What resistors do I need for a simple LDR triggered h-bridge?
From what I understand, you simply want to replace the switches in the H-Bridge circuit  with a photo resistor to trigger a motor while allowing it to be reversible. I used the Falstad circuit simulator to mock up this kind of circuit and I believe it may be what you are looking for. Below is just a schematic of it.







simulate this circuit &ndash; Schematic created using CircuitLab

As for the actual simulation, you can view it here. I could not find photo resistors, so I used potentiometers to mock the variable resistance of a photo resistor. To try out the circuit, use the Resistance sliders on the right side of the simulator. You will notice that the two outputs at the bottom of the circuit, which represent the motor output, will change voltages independently as the resistance of the photo resistors change. Keep in mind simulations do not often reflect reality, in this case it did. I built the same H-bridge circuit with potentiometers and two LEDs to show motor polarity (Their terminals are swapped relative to each other). Alternating the potentiometers randomly caused the the LEDs to light one at a time with a non-zero voltage reading through both LEDs.





Interestingly, when the potentiometers were swapped out for photoresistors the whole circuit failed. The potentiometers have a max resistance of 10k while the photoresistors have a max resistance of 8k. I doubt the 2k resistance difference is what causes this, but the nature of the two resistors themselves, please correct me if I am wrong. From this, it seems like the earlier comments are correct and you will not be able to use photo resistors to control an H-Bridge.





Using a microcontroller to interpret the photo resistor values and output PWM based on these values could be a solution as well for LDR H-bridge behavior. If you would like to get a head start on figuring out how to read circuit diagrams for the future, I would recommend this thorough tutorial by SparkFun.