The very simple, simplest BPW41 infrared reflective or trans photodiode interface to TTL or CMOS (uC, Arduino...) interface

Probably simplest BPW41 application.

 One day (in fact, it was a night) I needed simple and fast reflective transopto to precisely adjust a PWM RPM regulator. From scratch, like photodiode BPW41D i found. So I searched the net and found lots of schematics basen od op amps, bootstraps and so complex circuits. It seemed exaggerated to me. I measured that BPW generates 200-300mV static voltage  under IR diode light beam. Hey, it's very sufficient to open a transistor with a proper offset. So I made few tries and finally get a scheme like this. It worked!

The goal is to set the potentiometer, when dark diode, so base voltage is about 0,45V and Output voltage 4,5-5V.
Then with iluminated diode, base voltrage rises ca 100-150mV and output falls to 0,2 to 0,5V. Thats almost all I needed! Almost, beause of some noises on BPW input - I used speckled reflector which, together with fast (1-10MHz) diode, possibly made 400PMS counted as 17000 :).

(I suppose, when You use transreflective blend, it is not necessary)

So  I did a kind of filter to match the right range - just adding a cap (and resistor) to base. Worked great! With 100nF range is 0-2000rpm, with ab 33n 0-10000rpm. The 1K resistor is good idea, anyway, to protect BPW in a case of not proper mounting and/or setting 1,5M pot to 0.