Activity 6
Sending a Message with Light
Background Information
In this activity the source of the light is a large light-emitting diode (LED). The output of the LED is modulated by the signal from a portable AM radio. That is, the light emitted from the LED changes according to the signal from the radio. The LED is a highly non-linear circuit element. Its graph of current vs. voltage is sketched here.
The LED is a diode, a one-way device. An applied positive voltage generates a current. An applied negative voltage generates no current. Moreover, notice that when the voltage is positive, the current is not a linear function of the voltage. As the current increases, the slope of the current vs. voltage graph increases too. You can imagine the signal from the radio as a little back-and-forth wiggle on the voltage applied to the LED. To obtain the largest change in light output for a given wiggle of voltage, the LED must be operated at as high a current as possible. This is because the LED’s light output is proportional to the current, and the current vs. voltage curve has the greatest slope at the highest current. That means changing the LED voltage will have the greatest effect on the light output when the LED is already very bright. The maximum current for these big LEDs is about 100 mA. The 47 ohm resistor limits the current to about 60 mA, well below the maximum. If you have an oscilloscope, you can change the value of this resistor (try 100 ohms) and watch how the size of the AC signal from the radio varies as you change the current through the LED. An unfortunate consequence of operating the LED near its peak output is that it is impossible to see the brightness change as music is played. If you double the value of the resistor, you will see the brightness change, but the AC output signal will be reduced, along with the quality of the transmission.
Sending a Message with Light
Background Information
In this activity the source of the light is a large light-emitting diode (LED). The output of the LED is modulated by the signal from a portable AM radio. That is, the light emitted from the LED changes according to the signal from the radio. The LED is a highly non-linear circuit element. Its graph of current vs. voltage is sketched here.
The LED is a diode, a one-way device. An applied positive voltage generates a current. An applied negative voltage generates no current. Moreover, notice that when the voltage is positive, the current is not a linear function of the voltage. As the current increases, the slope of the current vs. voltage graph increases too. You can imagine the signal from the radio as a little back-and-forth wiggle on the voltage applied to the LED. To obtain the largest change in light output for a given wiggle of voltage, the LED must be operated at as high a current as possible. This is because the LED’s light output is proportional to the current, and the current vs. voltage curve has the greatest slope at the highest current. That means changing the LED voltage will have the greatest effect on the light output when the LED is already very bright. The maximum current for these big LEDs is about 100 mA. The 47 ohm resistor limits the current to about 60 mA, well below the maximum. If you have an oscilloscope, you can change the value of this resistor (try 100 ohms) and watch how the size of the AC signal from the radio varies as you change the current through the LED. An unfortunate consequence of operating the LED near its peak output is that it is impossible to see the brightness change as music is played. If you double the value of the resistor, you will see the brightness change, but the AC output signal will be reduced, along with the quality of the transmission.