50$ Robot Background Info

[spizzak]

Hey everyone, I'm working my way through the 50$ robot, and its not very difficult to follow instructions/ wiring diagrams but I would like to know why I'm doing what I'm doing.

I've read through all the additional links provided in the guide but they all seem pretty generic. I was wondering if there is some sort of writeup on why these parts were chosen, what each of their purposes are in the circuit, etc.

For example how would you know what size capacitor to use if designing the circuit? Also, I don't really know what the ATmega8 does, or how it works, or what each of the pins are for beyond the short forms on the diagram.


Anyway, that's the kind of info I'm looking for, if anyone could point me in the right direction that would be great.  Thanks!!

[chelmi]

Regarding the ATmega, the answers your looking for are in the datasheet.

http://www.atmel.com/dyn/resources/prod_documents/doc8159.pdf

For the rest, you can have a look on the tutorials from SparkFun, it's very detailed.

http://www.sparkfun.com/commerce/tutorials.php

[SmAsH]

i think admin chose the mega8 because its a fairly basic microcontroller yet it still has enough general I/O to let you go out into the open world... plus, you can use the arduino code with a few modifications!
things like the led are just basic in their operation so you don't really need to have a reason...
the capacitor (220uf) is used to help with current spikes caused by the servos so the rest of the circuit doesn't go spaz,
when you turn a servo on, it draws heaps of current.... same with when it switches direction fast.
the capacitor acts as a reservoir so when the servo starts up, you don't see much noise. look at the big one on the axon!
the size of the capacitor depends on how much current you think you will draw, if your servos/motors are bigger/ you have more of them you will probably want a bigger capacitor, right?

the voltage regulator is another basic component, if you didn't have it your atmega8 would not be happy with you at all!
the headers are there because... well... what else is your servo going to plug into?
thats about all the parts on the board that need explaining... if i missed anything tell me.

[kpmcgurk]

If I am not mistaken when Admin made the $50 robot tutorial, the Atmega168 and Atmega368P where not yet on the market... grant it there where other AVR's he could of picked..

[paulstreats]

Also, I don't really know what the ATmega8 does, or how it works

Its like a little computer. You can write a program on your computer and then download it to the ATmega8. The programming can control the pins on the ATmega, so it can read information from the photoresistors and send information to the servo's

Once you get to the software section it will guide you how to do it, the idea probably sounds quite overwhelming if you dont have any experience with microcontrollers. (the light follower program is already written for you to download).

The important thing is to build it and run through all of the sections and you will learn on the way.

[VegaObscura]

Explain the resistors please.

[SmAsH]

the resistors provide resistance to the led along the currents path.
without the resistor the led would burn out because too much current would flow through it.
we know that resistance is voltage divided by current in amps so.
The formula for working out the resistors value is as follows:
supply voltage (+5V)-led's operating voltage (~2V)/current the led draws in amps (.02A)
so, the equation is, 5-2/.02=150. now, for these resistors, the value doesn't have to be spot on because
its just limiting the current so anything around that value will do, ie. 100-200 ohms will work.

[kpmcgurk]

EDIT: Smash posted as I was typing