The reason why I chose the QL-570 was that it was fast, had a built in cutter and a good review on print quality. The technicality of this project isn't high but there are some critical steps that need precision (like cutting a plastic gear correctly).
This project WILL void warranty and I take no responsibility for any damage done by following this instructable. I will try to help with thoughts and potential remedies though if needed.
You need to be proficient with soldering and precision drilling or cutting to pull this off.
Step 1: Tools
You don't necessarily need the DSO nano (or full oscilloscope) to replicate, though if you hit issues it may some in handy to have for debugging.
One thing that I didn't show is a drill press. I used this to help with milling the gear as you'll see in a few steps.
Step 2: Take It Apart!
The reason why we are going after a sensor is due to how the QL570 assumes paper. Each spool of paper contains 2 codes to let the system know what it has.
The first code tells the printer where it is on the paper. There are black marks on the non-print side. These marks (black squares) signify where the end of the label is or in this case how far the paper has gone. These are the marks this hack is replicating because normal paper does not contain these.
The first image shows where the sensor is. It's under the controls (power/feed/cut button).
The second picture shows the actual sensor. It's a simple IR emitter and receiver. When the black of the paper is under it does not reflect and causes the receiver to be open - disconnecting the circuit. When the page is white it bounces nicely and is closed completing the circuit.
The other coding that this printer uses is a bump pattern on the actual spool. I recommend buying the $17 worth of thermal paper (non adhesive) so yo have the bump pattern of the spool. You'll also get a idea of the design constraints from this purchase and how to make a custom spool holder.
Step 3: Piggy Back on the Sensor
The 4th picture shows up close to the back of the sensor. One thing I forgot to do at this step was to wire up a 5th wire to toggle on and off the original sensor. Cut the trace to the receiver (where you soldered the 5th wire to).
The reason why the trace is cut is to allow us to toggle between the custom sensor and the built in sensor. After all, the printer still will be used to print labels and it needs to know where it is on the printer. A switch (show later) was used to toggle between the sensors. Both should not be used at the same time otherwise it will not behave as expected.
Step 4: New Sensor Location
This part take a bit to work on so go slow here.
First, pull off the gear. To do this make sure all the plastic is out of the way. Then while pulling out lightly bend the small plastic tab so the gear pops out.
This gear is almost identical to the pattern that is being matched. We want 3 identical cuts in the gear to act as "white" and "black". I used a micrometer and found the gear is .95" in diameter. This makes the radius .475".
Now a bit of math.. There should be 3 areas to cut out, or 6 segments in total.This makes each slide 60deg. The nice thing about that number and the radius of the wheel this means that measuring the arc is not really needed which makes for easier cutting. A equilateral triangle make the measurement easy to know from where to where the cut is made.
Take a piece of paper and cut it to be .475" and place it in the wheel and use a knife to mark the edges. After it's marked use a marker to highlight which areas to cut out (remember there are 3). I don't think absolute precision is needed, there seems to be a bit of slack allowed in the system. I think as long as each slot is equal cut sizes and separated evenly it should work.
This is where it gets interesting and creativity may be needed. I used a drill press to cut out my holes which proved the safest for me. Which ever way is picked, make sure to go slowly! If the gear is destroyed, the entire system is done for unless a spare gear can be found.
Step 5: Custom Sensor
Take out the IR sensor an receiver from the board. These will be the new sensor core components.
Wire up the new components the same way as the ql570 - use multimeter to figure out which one is a diode and wire it up the same way, ideally with the same colors for simplicity. Only one device will act as a diode when no significant source of light is near by.
Make sure you get the receiver correct. It acts as a diode only connecting the circuit one direction. If you get it wrong try reversing the wires but leave the emitter the same way. If you get really stuck use your multimeter and check wiring again.
Here's a creative part. You could cut out the pre-made housing from the mouse or make your own! I used a cd cover and heat gun to mold a custom housing.
Mount the housing over to cut gear and make sure they line up so that the beam is cut fully when the gear covers the path. Otherwise it will not work correctly. There is a small screw that can be used to mount the plastic to the frame to make things easy.
Step 6: Switch
Wiring up is easy. On one of the outer poles wire a single lead from the new sensor. The middle pole wire the main wire (where it connects to the main board). The last pole wire the original sensor here (the one that was cut in the top shell).
If you follow the diagram it will give an idea on how to wire it up. Use the ohm meter to know the positions of the switch. I oriented this to have facing towards the spool as old sensor and other way as new sensor. It's just intuitive to me... that way in the future it's easy enough to figure out.
Step 7: Put It Back Together
Now get some custom thermal paper that matches closely (I used 2.25", original roll was 2 1/8") and make a caddy for it. This is where you need to follow the same hole/bump pattern as the bought caddy. These signify what type of paper is in the printer.