• Teardown time again: Woods 32555WD remote control outlet. Finding out how the channel is set (it's a little brutal)

    a day ago 0 comments

    As I've gotten tired (and/or lazy) about having to plug/unplug the outdoor Christmas decoration lights, I finally got an outlet that can be switched on/off via a remote.  The one I got was labeled as a "Woods 32555WD, but there's no Woods logo anywhere on the product, which makes me think it's a product sold under a variety of brands.  On the label of the remote control (and the outlet itself) a sticker stated "CH 1." There's no accessible switches for setting it for a particular channel, so I decided to open it up:

    From the PCB photo above, it became very obvious how it's done.  There's four traces: "A B C D." To set the channel,  on or more of the "A B C D" traces are disconnected, not by removing  solder bridge(s) in a civilized manner (or even better having DIP switches), but by grinding off a part of the trace(s .)  It's not exactly a nice way to do it, I suppose they decided on this brutal method to allow them to to make use of lower skilled and thus lower cost workers. Strangely enough, "CH 1" correlates to the D trace remaining connected. 

    Onto the outlet tear down photos:

    Inside, is a relay and the control circuitry is powered by a capacitive dropper circuit, not surprisingly.  Interesting enough, there's a second IC on a daughterboard dedicated to receiving RF. The remote only has one IC for everything.  There's a matching arrangement for setting the channel.  I did sit down and reverse-engineered it. If anyone wants to see the schematic I did, please do comment and I'll make a more legible version and post it. 

    It claims to be weather resistant, but there isn't much in the way of gaskets, so it would be best to keep it away from any rain.

    Good to know how to set the channel because there was no "user serviceable" option to select what channel I want.  If I get another, it's easy enough to set it to either a matching or different channel, depending on my needs.

  • Vintage sewing machine foot pedal teardown

    5 days ago 0 comments

    It's been a while since I've done a post. I'm still active, but I've been busy with a lot of things as of late.

    Anyways, something that I've wanted to see the inner workings of, was an old sewing machine foot pedal that's been lying around. It's a bit older than me. On to the exterior photos:

    As you can see, it's a bit on the crusty side, with some electrical tape as a bonus, safety third!

    Time to get into the pedal.  The back slides off, but a screwdriver is needed as leverage to get it off.  Oxidation made it challenging.

    Inside it's obvious (and along the lines of what I expected to find) what it is, a variable resistor with  8 fixed positions (including a shorted position and the "off" position.) The resistor is composted of segments of coiled wire and contact points for the wiper that's wrapped around a piece of ceramic.  End to end, the resistor is around 100 or so ohms. 

    The resistor isn't very accurate or precise (not that it needs to be) and resistance can greatly change if you look at it the wrong way (connections between the coiled wire and contact points are quite finicky.)

    This thing is very much "safety third!" There's two large potential hazards. There's no grommet to prevent the wires going in from being nicked (although it may have had one that perhaps crumbled away.) The second big hazard is that the metal housing isn't grounded. If one of the coiled wire segments were to break or pop off, it could make contact with the housing, potentially making the housing itself live.  It wouldn't pass safety standards today.

    I also have the sewing machine this foot pedal goes with, perhaps I'll do a teardown of it in the future.

  • The tale of (effectivly) killing a $30-40ish USD chip

    06/15/2023 at 12:14 0 comments

    The other week I got a few samples of some MAX187 ADC chips, as I was in part curious what was so special about them that warranted the high price tag. Looking into ADC chips all started when I wanted to find a solution to the ESP8266's fairly poor internal ADC. Somewhat off-topic notes:  If anyone wants the board files (and code)for the MAX187 PCB I can post them, although it's a bit cost prohibitive if your not getting MAX187 samples.  I've done a more practical design around a MCP3304 ADC chip that I will be posting in the future.

    During the process of testing the code that I wrote to use it with an ESP8266 on a Wemos D1 board, I mistakenly connected the VIN pin instead of the GND pin to the ADC input pin.  This by itself wouldn't have been a huge deal by itself, except I was powering the board via a wall wart, which sent around 12V to the poor ADC input pin.  I discovered the issue as I started to smell something and then found the ADC chip to be streaming hot.  After correcting the issue, the ADC chip appeared to become unscathed.

    Or so I thought.  Last night, I was writing code to use the MAX187 with a ATmega328P (I did this to do a comparison with the ESP8266 in terms of noise the ADC picks up and found that the ADC picks up a bit more noise from the ESP8266.)  Upon testing the code, I noticed when using a 50K pot, the voltage was staying at full scale during most of the rotation and only dropping off close to 0v when hitting one end. I thought there was possibly an issue with my code and busted out a multimeter to confirm the strange readings.  Strangely enough, the readings were correct.  I then thought perhaps there was something wrong with the pot.  I tested it with the wiper pin disconnected from the MAX187 and it behaved normally.

    I then checked on the PCB for the MAX187 to make sure there wasn't any short between the ADC input pin and VCC. There was no short, but I did discover that the socket that I used was very slightly conductive, in the 6-18 or so megaohm range.  The plastic itself was ever so slightly conductive.  Good thing to note, as that can be problematic with dealing with high impedance sources.

    I didn't think that the very slightly conductive socket was the problem so I turned to the MAX187 chip itself.  I measured the ADC input pin voltage left floating, at it was around 5V. There was my problem.  A low impedance or dead short must have formed between the ADC input pin and VCC pin. That would explain why the input kept reading full scale until the pot was in the position that the resistance was low enough between the wiper and ground pin that it was able to shunt the supply voltage that was going into the ADC input pin. 

    Further investigation with a multimeter showed  sure enough,  there was a low impedance connection between the ADC input pin and VCC pin with a reading of 117 ohms.  Making an educated guess there's and ESD protection diode between the ADC input pin and VCC pin. That diode protected the input when it got 12v but sometime or after that diode failed in the way of forming a low impedance connection.  The diode that protected the MAX187 chip ended up rendering the chip useless in the end.

    That was quite a journey. I believe in the years that I've gotten into electronics, this is my first chip that I've effectively killed.