More H Bridge work

March 20th '06

Today I made the double NOT gate circuit for the motor controller.

For the first time I tried one of those blank predrilled PCB's from radio shack. What a total pain in the butt. It looked nice though and was less expensive and more sturdy than the acid etching stuff I've done before.

Here are the plans and the finished board.


Here it hooked up. It seemed to work for a bit but something went horrible wrong and fried the L298 H bridge chip. *cry* What a huge waste of time, I ordered a handful of NOT gate IC's that afternoon, along with a replacement L298 chip. The not gates are about 60cents a piece for a 8 gate chip, so why fight for hours building my own?


Thank goodness for the suction iron....


Per a request here are the notes I made for the H bridge and NOT gate stuff.
http://www.pasqualy.com/landraider/stepperplans.jpg

There may be errors on that page so make sure you double check anything I wrote down before you use it yourself.

Replacing the RDFR with a dual H bridge

March 17th '06

Alyssa and I worked on the robot today. I'm taking out the underused and overpriced RDFR speed controller and replacing it with a simple dual H bridge.


Alyssa says, "That's a price savings of about 300 bucks!"

The unit will mount in the front bay like the old one did, but forward of the motors.


Its is much smaller as you can see.


"Dad! Those motors draw 980 milliAmps each! You should add a heat sink!"


Right, here is the board with heat sink mounted on the L298 chip.


Fortunatly the motors have little fans on the back that kick out air in the direction of where the H bridge will be mounted, and there is an air vent just underneath of where the heatsink will stand.


Here it is mounted.


Here is the SV203 board driving the inputs on the H bridge, Alyssa was pretty close with her current draw estimate. The L298 can handle 2 Amps per motor.


Now here comes the tricky part.
The L298 has 6 inputs.
Left motor Forward
Left motor Backward
Left motor Disable
Right motor Forward
Right motor Backward
Right motor Disable

My SV203 controller only has 8 inputs total and I need 2 of them for the gun's pan tilt and another one for the trigger device on the gun. Plus I'd like to have another two for a second camera's pan tilt.

So I had to use some electronics wizardry(joke) to minimize the number of outputs used up by the H bridge (the RDFR used only two outputs Left/Right channel and Forward/Back channel all with automatic mixing).
So what I did was get it down to three outputs like this:

Left Motor Direction
Right Motor Direction
Left and Right Disable

For the left/right motor direction I simply used one line going to forward, and then did a logic inversion of that line same going to backward. So if forward is on, backward is off, if forward is off then backward is on.
Then I connected both sides disable to a single line, so either both are on, or both are off.

You can see me testing this with a NOT gate (or inverter) built from a resistor/transistor circuit built on the bread board. It worked great so all I have to do is build a board for it and stick on the robot.