black wire trailing from the innards goes to the battery negative terminal, with the aluminum body providing the positive terminal connection to the wavy-washer spring contact visible atop the rear PCB inside the front shell.
The switch connects each red wire to the battery negative terminal, so there’s a color code issue in full effect. The two red wires burrow through holes in the rear PCB (shown above) and connect to the negative terminal of the laser module (the brass cylinder near the top) and the negative terminal ring on the front PCB holding the seven white LEDs:
Small Sun flashlight – original wiring – LED laser board
Continuing the color code issue, the black wire from the laser is its positive terminal. The out-of-focus wire (an LED pin) sticking up near the top of the picture carries the positive connection to the LED ring. The red wires from the switch are the negative connections for the LEDs and laser.
Voltages applied to the LED ring and the currents flowing therein:
Small Sun flashlight – 7x white LED current vs voltage
Seven LEDs at 20 mA each = 140 mA, so the voltage booster must crank out slightly more than 3.2 V. They’re not the brightest white LEDs I’ve ever seen, but suffice for a small flashlight.
A crude sketch of the PCB layout, with a completely incorrect schematic based on the mistaken assumption the SOT23-3 package was an NPN transistor:
Small Sun flashlight – schematic doodle
Obviously, that’s just not ever going to oscillate, even if the 2603 topmark meant a 2SC2603 transistor, which it doesn’t.
A bit more searching suggests it’s a stripped-down Semtech SC2603A boost converter, normally presented in a SOT23-6 package. If you order a few million of ’em, you can strip off three unused pins, do some internal rebonding, and (presumably) come out with an SOT23-3:
Small Sun flashlight – correct schematic doodle
That topology makes more sense!
Before going further, I had to rationalize the colors:
Small Sun flashlight – rewired LED laser board
Soldering longer leads to the PCB allows current & voltage measurements:
Small Sun flashlight – LED current test
With the LEDs and laser disconnected, the converter seems to be struggling to keep the capacitor charged:
Small Sun flashlight – V boost I 200mA-div – idle
Those purple spikes come from the current probe at 200 mA/div: maybe half an amp in 5 μs pulses at 6 kHz works out to a 15 mA average current, which is pretty close to the 11 mA I measured; it’s not obvious the Siglent SDM3045 meter was intended to handle such a tiny duty cycle.
Obviously, the output capacitor is junk and, after removing it, the AADE L/C meter says NOT A CAPACITOR. Perhaps it never was one?
Measuring the cap in the good (well, the other flashlight) suggests something around 100 nF, so I installed a random 110 nF cap from the stash. The current peaks are about the same size:
Small Sun flashlight – I 200mA-div – 110nF cap
The cap voltage (not shown) is now nearly constant and the 50 Hz PWM rate reduces the average battery current to 100-ish μA:
Small Sun flashlight – I 200mA-div – color-grade – 110nF cap
Not great, but tolerable; a 1000 mA·h battery will go flat in a few months.
The LED current runs a bit hotter than I expected:
Small Sun flashlight – I 200mA-div – LED current – 110nF cap
The bottom is about 200 mA and the average might be 350 to 400 mA.
Compared with the other flashlight:
Small Sun flashlight other – I 200mA-div – LED current
So the cap is maybe a bit too small, but it likely doesn’t matter.
And, because they’re firmly attached to the fairing mount, there’s no way to tilt them to extract the 18650 cell.
This took entirely too long to figure out:
Lithium 18650 Cell Extractor Tab
The LC40 end caps have a recess exactly where it’ll do the most good: capturing the tab inside the cap means it can’t interfere with the rear contact spring:
Being that sort of bear, I (sometimes) note the date on cells when I change them, as with this notation on the AA alkaline cells in the Logitech trackball:
Amazon Basics AA cell – mouse runtime
These Amazon Basics AA cells lasted almost exactly two years, compared with 15 and 20 months from the previous two pairs of Duracell AAs. A few months one way or the other probably don’t mean much, but the Amazon cells aren’t complete duds.
The new Amazon Basics cells have a gray paint job, so they’ve either changed suppliers or branding.
