Posts Tagged Memo to Self

Digital Tattoo Power Supply: Polarity Doesn’t Matter

When I rewired the guts of the digital tattoo power supply to eliminate the series foot switch, I kept the original wiring polarity, with the black wire to the sleeve and the red wire to the tip:

Tattoo Digital Power Supply - internal view

Tattoo Digital Power Supply – internal view

It’s the same color code I (strongly) recommend in the Squidwrench Electronics Workshops: use any color for the ground / common wire as long as it’s black, then, if you have a red wire, use it for the positive supply. You can use yellow for the higher supply voltage, but stop being clever.

I put suitably colored Powerpoles on the far end of the cable to replace the standard tattoo machine spring clip connector, so I can attach clip leads, battery test fixtures, and so forth and so on.

We wired the supply into a clip-leaded diode measurement setup with a current limiting resistor and a pair of multimeters to measure the diode current and forward voltage, whereupon we noticed all the meters displayed negative voltages and currents.

After a frenzy of wire-checking verified their setup was all good, I forced the simplest possible test, herein recreated on my bench:

Tattoo Digital Power Supply - polarity test

Tattoo Digital Power Supply – polarity test

Which produced this display:

Tattoo Digital Supply - reverse polarity

Tattoo Digital Supply – reverse polarity


After a brief exploration of “Trust, but verify” territory, we swapped the clip leads from the power supply and continued the mission.

Back on my bench, I pulled the supply apart and measured the voltage at the jack terminals:

Tattoo Digital Power Supply - jack wiring

Tattoo Digital Power Supply – jack wiring

Still negative. Huh.

The bottom of the power supply PCB shows exactly what you should expect by now:

Tattoo Digital Power Supply - reversed color code

Tattoo Digital Power Supply – reversed color code

The red wire near the top of the board is, indeed, soldered to the trace labeled GND and goes to the jack’s tip terminal; the adjacent black wire goes to the front-panel LED. Similarly, the black wire just below it, soldered to the same trace as the yellow wire, goes to the jack’s sleeve terminal; that trace also connects to a resistor leading to the trace labeled LED+ and the LED’s red wire.

Although tattoo machines run from DC supplies, their motors or vibrators don’t depend on any particular polarity and will run fine with a backwards supply.

Resoldering the red and black wires where they should go produces the expected sign at the jack:

Tattoo Digital Supply - meter leads

Tattoo Digital Supply – meter leads

Although measuring and plotting diode voltages and currents may seem tedious, actually wiring stuff together and taking data reveals how difficult the real world can be.

I trusted the supply’s internal color code and, although I’m certain I tested the Powerpoles, I obviously didn’t notice the meter’s sign.

Memo to self: Sheesh.


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Tour Easy: Front Derailleur Cable Angle

Spotted while in the midst of replacing my Tour Easy’s rear grip shifter:

Tour Easy - front derailleur cable angle

Tour Easy – front derailleur cable angle

As you might expect, the cable saws through the side of its ferrule and the brazed-on frame fitting, because it’s been basically impossible (for me, anyhow) to find a replacement derailleur duplicating whatever the good folks at Easy Racers shipped back in 2001.

On the upside, this derailleur’s cable entry has a nicely rounded ramp eliminating the need for my brass cable pulley widget.

Memo to Self: Perhaps running the cable around a bearing anchored to the frame fitting would help?

I’ve obviously forgotten to fix this for several years, so putting it here may serve as a Round Tuit.



Subaru Forester Fuse Boxes

Speaking of automotive fuses, our 2015 Subaru Forester has a pair of fuse boxes, hereby documented in case of need.

One under the hood:

2015 Subaru Forester - engine compartment fuse box

2015 Subaru Forester – engine compartment fuse box

Note the white fuse puller near the top.

