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I recently had the same problem and just solved it. Push on the microcontroller a slight bit where it connects to the sensors. Sometimes the chip bends up slightly at the ends, and the connections between the pins on the atmega and those on the sensors data bus aren't secure enough.
...What do you mean check the voltage at the photoresistor input pins? Are you talking about the voltage that's going into the photoresistor or the voltage that's being outputted and going into the microcontroller? and how would I go about measuring this...
I was also wondering, should there be resistance across the + and - leads of the sensor bus when i use the multimeter? This is when the battery pack is unplugged but it goes to inifinity when it is plugged in.
OK next test Take your sensors off the robot. They should have three terminals a 0V, +5V and sensor output - like at the bottom of this tutorial page. For each sensor make the following resistance measurments: Between 0V and output.Between 0V and +5V.Do this with each sensor in the light, and again the dark, so there should be 8 readings. Post the results (you get merit for clear labeling ).
The resistor I used for the photosensors were actually 1.62kohm. Sorry, for sensor 1 I did mislabel =P. Is it okay for me to follow the same methodology for the bias resistor if my resistor is already so large?
Also I did the calculations for the requried resistor. I determined that the resistance of the photo resistor for the darkest light was 56k ohm and the brightest light was 10k ohm. This gave me a requirement of about 24k ohm. Is this a reasonable value?