Hi,
I believe this is a pretty basic question but I haven't found anything that entirely covers my question in the forum so here I go:
I'm prototyping a feature that requires obtaining the approximate ambient light measurement in lux with a phototransistor and I need some help interpreting the datasheet.
My circuit is a simple common collector configuration (please see the attached file).
I need to adjust the range in a way that it will saturate at 65000 - 70000 lux
I'm trying to make sense of the datasheet (images attached) to choose the value of my load resistor.
The datasheet says:
- ON State Collector current (typical) = 2 mA when Vce = 5V and the irradiance is 1mW/cm2
- Collector-emitter saturation voltage (typical) = 0.4V when Ic = 2mA and the irradiance is 1mW/cm2
I don't have a lux meter to verify or calibrate, so I need to extrapolate this information to my particular case:
- 3.3V supply
- Saturation at 65000-70000 lux
What confuses me is the fact that the irradiance vs collector current chart is relative and not absolute.
So my question is: how to proceed?
Here's my guess:
- Apparently, 1 mW/cm2 = 6830 lux at 555 nm
- If I understand correctly, the relative irradiance vs collector current chart in this case just means that the relationship between irradiance and current is linear.
- If at 5V, 6830 lux is 2 mA, then at 3.3V it is 0,132 mA (3.3V/5V = 0,66 factor)
- Therefore if we put 68300 lux into the sensor (6830 x 10) then the current would be 10 times as much (1.32 mA)
- If the saturation voltage is 0.4V when Vce=5V, then in my case it would be 0.264V (x0.66 factor again)
- Therefore my resistor should be ((3.3V-0.264V) / 1.32 mA) = 23K
- And the ADC voltage reading would linearly scale with the lux reading from roughly 0V (there is a small voltage produced by dark current) and 3.036V, and that would be roughly 22496.7 lux per volt.
- With a 3.3V-referenced 10-bit ADC, I would have about 3.2mV per count or 72 lux per count.
Is this correct or I'm misinterpreting the information or the workings of the phototransistor?
Thanks in advance!
I believe this is a pretty basic question but I haven't found anything that entirely covers my question in the forum so here I go:
I'm prototyping a feature that requires obtaining the approximate ambient light measurement in lux with a phototransistor and I need some help interpreting the datasheet.
My circuit is a simple common collector configuration (please see the attached file).
I need to adjust the range in a way that it will saturate at 65000 - 70000 lux
I'm trying to make sense of the datasheet (images attached) to choose the value of my load resistor.
The datasheet says:
- ON State Collector current (typical) = 2 mA when Vce = 5V and the irradiance is 1mW/cm2
- Collector-emitter saturation voltage (typical) = 0.4V when Ic = 2mA and the irradiance is 1mW/cm2
I don't have a lux meter to verify or calibrate, so I need to extrapolate this information to my particular case:
- 3.3V supply
- Saturation at 65000-70000 lux
What confuses me is the fact that the irradiance vs collector current chart is relative and not absolute.
So my question is: how to proceed?
Here's my guess:
- Apparently, 1 mW/cm2 = 6830 lux at 555 nm
- If I understand correctly, the relative irradiance vs collector current chart in this case just means that the relationship between irradiance and current is linear.
- If at 5V, 6830 lux is 2 mA, then at 3.3V it is 0,132 mA (3.3V/5V = 0,66 factor)
- Therefore if we put 68300 lux into the sensor (6830 x 10) then the current would be 10 times as much (1.32 mA)
- If the saturation voltage is 0.4V when Vce=5V, then in my case it would be 0.264V (x0.66 factor again)
- Therefore my resistor should be ((3.3V-0.264V) / 1.32 mA) = 23K
- And the ADC voltage reading would linearly scale with the lux reading from roughly 0V (there is a small voltage produced by dark current) and 3.036V, and that would be roughly 22496.7 lux per volt.
- With a 3.3V-referenced 10-bit ADC, I would have about 3.2mV per count or 72 lux per count.
Is this correct or I'm misinterpreting the information or the workings of the phototransistor?
Thanks in advance!
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