I want a high duty cycle...
No, you don't. You want a duty cycle that will limit the voltage coming out of the h-bridge chip to your motors to a maximum of 1.5V (presuming that's still the top voltage you want to run the motors at). What you're thinking of is the PWM
frequency, which does have to be relatively high to keep the motor rotation smooth (capacitors excepted to illustrate point). For instance, if the PWM frequency was 1Hz and the duty cycle was 50%, the power to your motors would be turned on for a half second, then off for a half second, then on again, and over and over resulting in jerky robot movement. The link you gave suggests a PWM frequency of about 150Hz, which would be okay for your application, but know that PWM frequencies for motors are usually in the 15,000 to 30,000Hz range. That way the PWM frequency is out of range for human hearing, so the impression is that your motor is running quieter.
I was just wondering, which potentiometer / trimmer should I go for?
At the heart of the 555 as an astable multivibrator (or oscillator) is the RC circuit. No, I don't mean
radio-
controlled. What I'm meaning is
Resistance-
Capacitance, which is a basic building block of electronic circuitry that is worth every tinkerer's time to check out, so do a search and read up. The nature of an RC circuit is that it oscillates the charging and discharging of the capacitor in a manner that can be explained in a simple equation:
where T is time in seconds, R is resistance in ohms, and C is capacitance in farads. Multiply the resistance times the capacitance, and you have the amount of time it will take the capacitor to charge and discharge once. This is what controls the rate at which the 555 oscillates, and consequently what your PWM frequency will be. Because of the nature of the equation, you can use a greater resistor and lesser capacitor, or a lesser resistor and greater capacitor, and still end up with the same frequency. Everything after that is just fine tuning. In short, the pots you have now should work fine as long as you can match them up with a capacitor lying around the house that will result in a PWM frequency you can live with. I still say go for the pack of 12 assorted trim pots from the Source, since they're nice and small in size.
I have some issues with the way your link set up the 555 timer, not that it's wrong, it's just not the way I'd do it. It just looks a little more complicated than it has to be. Search for some schematics demonstrating how to set up the 555 as an astable and compare. I've got a 555 here I'm tinkering with to PWM my nose clippers, so get one (a 555, that is) and start playing with it and we'll swap notes. Remember, it's the duty cycle you want to have control over, not the frequency, although you can use pots to control both if you wish. Use your voltmeter on the 555 output pin to verify the change as you adjust the duty cycle. Check the output pin of the 555 with a 330ohm resistor and an LED, and it should dim and get brighter as you adjust the duty cycle length (presuming the frequency is greater than 30Hz). If for kicks you drop the PWM down to 1Hz, then the LED will flash once per second, and the longer you make the duty cycle the longer the time the LED will stay on for each flash.
I was just randomly wondering (as I am breadboard person, not proto PCB person) that if you can just stick stuff into the proto PCB holes and have it connect, instead of having to solder it down. I've never used them and have always wondered!
Although I've seen proto-PCBs that have a configuration similar to that of a breadboard, even then you'd need just as many jumpers as you would need for the same circuit on a breadboard, and yes, you'd have to solder whatever you're connecting to it to make it "stick." So it's time to buy a soldering iron. I've never used proto-PCBs either, I'd just rather use a breadboard and then design a custom PCB if I needed it. I imagine you're going to high school soon, maybe next year? Technology classes vary widely from school to school in Ontario, so make sure whatever school you choose has a good electronics program and lab. Call the school to ask what they have, and try to arrange a tour/interview with the electronics tech teacher. A good high school lab should have its own PCB design and production equipment. Using your school's lab will save you the overhead of buying chemicals, boards, hardware and software for your own lab. Also, if you can convince your teacher to let you integrate your robot project into the course curriculum, you can work on your robot during school time
and get credit for it!