.. 3.13

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.

.. image:: schematics/opto-electric.svg
	   :width: 300px
.. image:: pics/opto-electric-transmission.png
	   :width: 300px

**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 :math:`5,1~k\Omega`
resistor. The output of the photo-transistor at :math:`1~kHz` 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
:math:`0,2~V`. 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.
