I've been trying to make a light sensor circuit that uses a photoresistor and transistor to turn an LED on when it's dark and turn it off when it's light. I've learned enough about transistors to know the general layout of the circuit, but I'm kind of confused on how to go about calculating the necessary resistor values.
The photoresistor I'm using has about 8kΩ resistance in normal indoor light conditions and I want the LED to turn on when it reaches about 80kΩ resistance (it goes up to about 150kΩ when it's totally dark). Fooling around in Multisim I came up with the following circuit that works:
I made the circuit on a breadboard and it actually works pretty well:
Although I got it to work, I feel like I cheated by just fiddling around in Multisim until it started working -- I'd like to know how you would go about actually determining the resistor values computationally. I understand how to calculate the different transistor currents, my textbook shows how to start with known values given and then compute the other currents and voltages, but it doesn't show how to actually design a circuit from scratch.
Can anyone explain that process to me?
Thanks
The photoresistor I'm using has about 8kΩ resistance in normal indoor light conditions and I want the LED to turn on when it reaches about 80kΩ resistance (it goes up to about 150kΩ when it's totally dark). Fooling around in Multisim I came up with the following circuit that works:
I made the circuit on a breadboard and it actually works pretty well:
Although I got it to work, I feel like I cheated by just fiddling around in Multisim until it started working -- I'd like to know how you would go about actually determining the resistor values computationally. I understand how to calculate the different transistor currents, my textbook shows how to start with known values given and then compute the other currents and voltages, but it doesn't show how to actually design a circuit from scratch.
Can anyone explain that process to me?
Thanks