dear friends

Previous lesson the teacher mentioned a problem about our batteries. We use 4 times AA batteries,
so: 4 x 1.5V =6V. We need the 6V for our motors, and we convert the 6V to 5V to feed our microcontroller and h-bridge. But these are the values in perfect conditions. If the batteries are a few minutes in use, and has lost some power, it could be that we only have 4x1.2V = 4.8V left. This situation will cause problems. So there are a few solutions, we summarize 3:

1. In place of converting the 6V to 5V, we can also convert it to 3.3V. This will give no problem for our microcontroller, but our h-bridge needs to be feed with 5V. So we have to place another h-bridge. In the worst case scenario (4 x1.2V=4.8V) we also have not enough voltage for our DC motors. So the problem is not completly solved out.

2. We can also use a DC-DC convertor, it will convert the voltage we get from the batteries to the wanted 5V. We can buy a DC-DC convertor in an ic, but they are very expensive. An other solution is to make the circuit ourself. But this will cost us a lot of space on our print, space we don't have.

3. The last solution is to place 6 batteries instead of 4, then we have in perfect conditions 9V, and about 7.2V (6 x 1.2V) in the worst situation. On our print we have a high class voltage convertor who can convert these voltages to 5V. The voltage for our h-bridge and microcontroller is already insured.
Now is the problem that the motors get in the worst situation 9V, which is 3V to much. In fact it's not a big problem, because we are forced to use PWM (pulse width modulation). Our motors will run in freewheel at 4m/s, but the top speed only need to be 2m/s. So it's necessary to use PWM to lower the voltage and speed of the motors. With this in mind, the voltage of the motors will never be a lot higher then the allowed 6V.

The easiest and cheapest solution will be the third, place 6 batteries instead of 4. Another advantage is that the robot will lasts longer without chance the batteries. It seems quit clear that we go for the third solution.

greets
Stef