The layout chart doesn’t say what “SBF” might be, but we have a lot of whatever it is:

2015 Subaru Forester - engine compartment fuse ID

2015 Subaru Forester – engine compartment fuse ID

The spare fuses line up along the lower edge of the cover.

Another under the dashboard:

2015 Subaru Forester - dashboard fuse box

2015 Subaru Forester – dashboard fuse box

And their functions:

2015 Subaru Forester - dashboard fuse ID

2015 Subaru Forester – dashboard fuse ID

The string of fuses down the right side of the main block looks like a line of spares, but they’re not. What they might be isn’t documented anywhere, which seems to be very deliberate.

Memo to Self: Having never replaced an automotive fuse, I shouldn’t start worrying now.


Squidwrench Electronics Workshop: Session 2

Some ex post facto notes from the second SquidWrench Electronics Workshop. This turned out much more intense than the first session, with plenty of hands-on measurement and extemporized explanations.

Measure voltage across and current through 4.7 kΩ 5 W resistor from 0.5 V to 30 V. Note importance of writing down what you intend to measure, voltage values, units. Plot data, find slope, calculate 1/slope.

Introduce parallel resistors: 1/R = 1/R1 + 1/R2. Derive by adding branch currents, compute overall resistance, factor & reciprocal.

Review metric prefixes and units!

Introduce power equation (P = E I) and variations (P = I² R, P = E²/R)

Measure voltage across  and current through incandescent bulb (6 V flashlight) at 0.1 through 6 V, note difference between voltage at power supply and voltage across bulb. Plot data, find slopes at 1 V and 5 V, calculate 1/slopes.

Measure voltage across ammeter with bulb at 6 V, compute meter internal resistance, measure meter resistance. Note on ammeter resistance trimming.

Measure voltage across and current through hulking power diode from 50 mV – 850 mV. Note large difference between power supply voltage and diode voltage above 750-ish mV. Note power supply current limit at 3 A. Plot, find slopes at 100 mV and 800 mV, calculate 1/slopes. Compare diode resistance with ammeter resistance.

Review prefixes and units!

The final whiteboard:

Whiteboard - Session 2

Whiteboard – Session 2

Hand-measured data & crude plots FTW!


Squidwrench Electronics Workshop: Session 1

Some ex post facto notes from the first SquidWrench Electronics Workshop, in the expectation we’ll run the series from the start in a while. I should have taken pictures of my scribbles on the whiteboard.


  • Voltage – symbol E (Electromotive Force or some French phrase), unit V = volt
  • Current – symbol I (French “intensity” or some such), unit A = ampere
  • Resistance – symbol R (“resistance”), unit Ω (capital Greek Omega) = ohm

Introduce Ohm’s Law & permutations, postpone calculations.

Measure the actual voltage of assorted cells & batteries. Identify chemistry, internal wiring:

  • 1.2 = nickel-cadmium or nickel-metal-hydride
  • 1.5 = carbon-zinc or alkaline
  • 2 V = lead-acid
  • 3.0 = primary lithium
  • 3.6 – 3.7 = rechargeable lithium, several variations
  • 4.8 = 4 x 1.2 V
  • 7.2 = 6 x 1.2 V
  • 7.4 = 2 x 3.6 V
  • 9.6 = 8 x 1.2 V
  • 10.8 = 3 x 3.6 V
  • 12 = 6 x 2 V

Measure various resistors, favoring hulking finger-friendly sandstone blocks.

Introduce metric prefixes:

  • Engineering notation uses only multiple-of-three exponents
  • μ = micro = 10-6
  • m = milli = 10-3
  • k = kilo = 103
  • M = mega = 106

Discuss resistor power dissipation vs. size vs. location, postpone power formula.

Clip-lead various resistors to various batteries, measure voltage & current.

Introduce fixed & variable power supplies, repeat resistor measurements.

Now compute permutations of Ohm’s Law using actual data!

