Sunday, February 14, 2016

[FIXED] Fellowes DM8C Paper Shredder Motor Never Stops Running

Fellowes DM8C
A long while back I bought a Fellowes DM8C paper shredder.

Over the years, it has shredded just about anything I've fed it.

It is designed with an infrared LED emitter and receiver in its mouth, whose job it is to trigger the shredder's motor to start as soon as a piece of paper breaks the invisible beam of light that passes betweent the two components.

When shredding is complete, the document is no longer blocking the beam, and after a short delay shredding stops.

For the past few years, this automatic start/stop feature hasn't worked.  I've cleaned it several times, but without any luck.  A cell phone camera can usually 'see' infrared light, but I couldn't see anything coming from either of these guys with mine.  To be sure I could see IR light, I tried my TV remote, and my cell camera could see it as plain as day.  So, I figured maybe the infrared emitter had gone bad.

Location of the infrared sensor
What next, except to take it apart?

Disassembly is pretty easy, the silver piece lifts off of the catch bin, and on the underside of the silver piece there are about 7-8 Phillips screws.  Remove them all, and you'll be staring at the shredder assembly as well as a small control board.

With the unit taken apart I could get a clear view of the IR LED with my phone camera, and indeed I found that it was dimly lit.  So, the LED wasn't bad.

With the IR emitter and receiver both on long leads, I was allowed to bring the two components close enough to be touching.  At this point the shredder DID shut down as it should.  If I tried to separate them slightly, the shredder would turn on.

So, the circuit was working as it should, but it was too sensitive.  I needed the emitter and receiver to be far enough away that they could be mounted in their factory locations.

I tried my best to understand the circuit by taking a photo of the component side of the board, a photo of the solder side of the board, and then using GIMP to superimpose a flipped image of the solder side on top of the component side.  By rotating and scaling the solder layer, I could position it over the component layer and then kind of trace out a rough schematic.  This method was somewhat successful:

Using GIMP to try to understand the IR portion of the circuit.
Gimp isn't really friendly for making straight lines and technical notation.  For that, I think Inkscape is better.  Above you can see how sloppy my freehand mouse drawing is.

Here we find a few important components:

Shredder's control board

  • [A]  The connection to the infrared LED
  • [B]  The connection to the photo diode (IR receiver)
  • [C]  LM324N, a quad op-amp (datasheet)
  • [D]  Resistor "R30", value 1.8k ohms, that goes from the cathode (negative) side of the LED to ground

Now, not being a hardware expert but at least pretending to be, I determined that the op-amp part of the circuit was used as a comparator.  If the IR light is present, don't do anything (op-amp output low).  When the IR light falls below a threshold, trigger the motor to start by setting the op-amp's output high.

So, I thought of two approaches to solving my poor sensitivity issue:

Approach 1: Alter the sensitivity of the comparator by playing with resistor values.
Here's another circuit involving an IR emitter that is probably very close to what is done here.  The designer of that circuit wisely incorporated a variable resistor so that he could adjust sensitivity.  I didn't have a decent selection of variable resistors in stock, and I wasn't entirely sure which resistor to alter.

Approach 2: Increase the brightness of the IR LED slightly.
Again, I don't plan to analyse the entire circuit here.  This is a blog about tinkering.  I found that the resistor marked "D" above would just restrict current through the LED, so why not choose a slightly smaller value for this?

I chose the second approach, but would've preferred to figure out the first approach.  At the end of the day, I already spent a few hours messing with this and I wanted to wrap it up and have the shredder working again.

I swapped out the 1.8k resistor for a 1.5k ohm, which I had in my inventory.  Subjectively, I didn't notice any change in brightness with the cellphone camera, but I did notice that I could hold the emitter and receiver farther apart now.  It seemed like only a couple of millimeters but it was a noticeable difference.  Now I could install the emitter and receiver back into their factory holders, and the shredder remained off.

Well, the problem is solved and the shredder is sitting here quietly waiting for a job.  It was a waste of an afternoon but I learned a little bit about op amps.

