Bob Engelhardt wrote:
Tim Wescott wrote:
Bob Engelhardt wrote:
...


Blk Y Blue
| | |
+-+--+ | |
| | | |
+++ +++ +++ +++ Photo
LEDs | | | | | | | | Detectors
| | | | | | | |
+++ +++ +++ +++
| | | |
|____|_________|_____|
|
|
|
R
...


It is quite odd that the detectors should be 2K in both directions,
unless the circuit is designed to have a supply voltage connected
between red & black, and let you pick signal off of yellow and blue
without needing any pullup resistors.


That occurred to me right after I sent it. I.e., that there are
built-in pull ups & yellow & blue are read directly. That doesn't
explain to me why the resistance would be the same - unless the pull up
resistor is the back resistance of the detector.

...
I'd hook up black & red in the direction that seems favored for
current to flow through the LEDs, through a current limiting resistor,
and see if I could get it up to no more than 50mA at 5V. ...
... I'd see if there's output on the yellow and blue.


I did & there is. There must be a limiting resistor in there, 'cause I
forgot it & it followed the applied voltage, kinda. At about 5v (red
+), the emitters draw 30 ma & I get about .7v on yellow & blue. Not a
square wave, as I expected it would be. More sinusoidal than anything.

A chopper wheel will give you a trapazoidal to outright triangular
output waveform, because the transitions on the wheel wipe across the
sensor in a noticeable amount of time. I worked on a motion control
system where we used the analog edges to provide a fractional part of an
encoder count. It was kind of hairy to implement in assembly language,
but it worked well. We got about 6 extra bits of resolution for the effort.

The 4 resistors that are there must be 2 current limiting on the LEDs
and 2 pull ups on the detectors. Wait - this was made in 1976: did they
have LEDs then?


As a wild guess, could those sensors be Cadmium Sulfide sensors? I
always thought that they would be too slow for this task, but I believe
that they are effectively photo-resistive and not polarity sensitive.
All of the CdS cells I have seen have a good sized window on them and
and a noticeable squiggly line dividing the sensor in half.

Curious,
BobH