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.

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?

Thanks for your help,
Bob

--
Nota for President