For a project, I want to detect the red part on the moving disk of Ferraris electricity meter. I want to do this with an Infrared LED and a phototransistor that receives the reflected light. The amount of light received by the phototransistor will be lower when the red part is in the illuminated area. As I don't know how much light will be necessary and to cancel out ambient light, I want to dim the LED with a DAC from a Microcontroller, to let it automatically detect how much light has to be emitted.
So, I need a simple circuit to dim the LED from 0 - 70mA without using a PWM, as this would probably cause problems on the phototransistor. As simple as it seems, I couldn't find any suitable solution on the internet.
The DAC Output will be from 0 - 3V3 and this must match to 0 - 70mA through the LED.
I'm not sure if this circuit will work, especially I don't know which FET or BJT I should use.
The LED I want to use draws 70mA at 1.5V.
The sensitivity of the phototransistor will be configured by the internal comparators of the MCU.
My question now is whether this circuit works the way I imagined it would.
Thanks a lot!
I thought of using this structure for detecting the metal disk. That guy was successful using that structure:
This is the ADC Reading of that Phototransistor over time.
My structure would look similar, that's why I think it would work as intended.