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 DPRG: biasing & reading a phototransistor Message index sorted by: [ date ] [ thread ] [ subject ] [ author ] Previous message: DPRG: rechargable alkalines Next message: DPRG: Diffrent terrains for navigation Subject: DPRG: biasing & reading a phototransistor From: Dennis dld at cyberramp.net Date: Tue Aug 17 09:08:46 CDT 1999 ```Thanks to all for your replies. I'm learning a lot. Mike, thanks for the detailed explanation, and for taking the time to recover it after your network error. MIKE MCCARTY -- 93789 wrote: > With 10K you should have > > Dark: 5V - (100nA x 10K) = 4.999V > Light: 5V - (50uA x 10K) = 4.5V What I get with a 10K across the output: Supply voltage = 4.98V Dark voltage across phototransistor = 4.92V (dark room or black card over sensor) Light voltage = ~3.5V (Al foil 0.15" from sensor) > Just from the graph, I'd say 10K is an *upper* limit on the load > resistor. Even then you are going to notice slow transitions. The > load resistance and collector capacitance form a low pass filter, which > slows down the transitions. At 10K you are going to have 150us rise and > fall times. That's pretty slow, and will need a comparator with > hysteresis. As noted above, with 10K you are going to get 1/2 V of > signal swing. A comparator set to 4.75 V with hysteresis of +/-0.1V might > do well (limits of 4.65V and 4.85V). It's a start, anyway. Now I just have to learn how to use a comparator... :^) Excuse my naivete, but can I do this with the 339 quad comparator I've got in front of me? It says its differential input range is equal to the supply voltage, but I don't see how to set up hysteresis with it. > How many stripes do you have? How many RPS do you anticipate? Suppose > your max. speed is equivalent to 5 RPS, and you have 20 stripes. That's > 100 stripes per second, or 10mS per stripe. 150us rise and fall times > adds up to 1/3 of a ms. Not too bad, but not too good, either. Ok for > speed sensing, not good enough for position sensing. You'd have to move > quickly, then slow way down for positioning as you neared your goal. I have two discs I'm playing with, one with 36 stripes and one with 90. I anticipate about 1RPS max. I'm just looking for speed sensing right now. I'll have to move to something better than a BS-II for position sensing (probably a Scenix with SX-Key & C2C). > > Here's what I recommend: > > Put in a 10K load resistor on the collector, and remove the emitter > resistor. Done. Never had an emitter resistor. > > Put a 91 ohm resistor on the diode. Didn't have one, so faked it with 150 ohms in parallel with 220. > > Stop all current to the diode. See what your leakage current is. Looks like about 1.9 microA. > > Then turn off all lights in the room and see what it is. 0.2 microA. > > Now reapply current to the diode and see what happens. (Make sure > you've got close to 40mA in the diode. Do this still in the dark. I'd > start with a 200 ohm resistor on the diode, and gradually reduce it > towards 91 ohms.) 5.0 microA. With Al foil in front of sensor, it's very variable with distance; best number is about 150 microA at about 0.15". With my disk (printed on an ink jet printer, backed with white card stock) in front of sensor, the best I get is something like 25 microA. You're right, I guess my discs are pretty bad. I'll try a more reflective paper, & printing on a laser printer instead of an ink jet for a better black. Other than that, do the numbers above sound reasonable based on the device specs? Thanks, Dennis Draheim ------------------------------ ``` Previous message: DPRG: rechargable alkalines Next message: DPRG: Diffrent terrains for navigation Message index sorted by: [ date ] [ thread ] [ subject ] [ author ] More information about the DPRG mailing list