scavenging opto-isolators

After several attempts, I tried searching for

The following occurred to me, but might be nonsense: Maybe the fact that TR is transparent and BL is black is partly irrelevant. In other words, maybe both are essentially the same device, with a bulge that emits and a recessed disk that receives. Maybe TR is a lousy receiver and a good transmitter, while for BL it is the other way around, for reasons having to do with one being transparent and the other not. Taking a wild guess, the output pin is the signal received by the recessed disk. On a device that receives badly, the output should be very low and it might be safe to tie it to ground. That might be consistent with the way some pins are tied together on the two opto-coupler pairs. And it would also imply that the pins have the same meaning on both TR and BL.

Anyway, that is just speculation. I'll keep looking for data sheets. I'm also trying to find books on these optical devices.

It occurs to me that there is one question I should have asked before getting so fascinated with my old Hewlett-Packard mouse. People have suggested that I obtain my opto-isolators from mice, floppy drives, old printers or fax machines and other devices. They've also suggested ways to extract the opto-isolators from these old devices.

How do you figure out what is the right way to use the particular opto-isolators you wind up with? What voltages get applied to what leads, for example? What do you do if they don't have identifying marks that would let you find data sheets?

In other words, how exactly are you any better off after you have removed them from the device they came from than before, if you don't know anything specific about them?

In a word: experience. When you're interested in how stuff works, you read and read and read. And then you play, play, play with various devices. Almost without realizing it, you retain information such as package styles and (ballpark) working voltage/current levels. This doesn't *always* give you enough information to use the part without having its specs. too but it can be a great help, as any of the experienced folks here could tell you.

It is possible to deduce the pin-out of a relatively simple device e.g. opto-isolator by judicious use of a resistance resistance meter. When you think you've got it, you prove that you're correct by connecting a current-limiter (resistor) in series with the light-emitting device, applying variable voltage, applying an input voltage, and measuring voltage changes at the output(s). What size resistors? Experience tells you that a 1M current limiter on an LED is probably way too large and that 10 ohms is probably not big enough.

Wiring up an opto-isolator "ain't rocket science"; you have some leeway regarding voltages and currents. While I probably would not incorporate an "unknown" part in a design for production, I do not hesitate to try one in a one-off, personal project.

The most salient advice I can give you (Allan) at this point is to get a resistance meter if you're gonna spook around in "black boxes".

OK, that makes a lot of sense. If you don't mind my asking, what are some of the things that you (Michael) read and read and read and have read and have read and have read?

OK. That sounds like it's within my capabilities.

That's good to know.

OK. I'll get a multi-meter. In an earlier posting, I mentioned that I had scavenged the innards of a discarded PC, including its power supply. So, I can use the power supply as the voltage source.

One reason I've been so cautious with this particular mouse and its components is that it is the only discarded mouse I have. So, another thing I should do is to try to collect more discarded devices. The other reason I've been so cautious is that it is at least conceivable that there might be some advantages to not removing the opto-isolators from the PCB, but in possibly removing some of the other components; trying to understand the PCB is part of studying that possibility.

Here is one concrete idea for how to use the mouse for a different purpose: remove the ball; drill holes in the bottom of the mouse and attach some string to the shafts of each of the rotating disks that interrupt the light beams; connect the strings to something whose xy motion you would like to keep track of; write a program to read the mouse port to keep track of the xy-motion. Or it could keep track of two rotation angles of some other device, the angles being turned into rotation of the shafts somehow.

-- Ignorantly, Allan Adler

• Disclaimer: I am a guest and *not* a member of the MIT CSAIL. My actions and
• comments do not reflect in any way on MIT. Also, I am nowhere near Boston.

Data/spec. sheets. Digest several Microchip uC ones, for example, and you'll have a good idea how many other Microchip uC's are designed. Read a number of them for various opto-isolators and you'll have a pretty good idea what you can and cannot do (voltages; currents; speeds) with your currently unidentified one. To use a military metaphor, know your specific enemy whenever possible but also know general tactics and strategies of war.

Here's one real example of the utility of variable voltage. I was given an HP-35 calculator (world's first "pocket sliderule"; circa early 1970's) that had no battery, and I wanted to see if it was functional or garbage. Not remembering the voltage of the proper battery pack (I have owned several HP calculators of that vintage), I hooked up a VARIABLE supply to the calculator and set it to about 1v, turned ON the calculator, slowly raised the voltage until the display lit up. Raising the voltage from that point while checking function told me a range of acceptable voltages. Studying that range for a bit jogged my memory, clued me to the proper voltage (I had EXPERIENCE with HP calcs., remember?).

