Free-Space Optical Communications

High-Intensity LED Driver

Current Source and Driver
Carrier Generator/Modulator and Sewer-Pipe LED Projector

A switching regulator supplies the current. A power MOSFET switches the LED on and off. LED current can be monitored with a voltmeter (or multimeter set to read volts) connected across the 2-ohm current monitor resistor. Current flow from the current regulator is one-half the instantaneous current flow through the LED, since LED current flows for exactly one-half of the time. The 2-ohm resistor establishes a nominal one-to-one voltage drop to instantaneous laser current relationship. For the Cree XR7090RO, the instantaneous current is set to 500 mA (500 mV).

lm2675 regulator circuit

Construction note: Return grounds directly to LM2675 pin 6

Some Part Numbers

  • Switching Regulator, LM2675N-ADJ - Digikey LM2675N-ADJ-nd
  • Inductor, 47 uH, 2.1 A - Digikey 811-1224-nd
  • Schottky Rectifier, 1 A, 20 volts - Digikey 1N5817FSCT-nd
  • Capacitor, 330 uF, 25 volt, low impedance - Digikey 493-1551-nd
  • Capacitor, 100 uF, 25 volt, low impedance - Digikey 493-1548-nd
  • Resistor, 2 ohm, 1 watt - Digikey PPC2.0W-1CT-nd
  • Almost any n-channel power MOSFET should work in the circuit,
    IRFD220, for example.

The LED Driver Prototype

The LED driver is on the right. The circuit was an afterthought. The LED driver prototype was built on a separate board and mated to the modulator prototype (left) with an interconnecting cable.

LED driver prototype

Related Links

Sewer-Pipe LED Projector

Subcarrier Generator/Modulator

Other Related Optical/Laser Links

laser beam concentrator (photos)

laser beam concentrator (schematic)

laser shotgun

music on a light beam - the system tested

Range Calculator/Estimator

The Photolistener