Interesting, that would save me lots of hassle. Do you know if it's sensitive to infrared light?
The "Normalized Spectral Sensitivity" chart on page 5 shows the peak sensitivity is 850nm with broad coverage from 700 to 1000 — yes the OPT101 is nicely sensitive to infrared.
I'm trying to decode the circuit diagram on that url on first message. What is the gap with o/p in it?
I have no idea what "o/p" means, but this is the output of the circuit.
There is one resistor with 120 written, but 120 what? (no units) or could that be the potentiometre.
Resistors are always in ohms unless otherwise indicated with a "K" for thousand ohms or "M" for million ohms.
Capacitors are in farads unless otherwise indicated, but a 1F capacitor is so huge that you'll never see it in normal circuits like this.