Electronic Instrumentation: Project 2 Information

Fall 1998

Goal: Using the given design of an optical transmitter/receiver, design, build and test an optical link for a transducer, such as a guitar pickup, a microphone, or a speaker used as a microphone that produces an electrical signal in the audio frequency range.

Group Responsibilities
• Complete the pre-project report by Friday, 23 October. Since we are using some new parts this term, it was not possible to show the completed standard transmitter/receiver pair until 19 October. We, therefore, moved the date for this report back a few days. Please note that this report is still due separately from the final report, even though we combined the two in project 1.
• First build the circuits as they are shown on the diagrams distributed in class. The component choices in the diagrams can be changed, but we will have only a limited number of components available. That should not deter anyone from thinking about how to improve the circuits. Be sure that you understand the functionality of each part of these circuits and test each part before you try the whole circuit.
• Test your circuits using the audio source for which the transmitter was designed: a portable tape player or CD player (connect to the earphone jack). Voltage levels at all points of the transmitter and receiver circuits must be measured and documented. These measurements will be made with two loads: a speaker and a resistor with the same value as the speaker. For each measurement made, there should be some discussion regarding which features of the data make some sense, which do not and why.
• Consider methods to improve transmission and reception of the optical signal by, for example, blocking unwanted light.
• Determine the Thevenin equivalent source representation for the walkman and the transducer. Also, determine the equivalent Thevenin source representation for the phototransistor.
• Something to think about for each circuit: Since it is required to look at each circuit with different sources and the receiver with different loads, it should be possible to determine the input impedance of the transmitter and the output impedance of the receiver.
• Each group should find some information that relates to their circuit on the web and the URL included in the report.
• Once the operation of the circuit has been documented, test the transmission of the signal from your transducer over as large a distance as possible. In a darkened room, it should be possible to transmit and receive the signal over at least several feet.
• Specific Issues Some suggestions for your report follow. Many are implied or obviously included in the tasks listed above. However, they are listed here to provide ideas if you need them.
  1. Include a diagram of the circuit(s) you are going to build. That is, a diagram of the receiver and transmitter and the appropriate variations for different sources or loads.
  2. Discuss the operation of the circuit
  3. Discuss the operation of the ICs (integrated circuits) in your circuit.
  4. What function do the components perform?
  5. Over what frequency range will your circuit work?
  6. How is the audio information transmitted throughout the circuits (from the tape player to the speaker)? What kind of modulation is used?
  7. What types of signal might you input into your circuit?
  8. Describe your input and output signals (e.g. units).
  9. How might you simulate your circuit on PSPICE? What components might you have to approximate?
  10. How will you test your circuit? Write up a simple procedure that you will follow.
  11. What parameters might you use to quantify the fidelity of your setup?
  12. What variables/factors do you expect to affect the operation of your circuit?
  13. What changes in the circuit design would you suggest that could improve its performance?
  14. What kind of noise might you expect to find and how might you minimize it?
  15. Provide some useful information about your circuit or integrated circuit that you found on the WWW. A good idea would be to look for a circuit that seems to be similar in design if not similar in purpose.
  16. Include some information on the properties of typical small speakers. How does a speaker work? What drives it? Why can it be used as a microphone? What is their nominal impedance, power rating, inductance, capacitance or whatever else you can find out.
  17. Develop a simple design for improving the transmission of the LED or reception of the photo-transistor.