Noot / Erwt Notes
Note #3 Saturday 17 September 2011
The 2xAA supply makes for some interesting power choices. If I work on the assumption of Alkalines, that’ll give a workable supply range of about 2-3V. Most microcontrollers will run down to 1.8V so that shouldn’t be a problem, but a most H-bridges are 3V+. This is especially true for the common through hole part: the L293D / SN754410.
Pololu claims 800mA @ 4.5V stall current for the motors, so in theory I’d need to be able to supply max 1.5A.
As a result, my two choices seem to be:
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Boost the battery voltage up with a buck converter and use that to supply the motors and logic. The Pololu 3pi Robot does this.
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Build an H-bridge from discrete parts. The BEAM guys do this, and for small motors it shouldn’t be a problem.
Noot
I think the deciding factor for the Noot is the FT2232 which doesn’t operate below 3.3V, requiring boosting. This shouldn’t be much of an issue actually, as I can use a relatively cheap buck converter to get me something in the 5V range, then regulate that back down to 3.3V with an LDO. There are a lot of compact 3V to 5V boost converters that switch at MHz frequencies (meaning tiny external inductor and cap) for Li-ion to USB conversion. I’ll use either the TB6112FNG (used on the Foxbot) or the LB1836M dual H-bridge ICs.
Erwt
Unfortunately, boosting doesn’t appear to be an option for the Erwt, as all the through-hole converters are either low current (won’t be able to drive the motors) or low frequency (huge inductors). However, you can do a super-simple 4 transistor bridge (from an old BEAM design):
This will be able to handle the 3V supply and 800mA stall (with the right transistors). The crossover design means I only need 2 drive lines per bridge, and its impossible to drive the bridge with magic-purple-smoke-releasing-inputs.
I’ve ordered a pair of motors and wheels from pololu as well as a LB1836M for testing.