2.3.1 Relaxation Oscillator
Figure 4 shows the circuit design of the relaxation oscillator used to produce the square wave. The time period of the output square wave is governed by the equation: T = 2CRln(3), where C = C1 = 1 uF, R = R3 (trimmer). The frequency can be found by f = 1/T, and is thus inversely proportional to the trimmer resistance value. The actual frequency-resistance relationship is shown in Figure 5, which presents the characterization results of the relaxation oscillator. From this characterization, we can see that the higher the resistance, the lower the frequency of oscillator square wave. Therefore, when we can control the frequency of this generated square waveform by adjusting the trimmer resistance. And the square wave is then used as a frequency carrier for the voice signals.
Frequency vs. Resistor Value Graph …show more content…
Additional Feature – LED Volume Display
In our project, a Light-emitting diode (LED) volume display circuit is to indicate the volume level of the microphone analog signal. More LEDs turning on indicates higher volume of microphone output.
Figure 7: Schematic of the LED Volume Display
To achieve this, 7 comparators are used to control 7 respective LEDs. Firstly, 8 resistors were connected in series between 5V (Vcc) and 0V (ground). By purposely selecting the resistance for each resistor, 9 different voltage levels are predetermined as reference voltages between 0V and 5V. Secondly, these different voltage outputs are connected to the negative input terminal of each comparators (LM358). And the positive input terminal is connected to the microphone voice signal after the amplifier. Thirdly, the 7 comparators will then compare every reference voltage level with the voltage of the microphone analog signal. When the speaker’s voice volume is higher, the microphone analog signal will generate a higher voltage. Once this voltage is greater than the reference voltage from the negative input terminal, the …show more content…
Therefore, the number of LEDs being turned on indicates the voice volume of the microphone signal from the speaker.
Debugging of the volume display
Initially, 8 resistors of 1k Ohms are selected to evenly divide 5V into 9 parts so that the increment of adjacent reference voltages is the same of 0.625V. This results in a situation that some LEDs are always on while some are always off. It is because we ignored the voltage range of the microphone voice signal after the amplifier. When the reference voltage is higher than the maximum voltage of voice signal, the corresponding LED will be on all the time. To correct this error, we used oscilloscope to test the voltage range. It is found that this voice analog signal fluctuates around 1V and the maximum voltage is around 2.24 V. Therefore, it is necessary