Darker LDR means more light from LED BJT circuit

Current through the LED load of this circuit is mostly determined by the voltage across the NPN BJT’s emitter resistor, and that resistor’s value.

Darker light dependent resistor LDR equals more light 2N2222 NPN BJT transistor current source
Darker light dependent resistor LDR equals more light 2N2222 NPN BJT transistor current source
  • Voltage across emitter resistor is approx. 0.6V less than whatever the voltage at the Base is.
  • The LDR (light dependent resistor) is on the low side of a voltage divider in this circuit. Low side means that it is closer to 0V ground.
    1. More light falling on LDR lowers it resistance. Therefore it causes the the voltage divider output voltage to go down. There is an easier path to ground.
    2. Less light falling on LDR increases it’s resistance. Therefore it causes the voltage divider output (base input) voltage to rise. Current has a harder time getting to ground.
    3. There must be enough total (supply) voltage to power the emitter resistor, transistor collector to emitter, and the load.
      • White, blue, or green LED load will drop about 3V of the 5V supply, from the emitter resistor. That means that the emitter resistor will only be able to build up about 2V across it. There will be no difference in the amount of current that flows after the NPN BJT Base voltage rises to above 2.6V.
      • Red, orange, or yellow LED load will drop about 2V of the 5V power supply from the emitter resistor. That means that the emitter resistor will only be able to build up about 3V across it. No extra current will flow once the  NPN BJT Base voltage rises above approx. 3.6V.
  • The current through the resistor can be calculated with Ohms law formula for current. That is I=V/R. The current (in amps) through the resistor is the voltage (in volts) across it, divided by it’s resistance in ohms.  That is also the current that will flow through an LED load, or any other load that doesn’t drop/divide away too much voltage.
    1. 1V/220Ω = 0.004545A (4.5mA)
    2. 2V/220Ω = 0.009A (9mA)
    3. 3V/220Ω = 0.013636A (13.6mA)

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