Saturday, February 6, 2016

Dell Inspiron 3520 Wifi Upgrade (Goodbye, Broadcom 43142)

I bought this laptop in 2012 for $399, and the biggest complaint I've had about it, by far, is the Broadcom 43142 Wifi card in Linux.  I even completely switched distros once to get the updated BlueZ Bluetooth stack, hoping for better Bluetooth support.  Nope.  I switched back to Linux Mint.

In Windows the card has seemed to work well, but in Linux -- not so much.
  • Frequently reports signal strength as 100% but has no connection at all
  • Bluetooth will stop working after the laptop sleeps in Linux (a boot to Windows will resolve this problem, presumably because the card gets updated w/ fresh firmware)
  • Terrible wifi reception when the card is actually working
I wrote once before about my frustrations with this card, and I had another article written about my experience with BlueZ and Arch Linux, but I never published it, and probably won't.

You win, Broadcom, I can't beat you.  So you're sitting on my desk, and I bought the Intel 7260 from Amazon to replace you.

Removal of the old card was a snap:
  • Remove the keyboard
  • Disconnect the antenna coax cables
  • Remove one screw that secures the card
  • Tilt the card out
Installation of the new card is the reverse of removal.

wskellenger@marquette ~ $ sudo apt-get remove bcmwl-kernel-source

Booting into Windows it was almost as if I made no changes, the device was detected and drivers installed.  It just worked.

It was the same booting into Linux, the card was identified and started working without any intervention on my part.  I can't say the same for the Broadcom card, which required some Googling to initially get it running in Linux.

Update Feb-9-2016:
I had some dropped connections with dmesg showing a bunch of stuff like this:

[  415.105430] cfg80211: Calling CRDA for country: US
[  415.110984] cfg80211: Regulatory domain changed to country: US
[  415.110992] cfg80211:   (start_freq - end_freq @ bandwidth), (max_antenna_gain, max_eirp)
[  415.110996] cfg80211:   (2402000 KHz - 2472000 KHz @ 40000 KHz), (300 mBi, 2700 mBm)
[  415.111000] cfg80211:   (5170000 KHz - 5250000 KHz @ 40000 KHz), (300 mBi, 1700 mBm)
[  415.111004] cfg80211:   (5250000 KHz - 5330000 KHz @ 40000 KHz), (300 mBi, 2000 mBm)
[  415.111007] cfg80211:   (5490000 KHz - 5600000 KHz @ 40000 KHz), (300 mBi, 2000 mBm)
[  415.111010] cfg80211:   (5650000 KHz - 5710000 KHz @ 40000 KHz), (300 mBi, 2000 mBm)
[  415.111013] cfg80211:   (5735000 KHz - 5835000 KHz @ 40000 KHz), (300 mBi, 3000 mBm)
[  415.111016] cfg80211:   (57240000 KHz - 63720000 KHz @ 2160000 KHz), (N/A, 4000 mBm)

Typically after seeing some stuff like this the connection would drop.

Solution:  This post has the steps that resolved this issue for me. 

Chrysler 3.6L Pentastar V6 Oil Filter Replacement

Last weekend, when I went to perform an oil change on my 2014 Wrangler, I could not figure out where the oil filter was!  After consulting the interwebs, I was delighted to discover that the 3.6L V6 has a replaceable element oil filter, much like the 2.4L I4 in my 2010 Corolla.

Chrysler 3.6L Pentastar V6 Oil Filter Location

The benefits to this design are readily apparent:

  • No steel canister that winds up in a landfill
  • Cheaper and easier to produce, with fewer raw materials
  • Less unused oil gets disposed with the filter
  • Less weight
  • Can offer more filter surface area for less money

Needless to say, I'm a fan of this design.  On the 3.6L Pentastar V6, the oil filter is mounted on the top of the engine, so when it gets removed, the excess oil drains back into the engine and not all over the floor.  Unlike the 2.4L Toyota I4, you don't need a special tool for this one.  I used a 15/16" socket and a 1/2" drive ratchet.

A WIX filter is available from Amazon for under ten bucks.  (I got mine from my very capable local parts store and I paid only slightly more than this.)

I already had some experience with this type of filter, but I figured it is likely others haven't, so I made a short YouTube video documenting the replacement process: