Bi-directional motor drive on an 08M, brain failure.

[lauriet]

Hi. I've been thinking creatively about a little output pin shortage problem, so I've only got myself to blame for this:
I'm trying to run a model motor from an 08M via a L269D using PCM, and for space reasons don't want to go to a larger chip.

Essentially the L269D has two inputs for a motor.
Set one high and one low and the motor runs, set them low and high and it runs in reverse.
Feed PCM to the high input instead and the forward speed can be varied, as the input pulses from low to high

As the 08M only has one PCM output, I reasoned that that if I then set the second input high, the motor would run in reverse and its speed could be varied, using the Space part of the input instead of the Mark part as the pulse, as the first input could be seen as pulsing from low to high.

To my brilliant mind this should work. So far it doesn't. Could someone gently explain how unbrilliant my mind really is?

Thanks, and merry Christmas to you all.

 

[westaust55]

Can you please:
1. post a link to the L269D chip
2. Post the program code you are trying to use
3. post the schematic/circuit diagram for your project.

Then folks here will have the entire "picture" and will be far better placed to help you rather than guessing.

 

[SAborn]

Why would the PWM (not PCM) need to change it should be only a matter of toggling the other pin on the L269D high or low to change directions.

As westy commented we are really just looking at the Xmas tree decorations without some hard information to work with.

 

[hippy]

I couldn't find an L296D datasheet so a link would help there. If it's like an L239D then it should work. Perhaps also post your circuit and program.

 

[lauriet]

Thanks for your intimation that it should work.
I'll have to wait till after Christmas to get the data together (busy packing the sleigh with lots of toys for all the girls and boys).
Why I wrote L269D and PCM I don't know, I blame the fireplace brandy. I meant L293D and PWM, the standard setup as in the manuals.

So, once I get back I'll get the code and schematic checked and in order and get back if it still doesn't behave as expected.
Have a good one.

 

[MFB]

Think your having a problem that I often have, forgetting to engage brain before posting.

 

[SAborn]

With a L293 you dont PWM the input (A&B) pins you PWM the enable pins.

 

[westaust55]

A potential problem here is that in using the AXE020 protoboard for the 28X1 chip, the L293D enable pins are permanently enabled by default and the two PWM pins (PWM 1 and PWM 2) which are also Input 1 and Input 2 for the 28X1 are not connected to the L293D.

lauriet would need to be skilled at tracing PCB tracks and be willing to take a plunge to modify the AXE020 to conenct a PWM signal to the L293D pins.

 

[BeanieBots]

Think you might still be thinking about the 28X1 servo thread Westy!.
lauriet is using an 08M. They don't work too well in an AXE020 ;-)

 

[westaust55]

Think you might be right there BB - even if we stretch the legs out a bit

 

[SAborn]

even if we strech the legs out a bit

That might put a new meaning to "spread legged"