The question occasionally comes up as to why one would want a Tektronix A6302 Hall effect current probe and AM503 amplifier. The answer is simple: non-contact, essentially non-invasive current monitoring.
The scope screen in the background shows the two base voltages at the top, plus the overall battery current along the bottom:
Tek A6302 – Astable multivibrator – LED current 1 mA-div
The current at 1 mA/div shows plenty of noise, but the 200 ms LED pulse is barely 1 mA tall. The two AA alkaline cells have faded to 2.5 V, so the “wearable” white-LED-with-dyed-overcoat runs far under its nominal 3.6-ish V spec.
There’s basically no other way to get that result, because inserting a current-sense resistor into the circuit will alter the results, plus be intractably difficult to measure, particularly if you need the current in a non-ground-referenced branch of the circuit.
The AM503 has terrible thermal drift, by contemporary standards, but after the first half-hour or so it’s manageable for short durations. I’m thinking of epoxying a small knob to the screwdriver-adjustable twiddlepot to simplify the baseline adjustment.
Alas, even non-working probes and amps have become eBay collectables. You could, of course, buy new.
A package deal of two Tektronix A6302 current probes arrived from eBay, with one probe having a small crack across its case (shown in the description and bought accordingly).
The other probe worked fine and was quite clean inside:
A6302 B055461 – major sections
The cracked one couldn’t be balanced, with the twiddlepot on the AM503 amp unable to bring the signal down to 0 V from a positive offset on any of the ranges.
The current transformer might have suffered some stress on the upper-left corner of the main part (in the probe body), but it doesn’t have any obvious damage:
A6302 B032444 – ball – current transformer in place
The small ball to the left of the transfomer lid provides the slide detent; it’s an ordinary 3/32 = 0.094 inch bearing. Which, as it happens, is a Good Thing, because there’s another one exactly like it somewhere in the litter under the Electronics Workbench.
Protip: follow the disassembly procedure in the instruction manual and do it over a towel or, at least, a shallow dish. You have been warned.
Extracting the transformer from the body revealed a numeric value I didn’t recognize at the time:
A6302 B032444 – current transformer
The top slide contacts looked awful, but they’re actually covered in semi-dried contact grease and cleaned up easily:
A6302 B032444 – slide contacts
Swapping the “bad” transformer into the P6302 probe I got a while ago showed it wouldn’t balance, either, but the offset was far off into negative voltages. Putting the “good” transformer into the “bad” probe produced a similar too-positive offset. Conclusion: the transformer was probably good and Something Else was wrong.
Spending more time with the manuals produced this hint in the AM503 Amplifier circuit description:
AM503 manual – Hall offset – probe resistor selection
Fortunately, the AM503 probe connector has pin labels:
Tek AM503 Amplifier – Probe Connector – pin ID
Note the absence of pins G and I, probably to eliminate any confusion with “ground” and “one”, respectively.
Continuity checking reveals the left end of the 34.8 kΩ resistor connects to pin H:
A6302 B032444 – PCB 34.8k offset R
Huh. Even a blind pig occasionally finds a truffle: where have we seen that value before? Apparently Tek measured each transformer / Hall sensor and wrote the appropriate offset resistor value exactly where it’d do the most good.
Although I don’t pretend to know why the transformer offset has changed, if Tek can select a resistor to correct the offset, so can I:
A6302 B032444 – PCB – tweaked 82k offset R
The 82 kΩ value roughly centers the offset twiddlepot span around 0 V; it’s the result of a binary search through the resistor drawers, rather than a complex calculation.
With the resistor in place and the probe reassembled in reverse order, everything works the way it should:
Tek A6302 – 82k ohm offset – 50 mA
The lower trace is a square wave from the scope’s arb waveform generator into a (likely counterfeit) Fotek DC-DC solid-state relay, with the bench supply dialed to 5.7 V to put 5 V across a hulking 100 Ω power resistor, thus 50 mA through the probe. The purple trace comes from the repaired probe, with the other one turned off for pedagogic purposes:
Tek A6302 Calibration Setup
That wasn’t easy, but seems to solve the problem.
Dang, I loves me some good Tek current probe action …
The Smell of Molten Projects in the Morning 