I'm using the below circuit to build a auto dimming circuit. I was fine when I only use one 7-segment led, but when I connect more 7-segment led (parallel to L1) the light intensity is very low. How to make it brighter and function properly? Thank you.
What you need here is an LDR-controlled Digit Voltage ( not current). If there are segment current resistors to regulate the brightness dependant on supply voltage drop, this will work and change the brightness uniformly with Digit Voltage. If not , it wont.
Unfortunately, with a BJT the current regulation with Resistors only affects the current limit. As you add more current displays, all the resistors have to be scaled down to increase the limits. This is why a True resistance controlled designed current source works better.
However, the real problem is you are driving all the Common Segment voltages with a current source which means the LEDS go dim with more segments active.
Thus an Emitter Follower of LDR bias voltage might work, not a Common Emitter