Luce di cortesia per il baule dell'auto

Arriva l'auto nuova. E' dotata di sensori di parcheggio, retrocamera, frenata assistita, sensori di collisione, sensore di pioggia, sensore luci, cruise control, navigatore, sedili riscaldati, specchietti motorizzati, il profumo di auto nuova. Ha anche le ruote, i sedili ed un volante, certo. Fai un bel giro. Poi ti capita di fare la spesa e scegli il supermercato con ampio parcheggio coperto dove puoi lasciarla lontana dagli sportelli altrui.

Arrivi con le borse, apri il bagagliaio e.... buio. Nero! Sbirci dentro per cercare l'interruttore della luce di cortesia ma non c'è. Non c'è nemmeno la luce! Hai speso tante migliaia di euro per accaparrarti il frutto del lavoro di tanti progettisti e LORO si dimenticano di mettere un piccolo, economico ma utilissimo sistema di illuminazione nel bagagliaio! Tra l'altro, parere personale, apprezzeresti molto di più la luce nel bagagliaio degli specchietti retrovisori regolabili elettricamente. E costa pure meno. O sbaglio?

Ma oggi la soluzione c'è. Si tratta di installare una "luce LED notturna con sensore di movimento" che viene normalmente proposta per l'illuminazione negli armadi. E' alimentata a batteria ricaricabile entrocontenuta ed è leggerissima. Io l'ho fissata sotto la cappelliera, dapprima con i magneti in dotazione, e poi affiancati da velcro incollato con supercolla ("At...ck" o "Bos..k"): un lato sulla luce, l'altro sulla superficie interna della cappelliera.

E signori, questa soluzione non fa solo da luce di cortesia per il baule. Dato che si può spostare, può seguirci nel cofano motore se dobbiamo controllare i liquidi o accompagnare la lettura di un libro/giornale senza scaricare la batteria dell'auto (che se poi non parte son dolori).

La batteria dura almeno un paio di mesi e si può ricaricare mentre si è in viaggio usando un cavetto USB adatto (la mia barra LED è arrivata con il suo cavetto).

Spesa totale? 5 euro più dieci minuti del proprio tempo.

Nota sull'utilizzo di questa versione. La barra luminosa si controlla con un singolo pulsante: alla prima pressione resta sempre accesa, con un'altra pressione (fa un lampeggio) si accende quando rileva movimento ed è buio, con un'altra pressione (fa tre lampeggi) si accende quanto rileva movimento indipendentemente dalla luminosità esterna. Con la quarta pressione si spegne. Tenendo premuto il pulsantino si attiva il dimmer per regolare la luminosità. Quindi per l'utilizzo nel bagagliaio, partendo da spenta, premete due o tre volte il pulsantino.

Per i curiosi, l'auto in questione è una Suzuki Ignis del 2017. La stessa auto prodotta nel 2020 ha la luce di cortesia nel bagagliaio, ma questa soluzione fa più luce!

Battery of Braun 5514 shaver

I have a cordless hair&beard "styling kit" from Braun, model 5514, that doesn't hold the charge anymore. I use my soldering iron more often than this shaver so I have no idea how long the stand-by period is. Still, loosing the charge in one week is not acceptable.

There is a good video on YT showing how to open it. You need a thin and a thicker plastic card/tool to pop open the top cover and a small straight screwdriver. 

There are two batteries wired in series, in my case they are marked as "SUPPO HS-AAA0.75" NIMH 1.2V. The guy in the video shows the same model.

Now, if you search for that string you can buy a replacement at 1/3rd of the price for a new tool. But those batteries are just AAA NiMH rechargeable cells with solder tabs, with a capacity of at least 750 mAh (so if you buy them of 1000 mAh that's fine, it will take longer to reach full charge).

Then it's a matter of cutting off old batteries and soldering in new ones.

Someone might mumble that it should not be so hard to replace a battery. This tool is designed to be used in wet environments so it is waterproof. This means that it's already gold it is not sealed. Also the motor in the shaver creates vibrations and it's better to have batteries (and everything else) held together as strong as possible.

High-Voltage High-Z DIY probe

As a self-assignment for troubleshooting the Hameg oscilloscope, I needed to measure kiloVolts. The world agrees that you need to arrange a resistive voltage divider using an high impedance (10 Mohm) voltmeter. 

