Hello, i have made a laser trip system consisting of arduino, light transistor (2 leads), NPN bjt transistor, and some leds, and resistors... I was hoping someone can help me analyze the circuit, i already did an analysis, however i simplified it by assuming the base current is zero going into the NPN. The circuit is shown below:
This is what i have tried so far:
I have drawn a load line for the phototransistor branch, and designed the resistors so that when i get 0.7 volts or above going through bottom resistor, the NPN turn on and lights up the green LED. By varying the resistor, i can get the LED to turn on for a dimmer light, or a brighter light.
My problem is, is that when i designed it, i calculated what current i would be getting out of the phototransistor for a given lux, and given resistance, and tried to get the voltage drop to be 0.7 . The problem with this is that it assumes all the current from the phototransistor goes through the bottom resistor, but some must go to the base of NPN for it to turn the other branch on. How do i find what the base current would be? I can't just assume the NPN is in active mode, which would then tell me what the base current is. If its not active, but saturated, then the base current is no longer related by B....
Is there any way to simplify this circuit, maybe do a thevenin equivalent? How do you usually deal with circuits that have transistors, or diodes in series/parallel and you want to do equivalent circuits? Is it possible? It seems like i would not be able to use superposition method to help me either...
Can i replace the transistors and replace them with variable resistors? Can i replace them with current sources? Whats the best way to do this...
I did the whole analysis, and i got numbers that are pretty good and my circuit does work, i just wanted to know the proper way.