I'm trying to figure out how to use an ALS-PT19-315C/L177/TR8 phototransistor (or something like it).

The datasheet has this sample circuit for converting current to voltage so that I can measure it with an IC.

enter image description here

What does CL do? How do I select the appropriate value for it? I've also seen similar circuits where there is no CL so I assume it's optional and for filtering/smoothing maybe?

The datasheet further describes the relationship with the capacitor:

Vout(max.) = IPH(max.) × RL ≦ Vout(saturation)=Vcc-0.4V

RL × CL ≧ 0.5 (empirical data)

What does the part about RL x CL ≧ 0.5 mean?


The capacitor determines how fast the circuit responds to changes in illumination.

The value 0.5 (which should actually have units of seconds) might have been appropriate for some particular application that the person who wrote the datasheet had experience with, or for what they were considering as the "typical" application when they wrote the datasheet.

You should choose the capacitance to fit the response time you need in your application.

If there were to be a step change in illumination (for example, the light is turned on in a dark room), the circuit will respond with an exponential curve toward the final value given by \$I_{ph}R_L\$ with a time constant given by \$R_LC_L\$.

  • \$\begingroup\$ If the time constant was .5 seconds, it would take 2.5 seconds for the capacitor to fully charge, right (I think I read that somewhere)? Why would someone want that? Is that just to smooth the response and reduce noise? \$\endgroup\$
    – D. Patrick
    Jan 19 '19 at 0:21
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    \$\begingroup\$ It all depends what you mean by fully charged. After 3x the time constant it will be 95% charged. After 5x the timeconstant, 99.3% charged. Either one could be a reasonable definition of "fully charged" depending on your needs. Yes, the usual reason to do this would be to smooth the response and reduce noise. \$\endgroup\$
    – The Photon
    Jan 19 '19 at 0:38
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    \$\begingroup\$ @D.Patrick. As another example, if you were using this to trigger something to happen when the lights are turned on or off, for example, smoothing the output might help prevent triggering if the lights flicker briefly. \$\endgroup\$
    – The Photon
    Jan 19 '19 at 0:44

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