Light Emitting Diodes (LEDs) require a DC forward bias current to output light. The characteristics of an LED is such that there is a operating region where the light output is linearly proportional to the forward current through the LED. This is useful for producing a light signal proportional to a current or voltage signal. A basic modulating circuit that produces a current proportional to the input voltage signal can be made using a single transistor. This post shows how to modulate an LED using a very simple circuit.
LED MODULATION CIRCUIT
Unlike the circuit provided in the above link, the proposed circuit put the transistor on the edge of conduction, such that the quiescent DC current through the LED is not enough to make it output light. To achieve that, the circuit is powered on and the potentiometer, used to set the base voltage, is adjusted so that the LED is on the edge of “conduction”.
When a signal is applied to the input, say from a signal generator, the voltage at the base will rise and fall according to the signal amplitude. This will cause a change in the rise and fall in the base current which will have a similar effect on the collector current of the NPN BJT, which happens to be the current through the LED.