Thanks.... I'm going to point another thread to this reply, where someone is looking to drive some serious motors.

Have a look at the schematic in this tutorial, which is a pull-down example.

The reason, by the way, that it needs to be a resistor not just a piece of wire can be seen from that diagram. If it was just wire right thru where the resistor is, when the switch was closed the switch will cause a dead short between the 5v and ground. That will be expensive.

On the other hand, as a resistor, when the switch is open there's "enough" connection to ground for the input pin to read low. Then when the switch is closed, the plain wire connection to 5v "wins" the battle and there's a better connection to 5 than to ground, so the pin reads high.

I should always use the internal pulldown/pullups on digital inputs?

Internal resistors are pullups only.

Does this mean I have grossly underestimated the total current necessary?

You haven't told us what the current draw of the motors is...check their data sheets for stall current, add it all up, and you'll be able to answer your own question.

Also check that the 12-6V DC-DC converter can handle the servo current.

so should i use some transistors? if yes which one

I have no idea... 24A strikes me as huge. You'll need to wait for others to respond.

Edit.... take a look at reply #3 to this thread... afaik, the L293 and your LN754410 are essentially the same.

Well that chip's datasheet says it can provide 1A per driver, or 2A per motor so it's half what you say you need.

BUT.... the even bigger problem is the 24A at stall- you have to cater for that.

The signal on the Arduino output is 5V, but voltage needs to be measured against some reference, usually a 0V aka ground. So the Arduino "knows" its output is 5V since that's measured against its own ground.

But if you take that wire to some other piece of kit, it can only be deemed 5V if it's measured against the same reference. So if you tie the grounds together, everyone knows what the ground is, and all measurements are taken against that. So what the Arduino says is 5V is also 5V to the other kit.

doesn't go off when there is no one around

Well, how would you know? 

But, more seriously, why would it go off when no one is around?- it's a motion detector isn't it?

You should post your circuit and sketch, and a link to a datasheet.

Has it ever worked? Does it work under some conditions and not others? What have you tried so far, to diagnose?

Jim, I cant tell you how much I have appreciated your efforts, and I am really sorry I asked you for your help, I had no idea that it would be so difficult.

You're welcome. Sorry I couldn't actually help- seems my piezo is one of those buzz only ones?

if you go to the main project you can download the entire thing as a zip

Great, backed out a few layers and did that. Thanks....

You said a while back, in response to my suggestions to write a really simple sketch:

thanks. will try that as soon as possible

So, did you try that? Do you yet have a simple code that at least proves to you that you can run the two motors at the same time under simple circumstances?

So apart from current, the usefulness of the 293 is that the quad setup manages a 4-wire stepper. But for DC motors, it's basically the same to run 2x DCs on a 293 or 298, again, current aside.

Ok, I'm pretty sure I'm being really thick here, but I cant figure out how to copy this folder to my laptop.

You'll have guessed I've never tried to copy a github folder before. If I click the link it opens and if I drag and drop or right click and save as, it copies the link, not the folder.

Can someone explain how to do this, surely trivial, task please!

You would need to do a quick and dirty calc (if you didn't already) to check the worst case of a fast moving ball right over on the far side from the ultrasonic sensor.... will the ball still be there when the ultrasonic pulse gets there? (If I get you right, that the y-dimension is the width of the lane?)

There's a library over in the Playground....

I assume you are referring to the position, momentum issue, and not the observer effect.

Of course