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background light control in Eye Inspection Machine (Glaucoma)

Mahabir Prasad
Intellectual 300 points
Other Parts Discussed in Thread: OPT101

Dear All,

In dome, we have two led light source (warm white, 2850 - 3200 degree kelvin). I need to control this light at 31.5 asb constant using PWM technique. I have designed light sensing circuit using OPT101 with 5V power supply, the output is fed to ADC of microcontroller. The mounting of OPT101 is done place which get sufficient light and can be change if needed.

Does any relation between the light out (asb) to current through OPT101 photo diode so that expected voltage can be calculate, scaling of it to feed 3.3V Full Scale range on ADC keeping the same location of opt101 and light source. I like to avoid any external component, like to use the circuit  figure 1 (Basic circuit) in its datasheet. Can any comment on it.

One more thing, why output voltage in negative, it must be positive

Thanks in advance

Mahabir Prasad

  • 0
    TI__Genius 14825 points

    Mahabir,

    I want to be sure I understand your questions, please confirm or correct me as needed.

    1. You want to measure the light output of an LED light source with an OPT101, powered by a 5V source, and need a 0V to 3.3V output to feed an ADC.

    2. You want to be able to calculate the current through the photodiode at different light intensities, so you can calculate the expected output voltage?

    3. You want to change the gain of the OPT101 without any external components added to the circuit, and without changing the physical position of the OPT101 in your system to get more or less incident light, presumably to have your most intense light read 3.3V and your darkest light level to read 0V?

    4. You want to know why the OPT101 outputs negative voltage?

    Regarding 1. This is not the first time I've seen this use case.  It can certainly be done.

    Regarding 2. It seems like you would like a plot like the one on page 5 of the PDS, Voltage Responsivity vs. Radiant Power.  This can't be provided for all possible cases as every light source type is different and has a different light emissions profile.  If you have the spectral intensity plot (power vs. wavelength), you could potentially overlap it with the plot "Normalized Spectral Responsivity" and see what components of the LED light source will have the most impact, but I don't think there is a way to really calculate the output directly.  You should probably perform at least a 2-point calibration, with the LED lights on at their brightest, and with them off.  This will give you the basic range of Voltage output.  The nonlinearity of the output can be estimated based on the spec on page 2 of the PDS.

    Regarding 3. This is difficult, as the incident light can really only be changed with positioning and filtering, and the output voltage range can really only be changed with additional components.  Figure 4a and 4b of the PDS (page 9) show relatively easy ways to change the gain of the OPT101 with only one or two additional passive components.

    Regarding 4. The voltage output is positive, as shown in the "Voltage Responsivity vs." series of plots.  To what are you referring that the output voltage is negative?

    If you have additional questions or would like to discuss further details of your project, please continue to post here or email me directly at bridgmon_jason@ti.com.

    Thank you for using TI products!

    Jason Bridgmon