Absolutely. I have a collection of them, all retrieved from PC "piles" that were left on the curb as trash. Lotta folks will get new PC's soon (Christmas!) and toss out their old ones.

Sometimes it is prudent (or just darned easier) not to desolder a device. When you don't care about the board, cut around the device with Dremel or coping saw or nibling tool or even old diagonal cutters. But I understand your theory too, a good one.

But again, sometimes the best thing is to just keep the unit intact, until such time as the parts are needed.

Time after time, people have asked about components they've taken off boards, and they want specs for them. But the boards often provide much of the needed information, and if you take it apart before you actually have a use for those parts, then you lose the information.

The more specific an IC, the more it can't be used for general purposes. But, if you actually need the function, the way it was used on the board is likely what you'll want. So the board can either be used intact or partially intact, or at least the "sample schematic" can be derived from the board.

ONe example is old cellphones, the clunky kind. They all have nice FM IF strips. SOmetimes they can be extracted as modules. But in most cases, if you reused the parts, you'd be using them as originally used. So the IF filters, you can guess the general bandwidth from its use (though if you extract the filter(s) and then leave them around with no mention of where it came from, that information is lost if you forget), and the terminating resistors will tell you the terminator resistors. Extract the filter without at least doing some tracing around the filter, and you lose that information. The IC may not be recognizeable (though those old clunky cellphones tended to use well known fairly general purpose ICs in nice DIP packages), but the circuit itself is what you want if you are using it for an FM IF strip. So either leave it intact (you likely will want the parts together if you ever use them), or trace it all out and keep the IC and peripheral parts that you know are unique to the FM IF strip.

The advantage of keeping things intact is that you don't end up wasting time just tracing things out that you will never actually use. Wait until you have the need, and then the effort becomes valuable. It may even be easier, since what you want to use it for may make it easier to trace the circuit.

MIchael

One way you learn is to keep reading, and that means other people's post in a newsgroup are as important or more important than any threads you start.

You don't spend time analyzing everything, just use it to build up a general base of knowledge. If you remember a tiny bit, it's far easier to look it up when you actually need the information, than starting from scratch every time, or getting distracted by too many details every time they come up.

So someone resurrected an old thread yesterday, quoting a post of mine about how computer power supplies may not be the best choice for a general purpose bench supply.

Computer power supplies are great for their intended purpose, relatively high current at a few fixed voltages, but they aren't the best choice unless you have the need for that current capacity, and general purpose experimenting will barely need any current.

And like I say in that post that was just quoted, computer power supplies become a liability for the beginner, because they usually won't work unless properly loaded, and by the time you get a good enough load, you';re better off starting with a lower current supply.

People suggested finding those zener diode voltages with a variable power supply, and that will not be available from a computer power supply. On the other hand, you won't need any real current capacity to test those diodes.

Michael

I've uploaded digital photos of the mouse PCB. That might make it easier for people to figure what I'm talking about or identify components.

--- WRT your comments:

The resistors are identified by a numerical code where the first two digits are significant figures and the third digit denotes how many zeros follow the first two digits.

So the resistors marked "330" would be 33 ohm resistors since the trailing zero indicates zero zeros follow.

The resistor marked "513" would be interpreted as 51 followed by three zeros, or 51300 ohms (51.3k ohms)

The resistors marked "0" are zero ohm resistors (jumpers) that's why they're labeled JPn

R1 appears to be either 62 or 620 ohms.

The capacitors are anyone's guess, and mine would be somewhere in the vicinity of 0.01 to 0.1µF.

The optoisolators with pins tied together are more than likely the emitters (IRLEDs).

While you've gone to the trouble to generate your matrix and all, much more useful documentation would be a schematic which you could easily generate by just examining the wiring and tracing all its connections to the various components.

Like this guy did:

-- JF

Thanks for the explanations. That would be 51000 ohms.

I'll have to take another look at the PCB, but I recall being confused by the fact that the manner in which pins were tied together was different on the two opto-isolator pairs.

