Questions:
1. Does the biasing set up seem correct? I seem to think i'm missing something.
2. For R2, i assumed i needed 60ma, so is it right to do this for the value of R2? for example, say i needed ie=ic=20ma, would i just do ve/0.020 = R2?
Wont the current thru R2 be a direct function of the voltage at the base of the transistor.???
Example: say when 'dark' the voltage on the base is 2V, then the emitter voltage will be 0.7V lower ie: 1.3V.
So if R2 was 50R then Ie would be 1.3V/50R =26mA.
If when 'light' the Vbase is say 1V then Ve would be 0.3V and the current Ie would be 0.3/50 = 6mA.
These values are for explanation only.
3. How would i improve the circuit to have a better 'snap on' time? for example, on the breadboard the leds are on in decent light, how would i modify ciruit so that they would be off unless Q2 was 1Mohm ish?
To get a 'snap' action its usual to provide positive feedback from the output to the input.
A Schmitt trigger would do just that, a second transistor is used.
Thanx for taking the time to read that.
Ross