Does it matter where the power source for the distance sensors come from, whether from the Arduino's 5V or from the battery pack?
I'm asking because I'm trying to modify the wiring to include 5 LDR (photoresistors) in front and the space is really tight. So I wonder what the harm is if the power source for the sensors came from the back rail instead of the front.
The reason I said to plug them into the front power rail (and to connect 5V from the Arduino to that) is that the sensor is only rated for 5V and the voltage going to the motor from the battery back may be more than that.
The voltage coming from the battery pack and going to the motors is from 4AA batteries in series, which should be between 4.8V and 6V depending on whether you're using rechargeable or non-rechargeable batteries (you can see the nominal voltage for AA batteries here). If you're using rechargeable batteries, then the voltage should be ~4.8V, which is fine, but if you're using non-rechargeable batteries then the voltage could be ~6V, which is above the sensor's maximum voltage rating.
In reality, most electronics are really durable, and even if you hook the ultrasonic up to 6V it will probably be fine. But anything above 5V and you're taking a risk..
p.s. BTW, thank you for the detailed instructions, they're the best I've found online so far
You're very welcome! Don't hesitate to ask if you have any more questions!!