Actually, I didn't generate the matrix. I just made sure that every pin had another pin somewhere that it was connected to and recorded the connection. I considered just looking at the connected components of the wiring and enumerating the pins on each component, but I was afraid of missing one, since the components move around so much. On the other hand, my method is also flawed, since it is possible that I could wind up generating a partition of the equivalence class represented by the connected component, unlikely as that seems (given a set S of 2 element subsets of an n element set X such the union of S is X, what is the probability, with fixed X and variable S, that the graph whose set of vertices is X and set of edges is S is connected?). From this anecdotal information about pin connections, it would be possible to generate the 95x95 matrix and all that follows from it, but I didn't actually do so.

Thanks for bringing this website to my attention. It is pretty impressive! I had been searching for mouse printed circuit boards in Google but didn't come up with this. What was your search string?

It's amazing the lengths this guy went to, replacing himself in the circuit by a bowl of water and some wires, before he finally figured out what's going on.

How likely do you think it is that the unidentified 16 pin IC on my mouse is a clone of the 16 pin IC on Colin Fahey's mouse? I've downloaded the data sheet of his IC and will take a look at it later.

His page shows that the Microsoft PS/2 mouse is a very sophisticated device. I had expected the HP mouse to be basically simple, which is one reason I thought that observation without measurement might suffice to make sense of it, but now I expect otherwise. On the other hand, even if I had the equipment, I doubt that I would ever have figured out what Colin Fahey did. I'm going to study his page very carefully.

The page he links to at the end of his page, regarding a similar approach to PS/2 keyboards, doesn't seem to exist anymore.

I took another look. They are tied together in the same way, i.e. the two leftmost pins on the emitter are tied together. It is the same for both emitters. I got confused because my original labeling system for pins was entirely in terms of the geometry of the PCB, not the devices themselves. I'll update the website on this point.

The different parts of the "wiring" on the PCB look like gerrymandered congressional districts. But the least I can do is try to count them. So, I'll do that.

The data sheet for the SPCP05A IC refers to the internal ROM as user program area and as an EPROM, but doesn't discuss any method of programming it. Anyway, if it really is an EPROM, if one pops the ceramic off the IC, maybe one can erase the EPROM. I don't know how one would reprogram it. The core CPU in the chip is said to be a 6502.

I've actually made a few schematics of the mouse PCB, but they aren't very neat. I'll try again and see if I can upload a digital photo of it.

I haven't forgotten that I told Michael publicly that I would get a multimeter. I just haven't done it yet.

I've made a legible schematic. Hopefully it is correct. I'll upload it later today and announce it here when I do. I use my own ad hoc notation for the opto-isolator pairs, since there is a wide variety of notations and I'm not sure which one is appropriate for mine.

Regarding Colin Fahey's page:

Although I can see the transparent transmitters in his pictures of the PCB, I don't see the receivers on it. He does have a separate picture of the receiver. He explains convincingly that the receive must have two receivers in it. So, maybe that implies that the receivers of my optoisolator pairs must be like that too. I can't tell by looking at them and there are very similar receivers (w.g. the Waitrony Infrared Receiver Module) that seem only to have one receiver in them, in spite of having 3 pins. The three pin receiver Colin Fahey describes contains two receivers, he says, and the two outer pins of the package are the outputs of the two receivers. In the Waitrony, one is output, one is ground and one is VCC, according to the data sheet.

One difference between the IC in Colin Fahey's schematic and the one in mine is that his has only one unconnected pin (pin 2) while mine has two, pins 2 and 7.

I did wonder about the wires but didn't realize he had used them to replace the receivers.

I thought I might be.

Thanks for the explanations. I'm not sure I understand. I accept that the Waitrony device has nothing to do with my mouse. Are the receivers on my mouse of the same type as those on Colin Fahey's mouse?

I've uploaded the schematic:

I discovered that I have another old mouse, a Belkin mouse. I've opened it up and will make a schematic diagram. I no longer have access to a digital camera, but maybe I can make a schematic using postscript and upload it to the website.

The IC on this one is a HMC chip, apparently with serial number HM8450AP. I haven't succeeded in finding a data sheet for it. Unlike the one for the HP mouse and Colin Fahey's mouse, this one has 18 pins, not 16. Pins 4 and

It also has a C945 transistor. I downloaded the data sheet.

Sounds like a SIP resistor pack. It likely has 4 - 15K resistors with pin 1 of the package common to all resistors (and probably connected to the positive supply on the board.)