Re: OT, but kinda on topic - optical encoder elex

Bob Engelhardt wrote:
I found a small DC motor at the dump with an optical encoder on it. I'd like to be able to use it, but I have no specs on the encoder. It's a Rae Corp motor, but the sticker is stamped "Aug 27 1976", so I have no hope of finding specs for it.

There are 2 micro ckt bds, but glued in place so ckt tracing is out. I can see 2 emitter/sensor pairs & there are 4 wires. My best guess is that the ckt looks like this:


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

(based upon using a VOM & seeing that Y-Blue = Y-R + R-Blue). The LED pair is 600k in 1 direction & 1.8M in the other. The detectors are about 2k in both directions. There are also 4 resistors: 2 fixed & 2 trim. Does this make sense? In particular, does it make sense that the detectors would have the same resistance in both directions?

Then, how would I hook it up?
- voltage to apply to Blk (LEDs)?
- a resistor on each of Y & Blue in series to voltage, signal taken at Y & Blue?
- if so, voltage & resistance values?

Thanks,
Bob

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.

Which direction do the LEDs read low? From the color code I would assume that red is +5V and black is ground, but black & red aren't always - and +.

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. That's high enough to damage something, and I'd be cautious, but I'll be damned if I can describe it step by step -- "looks like a forward biased LED" is what I'd look for. Once there, I'd see if there's output on the yellow and blue.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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