The circuit shown in figure 1 modulates a beam of light emitted by an LED (visible or infrared) from audio or pulse signals (PWM) for transmission in direct optical links or by fiber optics. The input of the circuit is high impedance and the maximum frequency of operation is a few tens of kilohertz.
Utilities:
• Direct optical links
• Optical fiber optic links
• Emission of modulated radiation in alarms and sensors
Constructive Details:
The circuit may be implemented on a small printed circuit board as shown in Figure 2. The LED may be provided with application-dependent optical resources. The field effect transistor accepts equivalents and the current in the LED is adjusted as a function of the amplitude of the modulation signal through P1. The maximum current in the LED for the transistor used is of the order of 20 mA.
Material list:
Semiconductors:
Q1 - BC245 or equivalent - JFET
LED - common, infrared or red LED
Resistors: (1/8 W, 5%)
R1 - 1 M ohms
R2 - 100 ohms
P1 - 1 k ohms - trimpot
Capacitor:
C1 - 22 nF - ceramic or polyester
Several:
Printed circuit board, wires, solder, etc.
