Analog and Digital Electronics

Lab 8: Supplement (April 11 and 18)

The basic idea of this experiment is to produce light whose intensity can be modulated or varied at a certain frequency. You will then try to detect this light. The light will be produced by an LED and detected by a photodiode. The goal is to see if you can detect the signal when you are a couple of meters away. As a result, the detector circuity and the transmitter circuitry (LED) need to be on different boards.

In this lab period I want you to set up a couple of circuits you will use in lab 8. The first is an LED or light emitting diode. If you run a current through one of these, it will emit light and the amount of light is approximately proportional to the current, and for our purposes we will assume the light intensity is proportional to the forward current through the diode. Our diodes are only designed to have currents of 10 to 20 mA run through them continuously. (They can take higher currents for short times, but not continuously.) Our diodes will emit light in the orange or red. For these the forward voltage to turn them on is about 1.8V. (What is the wavelength of a photon of energy 1.8eV?)

Actually we could drive them with the function generators directly, but try using a transistor as an emitter follower as shown at the right. We often use transistors this way to “boost” the current. The input Vin will come from the generator and connect to the base, B, of the transistor. The collector, C, will go to the positive supply voltage (about +12V) and the emitter, E, will connect to a resistor that in turn goes to the diode. The other end of the diode will go to the negative supply voltage (about -12V). The purpose of the resistor is to limit the current through the diode. You might want to reduce it later to get more light (i.e. more current), but stay above 1kW so the total current /

will not go much above 20mA.

If you have the transistors in the plastic packages, the pin configuration is shown at the right. It is assumed that the flat part is facing you and that the wires come out of the bottom. This is the pin out for a 2N3904 or a PN2222A transistor. It is good practice to try to relate the diagrams to the configuration on your breadboard.

Build the circuit above on your breadboard and let me check it before turning on the power. Then test it by grounding the base (with the generator disconnected) seeing if you get light coming out. Then disconnect the ground so the base is not connected to anything. At that point no light should be emitted from the diode. If this works, then attach the generator and use a sinusoidal signal of about 15V peak to peak and 1Hz to drive the transistor. (1 Hz is slow enough so that you should see a variation in intensity. At 50Hz you would probably not notice a change in intensity, it would occur faster than we can perceive and our eyes would just average it.)

On a second breadboard you should build the circuit at the right. It will require a photodiode. If you reverse bias a photodiode it will allow a current to flow through it when light strikes it. The current it proportional to the intensity of the light striking it. This current can be converted to a voltage by letting it flow to ground through a resistor. This voltage will need to be amplified and filtered in order to separate the signal due to the room lights from the signal from the LED. The detection and initial amplification can be done with the left circuit shown below. The circuit on the right will also filter the signal, but you must modulate the LED at the resonant frequency of the LC circluit.

The 2.2k resistor in the right circuit is there in case you put the photodiode in backwards. If you did that without the 2.2k resistor, it could destroy the photodiode. You will probably want a gain of 5 to 10 in the op amp.