Are you confident in the readings from the cell?

If there's any doubt, I'd be inclined to use serial printing to the monitor to confirm to you what the reading is, ala this tutorial. Then I would also use a print inside each "leg" of the if, to shout "I'm in the if leg" or "I'm in the else leg" so you can verify that your logic is taking you to correct parts.

Unless you are hell-bent on a diy solution, I'd recommend going for a turnkey solution from whomever supplies this kind of thing for marathons and cycle-races.

Can I connect the two L298N to  Arduino?

it looks like you run out of pins to use

But remember different Arduinos have different numbers of pins.. 14 on an Uno which Geoff presumably based his numbers on, in the absence of the OP saying what his board is, but 50+ on a Mega.

In the future, consider an online LED current calculator such as this one:
http://ledcalculator.net/

He did.... mentioned this one in first post.....

Not quite true, the voltage will drop as the current approaches the maximum that the batteries can deliver.

Yeah that's actually what I meant... I should have said "nominally" at 6V

I was trying to point out that in parallel, a number of motors are all still across the 6v terminals and are at 6v, but yes, subject to the drop as you say.

For more info on PID go here
http://playground.arduino.cc/Code/PIDLibrary

Or if you like your logic fuzzy, have a look here

[volts]

ok then, so each servo i should put how many volts? so one servo around 4 pieces of AA batteries? so the total is 24 batteries?

If you put 6 servos on one pack of 4 batteries they'll still be at 6 volts each.... but it's the current that will be the limiting factor.

You would need to experiment with that, because it depends how much current each draws compared to the current available from the batteries. You might find you can run all of them from one pack of 4, or might need say 2 packs running 3 each....

I gather that the PWM would be in the range of 0-255 when writing out to it from the ardunio?

Yes.... except that the Arduino PWM is under 500Hz, and the datasheet says it needs 25kHz PWM. The other ref I gave says it must be between 21 and 28kHz....

The datasheet shows the relationship between duty cycle and speed. PWM = 0 is 0%, 255 is 100%. As you'll see, PWM duty cycle below 30% (~80) still gives 1200rpm; above that it looks more or less linear up to full speed.

Generally it's marginal to run even a single servo from the 5V supply on the arduino

In this thread, post #19 there's a pic of my servo connected to a plastic hose with a string to bend it like a finger... it was drawing almost 500mA to do that.

warboyzzz, you should test this yourself: just put an ammeter in series with a servo power wire, and hold the servo and horn in your hands to try to stop it turning- the stall current is probably higher than you expect.

wildbill's right: even one servo is dodgy...

[to]

Still looking for code to read the PWM from the fan though....

Surely the PWM is the signal you provide to the fan to adjust its speed?

The green wire is the tach, which you would presumably read if you need to know what the speed actually is.

Have you seen this?

[absolutely never]

1.  Shorting a power supply should absolutely never be considered "Ok".  Don't do it.
2.  You should absolutely never change connections while powered.

Experience has shown me everyone ignores #2 until they finally blow up something they can't afford to replace.

And as a corollary to that, there was a thread recently suggesting always to load a sketch like bare minimum each time you're done, so that next time you power up you don't have to scratch your head wondering what pins are doing what. My bare minimum also puts pin 13 low- that LED is often on after a power up.

the site calculated 1/8W for both red and blue, but I can only find 1/2W. Would that be cool?

Yep... a resistor's wattage is the power it can handle, so by going up to a bigger one you're safe. You might waste a few pennies- which would matter if you were in factory production and buying zillions. Higher the wattage also means slightly larger, so if you're building a circuit in a restricted space, they do take up more room.

John said:

You might want to buy an assortment or at least get enough of each value you order that you will have plenty of spares. Perhaps 25 of each value.

Have a look at this "book" of 500x 1/4W resistors at Sparkfun

Hello all....

I have a salvaged Mitsumi M42SP-13NK stepper and have a pdf of the maker's datasheet.

Trouble is, the datasheet (see snippet below) indicates that there are both 12 and 24 VDC versions of this motor, yet there are no voltage indications on the motor itself.

Now, I don't need this motor to do anything other than run, to experiment with driving it, so I'm not fussed about torque, speed and so on. I have only a 12V supply. I'm guessing if it's a 24V version, and assuming it actually turns at 12V (I'm yet to test) then it's ok to use it like that at least for "demo" no-load purposes?

On the other hand, is there any way of telling if this is a 12 or 24V motor? (If I were to apply 24VDC (not that I have a source) and no smoke is released, for example, could I "deduce" it's a 24V one?)

(As an aside, the datasheet says 4.2 ohm coils; the sticker on mine, and my Fluke, both say 7.5)

Well they're certainly not words....

Yep I've seen that or similar definition before...

A word [it says]

But if...

You don't have to be able to sound it out,

.... then that's not a word, in my thinking