While electrons are undeniably cool, we humans sadly cannot see electrons and all the cool things electronic components do : (

Enter: the Oscilloscope (in my case, from stage left: the 80’s)

In a previous post I harvested some parts from inkjet printers, including the sensors that record the positions of the motors/gantries. Let’s see what sort of signals those sensors (called optical encoders) give by probing one with my Tektronix 2212 Oscilloscope.

In the above video I power the motor and pulley on this linear gantry from one of the printers. An optical encoder on the moving part that would hold the inkjet “reads” its position using the thin strip of plastic suspended above the track. That piece of plastic has alternating opaque and translucent stripes.

The encoder has a an LED that sends light to 2 photo-diodes. Those photo-diodes give a “high” signal when they receive light (in the translucent parts of the strip), but give a “low” signal when there is no light (in the opaque parts of the strip) – they’re kinda like little solar panels. Although in this sensor there are 2 photo-diodes to record not just velocity but also direction, I will only be probing 1 of them as I only have 1 probe for my scope.

To probe the sensor, I found a schematic so I could power up the LED and know how to access the signals.

Then I soldered some wires out from the ribbon cable:

Now we can run the plastic strip along and see the highs and lows as the light is interrupted. My scope is kind of limited, so it just looks like a sort of wave, but you can see how when I move the plastic strip faster, the period of the wave increases. And the period slows down when I decrease the velocity of the plastic strip.

It’s pretty cool to be able to see the flow of electricity!

Some control circuitry could come in and count the highs and lows and know the position and speed of the gantry, then use that to control the motor and where the gantry is.

Here’s a video explaining it more:

And of course Ben Krasnow has a post about doing a similar thing:

https://benkrasnow.blogspot.com/2010/02/linear-position-tracking-with.html