Brighter LED at less light circuit using LDR and NPN BJT Current Source by Electronzap
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NPN Bipolar Junction Transistor (BJT) with an emitter resistor becomes a current source. Current is the voltage at the emitter divided by the emitter resistor's resistance. The collector LED still drops about 2V, so there will be a limit of about 3V at the emitter with a 5V supply. Emitter voltage is approx. 0.6V less than the voltage at the Base. Voltage at the Base is set with a Low side Light Dependent Resistor (LDR) voltage divider. The Base resistor won't be able to reach 5V because of the LED voltage drop preventing the emitter from getting above approx. 3V.
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