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.

 

You need to increase the base bias. What number PNP transistor are you using? You can significantly increase the gain by using a darlington configuration.
SG

 

Maybe the LDR's resistance is too low in daylight. Put a resistance (maybe about 56Ω or a 100Ω variable resistor) in series with the LDR.

 

You need to increase the base bias. What number PNP transistor are you using? You can significantly increase the gain by using a darlington configuration.
SG
View attachment 170884

I'm using 2N3906.

 

That circuit can be made to work with a tungsten light bulb but if you are really driving and LED or LED array, drive it with current instead of voltage. Here is the reason:

As you increase the voltage across and LED nothing happens until you hit the
"knee voltage" after which voltage takes off like a rocket. What's worse is that as the LED heats up the knee voltage decreases, making the current higher.

It is far easier if you drive the LED through a resistor or better yet a current source because illuminance is nearly linearly proportional to current. You can make your existing circuit into a current source by placing a small resistor in series with the emitter of your 2N3906 and thus greatly reduce the chance of frying your LED display.

The value of the resistor depends on how much voltage you can get across it and on how much current you want the LED to have when it is being driven full on. I would start with a resistor around 300 ohms for 10 milliamps and go from there.

If all the segments are in parallel, the available current will be divided by the number of LED segments, and the peak amount of light will be reduced linearly.