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DPRG: biasing & reading a phototransistor

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

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