[Gghc-discuss] Proposed initial purchase - too much?

[Jonathan Manton]

I put together a Google Docs spreadsheet with my proposed initial  
purchase of stuff.
https://spreadsheets.google.com/ccc?key=0Atq-9wMS4xTIdGVpbnhuMmpOVmo2cGFHWEliYlBsbEE&hl=en&authkey=CNuInqUO 
  (read-only link - send me your email and I'll share it with you read/ 
write - didn't want to open it up to public edits).

As an explanation, the "base station" is the part that hooks up to the  
laptop, the "proto robot" is a fully-functional robot with motors and  
such, the "pseudo-robot" is a robot without a base or motor (but with  
battery/charger) that we can move around by hand, and the "training  
station" is an Arduino + XBee (no battery), for use in learning about  
the Arduino.  The "pseudo-robot" and "training station" can be turned  
into full robots later with the addition of a base, motors, motor  
controller, and battery (for the training station).

It comes out to $451.93, which is a distressingly-high percentage of  
our overall budget of $900.  And that is before shipping, which will  
probably be $30-$40 if we get 2-day shipping (the order comes from two  
different vendors, $15-$20 each, 2-day so we can start this weekend or  
Monday at latest).

Two things that have kind of high costs are the Arduino board ($30  
ea.) and the XBee module + shield + headers ($50).

We need to decide as a group how we want to end up spending the $900  
(at a high level).

(option 1)
On one extreme, if we went with the configuration I'm proposing for  
everything (we don't build custom hardware at all), then each base  
station costs $51.85, each robot costs $191.64, which means we would  
have enough for the two base stations and 4 robots.

(option 2)
At the other extreme, the base stations cost the same, but we design  
the robots from scratch (no prototype platform), and can probably cut  
the robot cost down to about $100 each.  We'd end up with two base  
stations and 8 robots.  But then we have no Arduino boards for people  
to learn with, and probably 2 weeks lead time (minimum) before we'd  
have the first prototype robot up and running.

(option 3)
The middle route is to go ahead with this initial order.  We'd have  
about $420 left (depending on how much shipping is).  That leaves  
enough for the other base station ($51), enough to turn the "training  
station" and "pseudo robot" into real robots (~$70, since they both  
already have an Arduino and XBee), and build 3 robots from scratch at  
$100 each.  At the end we'd end up with 6 robots and the base stations.

(many more options possible - this is just what I came up with)

My personal opinion on this is I don't want to buy everything (option  
1) because I want to help design the custom robot and make the circuit  
boards and stuff (this is what Erich articulated yesterday and I  
strongly agree with him).  But I don't think we should try so hard to  
save money by going with the cheapest route (option 2) that we run out  
of time (and have half the people on the project waiting around with  
nothing to do until the robots are built, and put pressure on the  
people doing the hardware design... in parallel with getting ready for  
Mini Maker Faire U-C).  Even though this consumes more than half our  
budget up-front, I'd advocate for the middle road (option 3), and go  
ahead and make this purchase now.

One thing we *could* do to save money is not get the LiPo battery  
packs, charger, or switching regulator, and instead use 4 AA batteries  
for each robot.  This would save about $80 from the initial order  
(which is then compensated by having to buy batteries).  That might be  
OK for development, but for the final product we want batteries that  
are light and can go through lots of charge/discharge cycles, rather  
than disposable AA batteries.  Even rechargeable batteries will be  
kind of a pain to deal with if they have to be removed to be recharged  
- 6 robots x 4 batteries each is a lot of rechargeable AAs to manage.

Personally I don't think getting AA batteries rather than rechargeable  
LiPoly batteries is a good idea, because we'd then end up spending  
money on something that is consumable (batteries).  In the stuff I'm  
recommending, everything is used for the end product - there are no  
parts that are used *just* for development (with the exception of a  
couple of USB cables maybe).