Hi, I managed to make this sensor work. However, I cant visualise the current flow in the circuit, maybe someone can help me?

The manual says to charge up the capacitor, then followed by discharging, and the rate of discharge will be determined by the reflectivity of surface.

If you apply a voltage at OUT, how would that charge up the capacitor? Since OUT=Vin wouldnt voltage across capacitor be 0V?

From how I see it, the capacitor should discharge instead because voltage is 0 and it shouldnt hold any charge, followed by being charged up and the rate of charging will vary, kind of counter logic.

Seems like a elementary issue but I cant really see it O.o


As you mentioned, applying voltage to OUT does effectively discharge the capacitor since the voltage across the capacitor becomes 0V. However, when this happens, the node between the capacitor, resistor, and phototransistor gets charged. Once you are no longer supplying an external voltage to the output pin, the node can slowly discharge through the phototransistor. You can read more about this in the “How it works in detail” section on the QTR-1RC product page.

- Jeremy