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phototransistor acting up, or is it me?

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jaydag71

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Hi folks! Today I am having some problems with sorting out some photo transistors that I've salvaged from a bunch of old mice' . They appear to be all the same- these are 3 leaded devices which has led to some confusion this morning while testing them out for a line sense array/edge detectors. Please find attached crappy jpeg with a picture of one.

Feel free to flame me, but my problem is that I have tried taking measurements in various configs and am getting the same results when I swap out what I am assuming is the base with the emitter. ie.

if 1=e & 2=c & 3=b
I tried 5V with 100ohm R in series to c, gnd the e and take some voltage readings under both ambient and with an IR led firing into the ptrans from both the b (4.96 amb -4.98IR) and the c (5.01amb -4.99IR),, but then I swap the gnd from e to b and take the measurements from e (4.96amb - 4.99IR), and again from c (5.01amb - 4.98IR).

I've noticed that most line follow bots take the output signal from the collector, but I tried the same tests this time from emitter:

This time swapped the 100ohm r over and placed it between e and gnd. Vcc into collector (5.01V) and take some voltage readings from both b (4.96amb - 4.99IR) and of course from e (0.00amb - 0.01IR),, swap the b and e connections and re-test,, e (4.96amb - 4.98IR) and b(0.00amb - 0.01IR).

I am now plainly confused. From where am I supposed to take the useable output signal? which pin is indeed the real emitter?? Does it even matter ,,if I am able to get a variable signal output from the device something is working right? Sure would like a datasheet though....any help much appreciated,, jd
 

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Look at the mouse circuit board. I think you will find that it is 2 phototransistors in 1 package to get a quadrature output.
Most have one +5v input (probably the center pin) and 2 outputs.
 
yeah, I was thinking that they might be 2 channel devices but I don't think this is the case,, even though on the mouse PCB's I salvaged them from both the disputed pins on each ptrans run into a separate pin on the main uC, so who knows- the trouble I'm having with the 2ch idea is that I cannot seem to find any evidence of part datasheets, or any links to parts.

How would they be structured? 2 emitter? The pcb's had them wired up differently than I do(the way you posted), but I am getting a variable value from the way I have them hooked up- the only way I get 2 outputs varying is taking the reading from the collector and whichever pin is left floating,, either side gets the same result(as collector) when the other side is gnded, so am still much confused. This doesn't seem to be typical transistor behavior? When I try to take the voltage reading from the leftover pin after whichever side I have gnded, I get the results as posted earlier! How would one make use of the second channel? , because I was getting voltage readings in the .01 region, harsh contrast to the other side! bummed.

I tried swapping these ptrans out with another known 2 pin device using my selected wiring (sampling from the collector after resistor tied to vcc, e to gnd) and the voltage readings are similar to what I was getting using these 3 leaded parts. From looking at the above posted v readings, can someone tell for sure whether the extra pin is indeed a second transistor or the base lead? I don't have enough of any other similar ptrans to use in the array so am stuck using these for now. Maybe things will look better tomorrow. cheers. jd
 
If it's a 2-channel device, then you need to load both channels equally. Assuming the center pin is the common, try pin 2 on Vcc, and pins 1 & 3 each through 1K to ground.

Repeat the experiment using the assumption that pin 1 is common, then with pin 3 common.
 
right you are. I'm getting identical v readings from both channels configed as recommended.

I'm thinking that the extra channel will be of little use in a line follow app , so would prefer to leave it out. Is there going to be issues if I leave the second channel floating? I only ask because it is a battery powered platform and I assume that the lack of the second channel being connected will save the additional consumption.

Thanks again for clearing the whole thing up! jd
 
There shouldn't be issues from leaving the second channel floating.

(Sorry answer took so long, was on vacation. Had access, er, dialup.)
 
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Thanks for the response! better late than never. I actually just finished designing the pcb and ended up leaving out the second channel- left it floating after all. I don't know enough about the internals of this device to know how this might affect the operation because I haven't been able to find of info about these types of parts, at all! Isn't that odd eh?

Anyways, thanks again for your input, at this point I'm just hoping for the best and that my time hasn't been wasted!!! regards,, jd
 
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