Where should I shop for electronic components?

[ArtOfWarfare]

I'm building an automated lawn mower with a group of people in school and we keep having a problem finding a good place to buy electronic components from. Some things that we need but haven't been able to find:

1 - A simple, large red push-on push-off button. Right now we just have wires that we physically have to connect / disconnect running between our main motor and our battery to turn it on and off. I want something with 2 terminals on it that I can just insert in the middle of the red line to connect / disconnect the battery. It's going to be used both as the standard way of starting/stopping the product, as well as the emergency way, so I don't like the buttons with the twisting mechanisms or the smaller buttons - I need to be able to reliably hit it in an emergency, meaning I should be able to hit it with an open or closed fist, or be able to step on it, or hit my head against it, or swing a hammer at it, or whatever - the smaller buttons are too small a target to hit (especially when you consider the fact that the lawn mower is driving itself.)

2 - Large stepper motors. They need to be able to drive the 7" wheels on our mower (which weighs about 70 pounds.) Thus far everything we've found is intended for much smaller products.

I'm having trouble finding these kinds of things anywhere online. Everywhere I search is giving me tons of totally unrelated products… I need someone to point me to a simple website that specializes in selling this stuff.

The button at the bottom of this page looks pretty good, but I don't know if the connections are adequately simple, plus how the heck do I buy it even if I want it? They don't list a price and there's no "Buy me!" or "Add to shopping cart" button or anything along those lines.

The page I'm talking about:
http://www.moeller.net/en/products_...mand/control_circuit_devices/stop_buttons.jsp

 

[ale500]

On the motors, I'd go with normal DC motors. You don't need steppers. You have to be able to move your mower at a predefined speed. You look for torque and not physical size. Your mower weights 70 lb... but how fast should it move ? that will tell you how powerful the motor(s) have to be. Controlling a DC motor with PWM brings low revolutions with high torque.
Those red buttons look ok.

You may get better answers in fora dedicated to building electric karts and scooters.

 

[ArtOfWarfare]

On the motors, I'd go with normal DC motors. You don't need steppers. You have to be able to move your mower at a predefined speed. You look for torque and not physical size. Your mower weights 70 lb... but how fast should it move ? that will tell you how powerful the motor(s) have to be. Controlling a DC motor with PWM brings low revolutions with high torque.
Those red buttons look ok.

You may get better answers in fora dedicated to building electric karts and scooters.

The motors need to be capable of moving the wheels both forward and backwards. We're starting from an existing lawn mower and just adding motors to its front wheels - we figure if we can make one wheel rotate forward and the other rotate backwards it'll allow it to rotate in place. It already came with powered rear wheels - we'll just use those to drive the mower forward.

 

[Astroboy907]

Sparkfun
Taydaelectronics.com
Digikey
Mouser
Jameco
Makershed
Adafruit
Element14 (Farnell internationally).

Let me know of you need more places!

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The motors need to be capable of moving the wheels both forward and backwards. We're starting from an existing lawn mower and just adding motors to its front wheels - we figure if we can make one wheel rotate forward and the other rotate backwards it'll allow it to rotate in place. It already came with powered rear wheels - we'll just use those to drive the mower forward.

Having the rotating wheels placed diagonally across the mower would likely be better.
Make: magazine had an issue where someone built an autonomous lawn mower, might be worth a look.

 

[chown33]

The motors need to be capable of moving the wheels both forward and backwards. We're starting from an existing lawn mower and just adding motors to its front wheels - we figure if we can make one wheel rotate forward and the other rotate backwards it'll allow it to rotate in place. It already came with powered rear wheels - we'll just use those to drive the mower forward.

Unless you can lift the rear wheels off the ground, I don't see this working.

To pivot with only two counter-rotating wheels, the other wheel(s) need to be casters, or not contacting the ground. The rear wheels won't roll in the direction that the rear end needs to move in order to execute a pivot. I suggest making a small non-powered model, maybe using a toy bulding-system like Construx or Lego, and observing the amount of drag perpendicular wheels have when trying to pivot only the two front wheels.

Years ago, I saw pictures of a device that had a 5th wheel (a caster) on a pivoted arm. The arm was raised and lowered by a medium-sized RC servo. It would roll perpendicular to the main drive direction when the arm was down. It raised the two rear wheels off the ground, enabling the two front wheels to act as pivots (tricycle gear). I think only one of the front wheels had a motor, but it was bidirectional. The other front wheel had a brake, so it was always the pivot point.