Sunday, July 10, 2005

Introducing...The H-Bridge!

At this point, our first robot has motors to drive the wheels, a spiffy frame, and nothing else. We're going to need a way to distribute power to the drive wheels, under the control of the processor. The first step in this is to create an H-bridge. An H-bridge is a circuit that allows one to send power to a device forwards and backwards. In fact, an H-bridge has four operating modes: coast, forward, backward, and brake. I could try to explain the H-bridge, but other sites, such as, do a fine job, so I'll leave it to them.

Here's our H-Bridge. It is based on the International Rectifier RFZ44N power MOSFET. Playing with these MOSFETs taught me something interesting. Unlike a "traditional" transistor, which is current-based, MOSFETs are voltage-based. When you supply a voltage to the gate of a MOSFET and then disconnect it, the gate remains energized and power keeps flowing from source to drain. So in my H-bridge are four pull-down resistors to force the signal low when power is not being applied. Also note the diodes to keep the motor from pumping current where it doesn't belong. We chose the MOSFET because it is the same one used by the manufacturer of the cordless drills, so we know they can handle the current. The one issue is that the RFZ44N requires more than 5V to close, so we'll need to use another transistor to switch on the RFZ44N.

Sunday night's all right for picking

whoo-hooo my first post, Anyways back to business being a 13 year old I don't have a lot of cash lying around. Many first time robot builders don't want to invest alot of money into a robot until they are sure that is what they want. To reduce costs we do things like buy at discount outlets, garage sales and search trash heaps. Monday is our trash day so Dragonfly and I ride our bikes around the neighborhood on Sunday night looking for prime targets like fans, tape players, microwaves and old remote control cars. Besides getting the electronic components its amusing (and a heck of a lot of fun) to tear out the devices innards. Usually if you can get a microwave it has at least 3 switches, a circut board and display panel and a motor for the turntable. Robot parts are everywhere you just need to know where to look.


Nice ride huh? You might notice I've found a CPU and have it in the wagon. Lets hope it works.

Sunday, July 03, 2005

Back from vacation...

Just back from vacation. Haven't done anything lately, but I ran across another blog by another robotics newbie. See