Testing the LED Indicator Circuit

In this section you will test the LED indicator circuit in your transmitter and determine if it operates close to the intended design from lab 5. The potentiometer is not to be connected in the transmitter circuit until later in the experiment.

1. Disconnect the SPST toggle switch from your breadboard. Connect the multimeter to the switch leads. With the multimeter set to measure resistance, toggle the switch and observe the resistance as you toggle the switch a few times. Put the switch in the ``off" position. Reinstall the switch in your transmitter circuit.
2. Set the DC power supply to 0V and connect it to the input terminals of the transmitter. Increase the voltage to 5V and then toggle the switch to the ``on" position.
3. You should see the LED glowing red. If the LED does not illuminate, remain calm. You can troubleshoot the circuit easily. Whether your LED illuminates or not, use the multimeter as a voltmeter and measure the voltage drops across the power supply, the switch, the current limiting resistor and the LED. Record all values measured. If your LED did not illuminate, try to figure out where the fault is. If the source voltage is correct (near 5V) and the voltage across the resistor is 0V, where is the 5V dropping? If the 5V is dropping across the LED, then it is reverse biased. Check the polarity of the power supply. If the polarity of the power supply is correct, then the polarity of the diode must be wrong. Remove the diode, invert the terminals, and reinsert it into your circuit. Once your diode circuit is operating, perform the voltage measurements again. Does KVL hold? Set the multimeter to measure current (ie. as an ammeter) and measure the current through the resistor and LED. Compare the measured current with its theoretical value as determined from Ohm's Law.
4. Toggle the switch to the ``off" position. Disconnect the DC power supply from the transmitter input. Connect the function generator to the input of the transmitter. Set the function generator to produce a sinusoidal voltage at a frequency of 10 kHz. Toggle the switch to the ``on" position. Adjust the amplitude of the function generator until a 10V peak-to-peak value is measured at the input of the transmitter.
5. The LED should be illuminated. Adjust the frequency to the range of 2-3 Hz. Observe the LED flashing. Increase the frequency until the LED appears to be emitting a continuous luminance. Record this frequency value. Reduce the amplitude of the function generator until the LED is barely visible. Measure the voltage across and current through the LED. How much power is the LED absorbing when it is barely emitting light?
6. Adjust the amplitude of the function generator to produce a 10V peak-to-peak voltage at the input of the transmitter. Use both channels of the scope to observe simultaneously the voltage across the resistor and the voltage across the diode. Explain why the waveforms are as shown on the scope.