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DPRG: biasing & reading a phototransistor

Subject: DPRG: biasing & reading a phototransistor
From: Dennis dld at cyberramp.net
Date: Sat Aug 14 12:44:02 CDT 1999

I'm working on adding encoders to Rover, the little Lynxmotion robot I
brought to the meeting last week.  I've made some black & white striped
discs & attached them to the wheels, and I found some "Reflective Object
Sensors" (Optek OPB741) at Tanner's yesterday.  These consist of an IR
LED and a phototransistor in one package, focused on a point 0.15" in
front of the detector.  The spec sheet is at:


I'm trying to figure out an appropriate value to use for the biasing
resistor on the phototransistor.  At Optek's site, they have an
application note (http://www.optekinc.com/book/pdf/BULLETIN213.pdf) that
describes basic circuits to use for emitters & detectors.  Based on
this, I initially chose a 220 ohm resistor for current limiting on the
emitter, and a 10K resistor to bias the detector.  With this, when the
detector is in the "off" state, I get about 4.8 volts at the output, but
when I put something reflective in front of the detector, the voltage
only drops to something like 4.5 volts.  I figured this meant the
transistor was partially conducting, but not saturated.  

Being the mathematically- and electronically-challenged person that I
am, I started trying to determine the appropriate resistance
empirically.  I went down to 4.7K ohms, but this made it worse, so I
started working the other way.  I tried 33Kohms, 47Kohms, 100Kohms, and
things got steadily better, in that the difference between the on and
off voltages became greater.  Finally at 220Kohms, the "on" voltage
dropped to 0.2 volts, which I took to mean I had achieved saturation. 
However, now the "off" voltage is only 3.5 volts or so.  The Optek app
note led me to expect a swing from 0.4 volts to 4.9 volts, but I can't
seem to find a resistor that achieves this.  Of course, the Optek app
note may have been for a photodetector with different characteristics
than the one in the OPB741.  So, my question is, does a 0.2 to 3.5 volt
transition sound reasonable for this photodetector?  Is it possible that
I'm going to damage it by using the wrong resistance?  (I tried swapping
it out for a different unit in case I had damaged it, but the new unit
showed the same results.)

Also, when I try to hook this open-collector output up directly to a
Basic Stamp II & read it using the "count" command, I get lots of noise
when transitions occur.  So I tried running the output through a 7414
Schmitt trigger inverter that I had laying around from when I used to
play with electronics 10 years ago, but apparently its input impedence
is too low, as the voltage swing at its input goes from 1.4 to 2.5
volts, which doesn't appear to be enough to get it to trigger.  I'm
going to go out to Radio Shack & look for a CMOS schmitt trigger to see
if it makes a difference, but I'm wondering if I'm going to have to use
something else -- a comparator? an op-amp?  Unfortunately, I have no
experience using those.

Optek appears to be a local company, located on Crosby Road in
Carrollton, so I may try to call them Monday.  But if anyone has
experience interfacing these parts or similar photodetectors, I'd
appreciate any advice you can give me.

Dennis Draheim


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