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.

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