For example if your voltmeter has an input impedance of 10 Mohm you need a 90 Mohm resistor in series to obtain a 1:10 ratio:

 (kV)------/\/\/\/\/------[Voltmeter]------(GND)

            90 Mohm         10 Mohm

I looked for suitable resistors at home and I found 4x 10 Mohm and 5x 5.27 Mohm, all 10% or more. So theoretically I could reach 66 Mohm which gives about 1:7,6 ratio. The chosen voltmeter can measure 500V, that's up to 3800V "input".

Since it is hard to accurately measure high resistances and their value may change with voltage I decided to determine the ratio with an experiment.

Given that the input impedance of an instrument might be unknown or different from the spec sheet, I empirically found the ratio of different voltmeters I own. First measure a DC voltage V1 using the voltmeter alone. Then measure again the same source adding the big resistor in series, you get V2. V1/V2 is the ratio of that resistor using that voltmeter at that voltage range.

A high-Z probe for HV.

I repeated the experiment at higher voltages, like 80V and 300V, to observe that the ratio increases 1-2% with applied voltage (300V vs 10V). I settled to 1:8.02 for my setup.

I completed the build with a label on the probe that reminds its usage and characteristics.

IMPORTANT. Whenever you work on live circuits that use high voltages prepare the measurement setup when everything is off and unplugged from the grid. Then put one hand in the pocket, reconnect power and press "ON". Before doing any adjustment switch off the device under test and unplug the AC cord. If you are unsure, let someone expert do it.

Why didn't I aim for the 1:10 ratio? First and foremost because I needed "now" to measure that voltage. Second because I wanted to use those high value resistors. Third because it's hard (and expensive) to get a perfect 1:10 ratio, so I would have used a calculator anyway: any ratio is good, then! Fourth because I had an excuse to use one of the vintage calculators from my collection.

Troubleshooting an Hameg HM203-6 oscilloscope - no trace

Right after fixing the high voltage being too high (-2500V vs -1900V) on the Hameg HM203-6 oscilloscope, the trace disappeared. Note that I was able to see the trace at the restored -1900V cathode voltage and I was after the ineffective un/blanking control.

HV was there. Un/blanking signal was there. What could have possibly gone bad, now?

When operating in XY mode (or component tester mode if your scope has it), there is no retrace or blanking: you should always get at least a dot somewhere on the screen (be careful with phosphor burning!). I was not getting a beam, even in XY mode. So something was blocking the electron beam into the CRT: either missing acceleration or blocking grid at wrong voltage. That is where I concentrated my efforts.

I knew that BF199 was bad. Even with a new one nothing happened. BF440 was good as well as the optocoupler. At least they tested as expected on the DVM at low voltage.

Don't forget that components may (will) show a different behavior or value depending on the voltage applied. That's why I changed three 68 pF 2 kV capacitors that have been pinpointed as being prone to failure in online forums.

When all voltages were measured as expected I moved on to in-circuit component testing. The device MUST be powered off and unplugged! I take no responsibility for whatever damage to animated or inanimate beings you may cause.

Remove socketed components as they can be tested individually. Then proceed with this methodology:

• resistors should exhibit the stamped value or lower, because they are in parallel with other resistors and alike: replace if value is too high or too low;
  
• diodes should conduct in the A>K sense and exhibit an open-circuit or high impedance on reverse polarization: replace if forward and reverse voltages are too low for the diode type, or if it's open

I did not need to test capacitors as I found two open resistors and one shorted diode. I mark them in red in the schematic diagram excerpt below. See the thread on eevblog for the whole story.

A part of Hameg 203-6 oscilloscope circuit diagram.

So the Hameg HM203-6 symptoms were:

1. trace too bright and not going dim/off with intensity control plus
2. not un/blanking 

They were caused by:

1. broken high voltage regulation, which probably lead to
2. un/blanking failure and
3. blocking grid control failure 

Components replaced:

• one 741 op-amp IC
  
• 3 high voltage 68 pF capacitors (perhaps not needed)
• one BF199 transistor
• two resistors
• one diode 
  

Cost about 5€(2023)  and 10 man/hours. Troubleshooting value: invaluable.