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Fake Flash

This 2 GB flash drive arrived with datasheets & sample files for a (computerized) sewing machine Mary eventually decided she wasn’t going to get (because computerized):

Fake Flash drive

Fake Flash drive

Being of sound mind, we reformatted it and dropped it in the bag o’ random drives. She eventually used it for one of her gardening presentations, whereupon the library’s (Windows) laptop said it needed formatting; she pulled out a backup drive and continued the mission.

Lather, rinse, verify a good format, verify presentation files on the Token Windows Box, and repeat, right down to having another library’s laptop kvetch about the drive.

Soooo, I did what I should have done in the first place:

sudo f3probe -t /dev/sdc
F3 probe 6.0
Copyright (C) 2010 Digirati Internet LTDA.
This is free software; see the source for copying conditions.

WARNING: Probing normally takes from a few seconds to 15 minutes, but
         it can take longer. Please be patient.

Probe finished, recovering blocks... Done

Bad news: The device `/dev/sdc' is a counterfeit of type limbo

You can "fix" this device using the following command:
f3fix --last-sec=25154 /dev/sdc

Device geometry:
	         *Usable* size: 12.28 MB (25155 blocks)
	        Announced size: 1.86 GB (3893248 blocks)
	                Module: 2.00 GB (2^31 Bytes)
	Approximate cache size: 511.00 MB (1046528 blocks), need-reset=no
	   Physical block size: 512.00 Byte (2^9 Bytes)

Probe time: 55'18"
 Operation: total time / count = avg time
      Read: 8'35" / 3145715 = 163us
     Write: 46'37" / 18838872 = 148us
     Reset: 350.7ms / 2 = 175.3ms


As long as you don’t write more than a few megabytes, it’s all good, which was apparently enough for its original use.

The front of the PCB looks normal:

Fake Flash - controller

Fake Flash – controller

But it seems they really didn’t want you to see the flash chip:

Fake Flash - covered chip

Fake Flash – covered chip

Given the two rows of unused pads, it must be a really small chip!

Memo to Self: Always examine the dentition of any Equus ferus received as a gift.


Baofeng BL-5 Battery Pack: Recharge and Reassembly

Separately charging all four cells from the Baofeng BL-5 packs covered the Electronics Bench with wires:

Baofeng BL-5 cell charging

Baofeng BL-5 cell charging

The cell sits on a ceramic tile as a nod to fire safety, although I doubt it makes any difference.

The discharge tests showed two nearly identical pairs:

Baofeng BL-5 Cells - Separate Charge - 2018-02-24

Baofeng BL-5 Cells – Separate Charge – 2018-02-24

Surprisingly, cells A and B (upper traces) were deaders in the original packs. Cells C and D (lower traces) were more-or-less fully charged, but now have a lower terminal voltage and slightly lower capacity. I have no explanation for that, nor for the voltage undulations.

The rebuilt packs pair up A+B and C+D.

Reassembling pairs into the pack shell and resoldering all the leads produces a good pack:

Baofeng BL-5 battery rebuild

Baofeng BL-5 battery rebuild

I later added a snippet of heavy manila paper under the nickel tape bent around the edge of the pack as a third level of insulation, in the interest of having the nickel tape not produce a dead short between the isolated – terminal and the + cell case.

Memo to Self: tape the long wiggly leads from the protection PCB to the radio contacts (at the left side) before soldering the PCB to the cell terminals, because an inadvertent short will convert the 8205A battery protection IC into a Light-Emitting IC, at least for a moment, and subsequently release the Acrid Smell of Electrical Death. A handful of charge PCBs are en route halfway around the planet, from which I intend to liberate one IC for this board; with luck, I didn’t incinerate anything else.

The pack works fine in the radio, as does the APRS interface:

APRS Coverage in Poughkeepsie - 2018-03-01

APRS Coverage in Poughkeepsie – 2018-03-01

Unfortunately, two APRS iGates vanished in the last year, leaving poor coverage south of Poughkeepsie.