The intensity measurement stage uses an active half-wave rectifier to ignore the negative voltages while monitoring the the peak voltage to create a DC voltage signal with minimal ripple across the sinusoidal wave peaks at the filter output. Because a few people were having problems with this part of the design, our instructor actually gave us the schematic to use. I hadn't actually gotten to this stage yet because we were still discussing how the robot should behave, but there was no sense in reinventing the wheel, so to speak, so I used it!
This circuit was also simulated in PSpice with the output of the previous two stages serving as the input. A plot voltage output vs. time is generated with the circuit being initiated at 1ms. It can be noted that the input and output have different center lines. This is because the the input of this stage is a sinusoidal waveform centered at 2.5 volts, while the output is desired to idle around 0V like a standard DC signal.
Although this circuit worked flawlessly, it was not enough to complete the tasks. A second microphone and identical circuit was created such that the robot would have two sound sources to compare. A phase comparator was also added to monitor the difference between the two sound waveforms. The output of this comparator is proportional to the offset angle of a line running between the two microphones and the sound source. Apparently, students were having trouble with this new circuit addition as well, so the solution was given to us yet again. Like before, I wasn't even to this stage yet, but decided to use what was given to us to save time. The complete circuit was built on a breadboard and secured to the top of the robot. The circuit is shown below with the individual stages noted.

Strategy

We had planned to use two microphones long before the "phase comparator" circuit was given to us, so we never even bothered with incorporating it into our strategy. The robot would be placed in the center of the arena in any direction. The sound source would then be placed in some arbitrary location within the arena with the speaker facing the center. To start, the robot would begin to rotate, constantly comparing the current sound level from each microphone with both the other microphone and the previous sound levels. If either mic were generating a consistently higher level, then the robot would continue to turn in that direction until that sound level began to drop. It would then rotate to the previous angle and begin to drive towards the sound source, occasionally stopping to recheck the levels and compare between the two mics, slightly rotating in either direction if necessary.

The total gain of the microphones had actually been designed to max out the A2D converter in the XBC when the robot had reached the correct distance from the sound source as no types of distance sensors could be used. To ensure the robot would stay within the arena, two light sensors were placed in dark tubes and pointed towards the ground. As long as they saw some reflection of light, the robot would be within the arena. If the sensors saw an absence of light, it would be because the robot was about to cross the black outline of the arena, and it would redirect its course accordingly.

Afterthought

Although our strategy seemed pretty sound at the time (no pun intended), there were many things we did not originally take into consideration. The microphone placement could have been better to more easily distinguish between the two intensity levels, and the phase comparator could have helped to determine what direction the sound was coming from. We also had to change the way the robot moved and recorded data because of the Doppler Effect. The biggest problem we faced was getting the robot to figure out what direction the sound was actually coming from. We humans take this ability for granted since we don't have to work at it, but it can even be difficult for us to determine the origin of a sound when one just one of our ears is covered, injured, or muffled.

In the end, we almost completed this task successfully, but ended up running over the sound source instead of stopping in front of it. This happened because the robot got a little lost at one point trying to find the source and after turning from the edge of the arena began to approach the sound source from the side. Although it was still able to drive directly towards the source, the expected intensity of the sound was much lower because of the approach angle, and so the robot failed to realize how close it actually was to it. Oh well, at least the light sensors kept us from driving out of the arena and straight into a disqualification!