![[ir_photo_transistor_amplifier.png]]

This amplifier, when IR hits the exposed base on the photo~transistor (marked as a diode above) pulls the OPAMP minus input down. To re balance this the OPAMP output swings up in voltage to provide current via R12. This 1uA of current flowing through the photo transistor will cause $1 \mu A \times 82\times 10^3 = 82mV$. This signal is buffered by the OPAMP and thus suitable for direct reading by an ADC. Also if the OPAMP is operated from 0 to 3V3 supply, there is no need for ADC voltage scaling. The 2k2 potential divider should be connected to a port pin, so that the ADC readings and MUX can be verified.