ELEC 241 Lab

The Receiver

The receiver consists of two stages. The first stage converts the current from the photodiode into a voltage. The second amplifies this voltage to a level capable of driving the earphone.

The Receiver Antenna.

The key to achieving a reasonable range with this system is the receiver antenna. This is basically a telescope which focuses an image of the transmitter LED onto the photodiode. It consists of a plastic tube with a lens at one end and a clear plastic sheet at the other.
The distance between the lens and the plastic sheet on the back of the antenna is equal to the focal length of the lens, so the plate is in the focal plane of the telescope. At the center of the plate is a hole the size of the photodiode. Attached to the tube is a cable with a miniature phone plug on one end and a three pin connector on the other. Bend the leads of the photodiode $90^\circ$ and plug them into the connector, with the long lead in the hole with the red wire and the shorter lead in the hole with the black wire. Fit the photodiode into the hole.

To use the antenna, hold it at arm's length, point it toward the transmitter, look through the clear plate at the rear of the telescope, and align the image of the LED with the center of the photodiode.

Photodiode Amplifier: Construction.

The first stage is a modified version of the circuit we used in Lab 4. The changes are:

  1. Higher gain.
  2. The anode of the photodiode is returned to -15 V rather than ground.
  3. Bandwidth reduction capacitor.

Also the photodiode is not mounted on the breadboard but is connected via a cable through the interface board. The diagram shows it wired to J1-5.

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Photodiode Amplifier: Testing.

Plug the photodiode directly into the breadboard. Connect the Scope to $v_r$ . You should see a small DC voltage. If you see a large DC voltage (about 15 V) check to be sure the diode is connected with the correct polarity. Verify that the voltage changes as you cover and uncover the photodiode with your hand, blocking and unblocking the light falling on it.

Unplug the diode from the breadboard and plug it into the connector on the receiver antenna (making sure to keep correct polarity. Plug the other connector into J1-5 of the interface board. Verify that that the circuit still responds as you point the antenna at various sources of light (and dark).

Earphone Driver: Construction.

Because of the wide variation in the amplitude of the received signal, we need more gain and a volume control for the earphone amplifier.

\includegraphics[scale=0.600000]{ckt9.5.ps}

Earphone Driver: Testing.

Connect the output of the photodiode amplifier to the input of the earphone driver. Set the volume control to midscale. You should hear a buzzing sound. To make sure that this is optical and not electrical noise, verify that it changes as you point the antenna in different directions or cover the antenna (front and back).