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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. enter image description here

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  • \$\begingroup\$ Crudely speaking, play with the resistor values. But you have a fundamental problem when controlling current to the common anode: brightness will vary with the number of segments lit. To avoid that, you either need to do this with a controlled current sink per segment line, or to switch to something like PWM brightness control with "safe" current (for full brightness at 100% duty) control provided by a resistor per segment, and the common anode simply wired to the supply. If your values ultimately come from an MCU you have a lot of software-based options for duty cycle variation. \$\endgroup\$ – Chris Stratton Feb 24 at 17:55
  • \$\begingroup\$ PWM is the usual approach and this can be done with a 555 timer and an LDR or else a 74122 and an LDR, for example. Either works okay for up to about 10:1 variations. In an aircraft situation, the need can be as much as 100:1 for the PWM and this makes the circuit slightly more complex. The minimum frequency period should be 100 Hz or faster, just as a note. \$\endgroup\$ – jonk Feb 25 at 8:14
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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

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