LED Brightness set by LDR Voltage Divider amplified with NPN BJT Emitter Follower Circuit learning electronics lesson 0029

Light dependent resistors (LDRs) make for a good light variable signal voltage for Bipolar Junction Transistors (BJTs).

Low light turns LED on circuit using light dependent resistor LDR aka photoresistor plus 2N3904 NPN BJT emitter follower power amplifier schematic diagram by electronzap
Low light turns LED on circuit using light dependent resistor LDR aka photoresistor plus 2N3904 NPN BJT emitter follower power amplifier schematic diagram by electronzap

When it is dark, the light dependent resistor in the diagram does not conduct. That makes the voltage at the base, almost the full 5V. The emitter will be about 0.6V shy of that 5V because of the Base to Emitter diode drop).

As light starts falling on the LDR more and more, the LDR conducts better and better. The voltage divider output voltage will lower as the LDR conducts better, and the LED will dim until it is off.

A red LED has a forward voltage of about 2V. Therefore it will very pretty dim when the Base drops to 2.6V. The LED light will go out altogether at about 1.5V, which will be when the Base is down to approx. 2.1V.

The load comes after the NPN BJT Emitter when considering conventional current. Therefore the circuit is wired as an Emitter Follower. Once current flows through the base, the collector conducts better. A small base current = a lot of collector current. So, the collector current will limit how much base current can flow, and the base will maintain the voltage set to it.

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