Q: Motor w/optical rotary encoder from an inkjet

Hi,
I'm a software guy groping around in EE-land, and I could use a little help.
A few old inkjet printers yielded up some nice motors with optical
rotary encoders built in. There are only six wires coming from the connector on the back of the motor/encoder. Two of the wires go directly to the DC motor. The other four are for the I-R emitter and the two I-R detectors. Here are some lovely photos, in case they help:
http://www.minsmithphoto.com/mrintj/encoder-top.jpg
http://www.minsmithphoto.com/mrintj/encoder-back.jpg
http://www.minsmithphoto.com/mrintj/encoder-bottom.jpg
Using a low-voltage diode tester, and squinting at the traces on the circuit board, I came up with the following circuit diagram...
http://www.minsmithphoto.com/mrintj/schematic.png
I've powered up the emitter, put an ohmmeter on one of the photodiodes (in forward polarity), and watched the resistance go from high to low as I slowly turn the motor shaft.
Even though it seems to work, I can't help but feel that something is wrong. I would have expected the two photodiodes to share a common anode or cathode, but that doesn't seem to be the case. Also, I'm puzzled as to why they would be connected in the direction that they are - so that I'd need a supply greater than +5V to read D1 and a negative supply to read D3?
Surely, I've made a mistake somewhere, but when I go back through the process of measuring and visual inspection, I end up with the same goofy schematic.
What am I doing wrong? I'm assuming that the detectors (on the bottom of the opto-interrupter assembly, closest to the circuit board) *are* two individual photo diodes. An EE at work suggested that they may be ICs with built-in amplifiers, etc...
Can anyone shed some light on this?
Thanks.
Mr. INTJ San Diego, CA
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Mr. INTJ wrote:

What manufacturer printer did they come out of? Are the motors from the paper path or the carriage?
Are there any part numbers on the sensor?
I am only familiar with the HP ones, but they did have an internal amp. Those parts had 4 terminals: +5V, Ground and two phase output signals. Some of them had logic level phase outputs and some of them had current mode analog outputs. HP's Opto manufacturing group got renamed to Agilent when the spinoff happened and then recently to Avago.
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wrote:

These are from HP inkjet printers. You were right about the logic levels - I just hooked both phase outputs up to individual traces on my scope, and there they are - textbook logic signals, with a phase difference according to direction. My mistake seems to have been assuming that these were individual photo diodes. :-(
Thanks!
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Mr. INTJ wrote:

Sure, glad to help. Photo diodes would only be used in very high speed applications I think. Normal low cost opto-interrupters use photo-transistors or photo-darlingtons with open collector outputs. The fancier ones have amplifiers and logic level outputs. The HP sensors are pretty sophisticated, there are a number of detectors at the proper spacing for the chopper wheel pitch, so that you still get a clean output if there is junk on the chopper wheel or the sensor.
Bob
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Me too.

I don't know much of anything about these, but reading the wikipedia article on photo diodes, it points out that a common mode of operation is to reverse bias the diodes. As they are connected, they would be reverse biased so it doesn't look backwards to me. You seem to be assuming they must be forward biased to work.
I can only guess they are wried that way to drive some type of differential sensor circuit.

Do they act like photo diodes when you test them without applying the power to the +5 and GND? Maybe by shining a bright light or sunlight (to get IR wavelengths) on them? If so, your schematic is probably correct. If it needs the power to operate, it might be some type of receiver circuit instead of the dual diodes you are assuming.
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On Aug 17, 9:56am, snipped-for-privacy@kcwc.com (Curt Welch) wrote:

(snip)
Good point, I missed that.
(snip)

After some testing, I found that I'm getting the expected 5V triggering signals out of both (phase differentiated), which makes more sense to me than just bringing the photodiodes out to the connector. I'm guessing that there's some magic inside the emitter- detector assembly that I couldn't see. The two connections to GND and +5V are correct, but internally, I think there's some kind of amplifier or conditioner such that I'm getting logic-level pulses out of the two remaining wires.
Thanks.
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Mr. INTJ wrote:

That's usually the case today. In fact, today you usually find a single component with an emitter, two detectors, and buffers for the detectors, so you get out a logic level. Once you figure out which leads are power and ground, looking at the others with a scope will tell you what you need to know. Power is usually 5VDC.
Some units have differential outputs, for noisy environments near larger motors, and there you'll have power, ground, +X, -X, +Y, -Y.
                John Nagle
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Well, that makes it easy doesn't it! :)
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