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

The following circuit shows how to modulate an LED and is based on the circuit by M. Gallant.

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.

Circuit Test

To test the circuit, a square wave with amplitude of 5V was fed into the input from a signal generator. The frequency of the signal generator was varied and at low frequency, the LED can be seen to modulate. As the frequency is increased, it is difficult to see if the LED is modulating; since our eyes have a limited FPS. A Si Photodiode (BPX65) followed by a transimpedance amplifier of ~200 kHz bandwidth was used to detect the modulated LED light. The output of the transimpedance amplifier was connected to an oscilloscope.

The following figures show the output of the transimpedance amplifier at signal frequency of 100 kHz and 140 kHz respectively.

Conclusion

The maximum obtainable bandwidth is limited by the bandwidth of the LED. The signal drops off fairly quickly after 140 kHz. The signal output from the oscilloscope did show overshoots in the rising and falling edge and this is due to an undercompensated transimpedance amplifier.

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