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John N. Power wrote:
> Phototransistors convert photons to electrons, which means=20
> current output.=20

I'd rather say - Photons form electron-hole pairs in the base which=20
creates a base current.

> You really should use a current-to-voltage converter. A simple
> one could be made by connecting the emitter of the phototransistor
> (in your circuit above) to the base of a transistor which has its
emitter
> grounded. Put a resistor from the collector to +5 volts. The collector
> voltage will be +5 volts minus the product of the collector resistor
and
> the collector current. The collector current will equal the photo
current
> times the beta (hfe) of the converter transistor.
> One problem would be saturation of the transistor by the ambient light
> current. This can be taken out with a resistor from base to ground
> (emitter). This will subtract a constant from the collector voltage;
adjust
> this resistor so that ambient light puts the collector voltage at +2.5
> volts...
> A good opamp solution can be made with the LM3900. It works from
> 5 volts and has current sinking inputs. Connect the phototransistor
> to the plus input of one of the amplifiers (the chip has 4), and a
> feedback resistor from the output to the inverting input. Additional
> current into the inverting input will subtract an offset from the
output,
> and choosing the value of the feedback resistor will allow setting the
> gain.


 *** "LM2900 schematic.gif" is attached ***

By the way, "plus input" current is about 0.02 ma for LM2900. Ever=20
seen a phototransistor with the 0.02 ma output current?=20

Not to say, two transistors without negative feedback (phototransistor=20
and "plus input" transistor) are real killers for the converter
linearity=20
and even stability. Temperature signal shift will be much more than
working signal range.=20

   I'd suggest another (if mentioned already then sorry, I haven't=20
red the hole thread) PIC-based approach:

 - a n-p-n phototransistor, collector to +V, emitter to the PIC's PWM
output with the appropriate resistor and capacitor (and transistor if=20
needed). Resulting voltage should be applied to comparator or A/D=20
input (perhaps over amplifier).

- PIC must keep the voltage about a half of +V, adjusting both=20
PWM parameters, duty cycle and period.

- for the better precision additional temperature sensor and=20
appropriate look-up table may be used.

Best Regards,
Mike.

P.S. On a non-related wave: For me Olin's stile of writing is like=20
president Reagan's style of  communication, John N. Power's=20
is like Bush II's, Russell's The Great Pacifier (or Pacifist - which=20
is better) is like Pope's :-)=20
Hope nobody is offended, if so a thousand apologies, guys.




 =20


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