Photo transistor in parallel


I'm a beginner and trying to figure out how this circuit works (its a simple circuit that glows an LED when dark) :

My confusion is when the light is present, wont the current still flow through the collector and base of 2N3904 ? This is connected in parallel ? How does this circuit work ? How is the LED portion of the circuit not closed when light is present ?


Best Answer

When light is present, the phototransistor will conduct better than in dark conditions. This results in a lower voltage on the base of the transistor (the phototransistor and resistor form a so called voltage divider).

When the voltage on the base of the transistor is lower than about 0.7V (transistor base-emitter voltage) + 1.8V (LED forward voltage) = 2.5V, the transistor will turn off and the led will be off.