Use your Discussion to analyse your results and to explain any communications principles utilised in the experiment.
SFH750V transmitter, SFH250V receiver, 8-inch length of plastic fibre, bread-board, power supply, 330Ω and 470 kΩ resistors, a function generator (with fixed TTL output signal), oscilloscope and assorted leads.
Pin-outs of Transmitter and Receiver.
The SFH750V transmitter is a LED that emits at around 650 nm and can output 20 uW of light. The device has a spectral width of 35 nm and requires a current, depending on the required intensity, of 10 to 45 mA. The transmitter is a four-pin device (only two of which are active) and comprises of a LED, glass-bead focussing and a threaded chamber into which an optical fibre is inserted.
The SFH250V is a photo-diode receiver. It is designed to be most sensitive at 850 nm, but will detect a 650 nm signal. When used to detect a signal, a photo-diode has to be used in reverse-bias mode.
The pin-outs for both devices, looking from the bottom, is:
A = anode
C = cathode
N/C = not connected
Method
Ω resisSet the power supply to 5V. Place the transmitter device (SFH750V) on the bread-board – making sure the A and C terminals are not in the same bread-board column – and connect the anode to a 330tor (Figure I).
Connect the other end of the resistor, via wire, to the positive terminal of the power supply. Connect the negative terminal of the supply, via wire, to the cathode of the transmitter. Look into the screw terminal – the LED should be on (red).
Now place the receiver (SFH250V) on the bread-board, ensuring the A and C terminals are not shorted. The device must be reverse-biased, so connect the cathode to the 470kΩ resistor, and the other end of the resistor to the positive terminal of the power supply, via wire (Figure 2). Connect the negative terminal of the supply, via wire, to the anode of the receiver.
Check to see the LED of the transmitter is still on. Now place one end of the fibre into the transmitter’s threaded cavity – this may be more easily done by temporarily removing the transmitter from the bread-board and replacing it afterwards. Push the fibre gently, as far as it will go and then tighten the threaded nut. Use your fingers only – pliers aren’t required. Now look at the other end of the fibre and check to see that light is reaching the end.
If you can see light, put the floating fibre end into the receiver’s threaded cavity, and tighten up the threaded nut as before.
Now disconnect the end of the 330Ω resistor that is connected to the 5V power supply. Note: the power supply is still required to provide a ground to the transmitter and 5V to the receiver.
Turn on the function generator and set the frequency to 100 Hz. To the TTL output of the generator connect a coaxial cable. Now connect the live end (positive) of the coaxial cable to the end of the 330Ω resistor of the transmitter that you have just disconnected from the supply. Connect the shielding of the coaxial cable to the negative terminal of the 5V supply. Connect the output of the function generator to Channel 1 of the oscilloscope and adjust until the 100 Hz TTL square wave is visible.
Connect Channel 2 of the oscilloscope to the cathode of the receiver. The signal displayed should be a square wave with slightly rounded edges. Sketch the waveform and note its voltage.
When you have recorded the output at 100 Hz, increase the input frequency to 1 kHz and record the waveform. Then increase the input frequency to 10 kHz and again record the waveform.
Report
Use your Discussion to analyse your results and to explain any communications principles utilised in the experiment.
