# Opto-electric signal transmission

Objective

Demonstrate the transmission of signals using light. An LED is powered by a 1kHz signal and the light is made to fall on a photo-transistor.

Procedure

- Keep the LED facing the photo-transistor and set SQR1 to 1000Hz

- Repeat the experiment by changing the frequency.

Discussion

The SEN input is internally connected to 5 volts through a resistor. The output of the photo-transistor at is shown in figure. The square trace is the voltage across the LED. When the LED is ON, photo-transistor conducts and the voltage across the collector drops to . When the LED is OFF the photo-transistor goes into cut off mode and the collector shows almost the supply voltage. The rise and fall times of the photo-transistor seem to be different. Find the upper limit of the frequency that the given photo-transistor can respond.

Repeat this experiment with a Fiber Optic cable to guide the light from LED to the photo-transistor.