Yep, problem number one encountered (I’m sure there will be more).
It turns out that I have underestimated the amount of current draw on the drive motor. I wasn’t able to get an exact current rating of the motor but measuring current with the battery directly connected showed a 1600mA draw (11.5w). The L9110 DC Motor Controller I selected may not be suitable for this particular application.
The specs of the L9110 driver show Imax = 1500mA @ Vcc = 6v so unfortunately this means Imax = 1600mA @ Vcc = 7.2v would probably be ok as a max current draw for milliseconds only, not a constant draw (that’s rated as Ic = 800mA). As it turns out, setting PWM to 50% speed (with no load on the motor), gives an Ic = 1200mA, causing the L9110 to burn up and start melting the breadboard after a few seconds. I suspect that if Ic = 1600mA then there would be a loud pop and I would be picking pins out of the breadboard.
Unfortunately I’m going to have to find a substitute DCMC that has an Imax = 3000mA @ Vcc = 6v, or an uber-efficient cooling system. This means that, for now, Ripley is immobile until I find a suitable DCMC.
Oh well, plenty of other stuff to sort out. Next up, steering followed by front & rear sweeping ultrasonics (don’t panic, all will be revealed soon).