I'm taking apart a Canon Pixma MX340 multi-function inkjet and I've found a lot of clever mechanisms that allow a single motor to activate multiple different functions. In this example, the paper feed motor does more than just feed paper.

There is a door in front of the printer below the control panel. It is usually kept closed when the printer is not in use. It help keep dust out of the printer mechanisms and also presents an aesthetically clean face when not printing. Once opened, the door becomes part of the paper output tray. I usually remember to pry the door open whenever I start a print job on this printer, but one time I forgot to do so as the printer whirred to life. Before I realized my mistake, I was startled by the sound of the printer popping the door open by itself. I had been curious how this was done, because I doubt it would be cost-effective to include a motor just to actuate this door. Now I can look at the details of this mechanism for opening the door using the paper feed motor.

Here's the mechanical linkage responsible for this magic. In these pictures, the camera is looking from the printer's side. The door is to our right and the paper feeds from our left. The picture on the left depicts the system when the door is closed state, the picture on the right is when the door is open.

Tracing it further back, I saw the pop-open mechanism is under this black plastic cover, removed after releasing two fasteners. Removing the cover also uncovered the motor driving this gearbox, confirming it is a DC motor and appears to be a commodity component.

How does the paper feed roller shaft pop open the front door? Power is transmitted via this pin in the shaft, rigidly coupling the right-most piece of plastic to the shaft.

I count at least three, possibly more, other pieces of plastic adjacent to the pinned piece. They lack the pin and thus could spin freely on the shaft. As the paper feed roll rolls forward, tabs on each plastic piece would have a bit of freedom to rotate before engaging the next piece in the sequence. The door opening mechanism is connected to a paddle (circled in red) that rests against the left-most segments.

As each piece spins and engages the next, we start seeing the slot that will engage with the paddle.

As the pieces kept turning, the slot (divided across two pieces) line up and the paddle falls in to the slot. This blocks the left-most two pieces of plastic from turning until the paddle moves.

But the motor is quite powerful, so it pushes the paddle away, which opens the door. As the door flop down to become part of the paper output tray, the paddle pulls further away from the shaft.

There weren't much else to the front door other than its automatic opening mechanism. I found it was clipped together and fairly straightforward to take apart into its component plastic pieces.

The open question I have about this mechanism is the fact it incorporated several interlocking pieces that all have to engage and connect before the door opens. In practice, this means the paper feed roller turns several rotations before the door opens. Versus an easy single-piece implementation that would have opened the door as soon as the paper feed roller starts spinning forward. Why does this complexity exist? I think the answer has to do with the gearbox behind the print head parking area, so it's a mystery I'll put on hold for the moment. There are other, more easily accessible, things to look at like the print head carriage motor and nearby components.

This teardown ran far longer than I originally thought it would. Click here for the starting point.