Help building REALLY simple brain. (I wouldn't even call it a brain)

[wajoe]

Hi everyone, fond this site while googling things for a project and though I could get some help here.  Here's a little bit of background stuff: I did the whole iraq thing instead of college, so now that i'm back in the swing of things school wise I've found I can't remember simple things from high school tinkering(both electronics and robotics classes).  I'm in an introductory engineering class and our first design competition is a cable car drag racer jousting thing of sorts.  Say that 5 times fast.  There are 4 cables (2 for us 2 for the other guys) maybe 40ft long, with a ring in the middle.  Whoever gets the ring wins.  Weight limit is 600grams(1.32lbs), so that means I need to harvest 20ft of raw cable traveling power from something the size of my boot.  Drive can be spring/rubber bands/whatever but electronics are limited to common AA's in no more than 6V setups.  I'm leaning towards electric drive for the sustained power over the distance it's got to go, but we're not allowed to do any controlling other than releasing the thing.  I'd rather not just have someone catch it at the other end or let it slam into the cable anchors

Is there a simple circuit I could build that just cuts power after x amounts of seconds?  Maybe with a potentiometer to adjust it to the performance of the machine.  That'd give me a coasting chance before blowing the gearhead motor apart when someone catches the thing on the other end.  Once again, I'm restricted to 6V of AA's, so within the weight limit at most I'll have 10 AA's, 2 sets of 5 in series linked in parallel.

It's either this or a small egg timer/clockwork motor that physically trips a kill switch:P

Thanks for any help and suggestions.

Roger

[waltr]

Lots of possibilities.
A mechanical timer could work or a 555 timer chip.
I would go with a small PIC processor. The 8pin PIC12F508 are a favorite of mine for simple timing tasks.
Check the gooligum tutorials for starting:
http://www.gooligum.com.au/tutorials.html

[wajoe]

Thanks for the jump start on my brain.  Just read wiki on the 555 circuit and it was bringing back vague memories of my electronics class 10 years ago, but mostly I remember the burnt out hippy teacher who smelled like stale coffee:P  The only recent electronics stuff I've done is bigger scale and a lot simpler, rewiring motorcycles.

Looks like I'd need to run a relay since wiki claims the max output on a NE555 is 200mA(and an IC pin is smaller gauge than I'd want to run for my motor wires anyway).  Any guidance there on what I should ask for at radio shack or will it be pretty obvious/no choice really based on voltage and current ratings?  The motors I'm looking at have stall current of 1.3a and I'll be running AAs in a 6v set-up.  With the relay, would I want to run the 555 on it's own battery or will it be ok piggy backed off of the main power supply?

Same thing goes for the resistor and capacitor, any suggestions where to start or will it be pretty obvious based on what's in stock for a potentiometer and the time=RC*Ln3 formula on wiki.

Sorry to be asking easy questions I should be answering myself with a bit more research.  My brain is just toast from trying to jump back into school like this and I don't really have much left in me for tutorials and such on the side(18 credits of physics/calc/engineering classes and my break from gears and numbers is Spanish 2).

Thanks,

Roger

[waltr]

There are many transistors both bipolar and MOSFET that would easily handle 1.3A. Either could be switched from the output of a 555 or a micro-controller.

[Admin]

Put a kill switch at the front, so that when it rams into something, the switch cuts the power.

Can't be any simpler.

[wajoe]

Yeah, that was one of our first thoughts actually.  I even have an extra pull-on switch around here somewhere, but if I can't find it I can just pull the headlight switch one out of my truck:P  We opted for some Copal motors though since we're going pretty light with this thing.  That gives us 2 motors for the weight of one of the burlier ones we we initially looking at, but now the concern is blowing the plastic gearheads in a sudden deceleration.  They also have higher RPMs and slightly better stall torque ratings at 6V than the "beetle weight motor" we were also considering, although the retailer warns "Avoid stalling, to prevent gear damage."  And they also recommend 2"drive wheels or smaller.  Usually I'm all in favor of the most overbuilt drive train possible, but speed is what's gonna win this so we're taking a lighter approach with a price tag in the durability department. It's not a fighting bot, so there shouldn't be any need to worry about serious impact unless we inflict it on ourselves.

Back on the subject: At this point a monostable 555 sounds like all we really need.  The master power button will still be a pull switch as a back up(if we get a little too cocky adjusting the Resistor Capacitor part of the 555 circuit) and then we'll put the 555 trigger switch on the side or bottom.  With those light duty Copal gear heads, is there any chance we need to ease them into full power?  I'd think no since we're only hitting them with 6 volts anyway.

[Soeren]

Hi,

You might consider a Sharp distance sensor to detect when your vehicle is a bit more than the distance from the end that it takes to brake it down.