noob needs help

[mikgol]

Hi Everyone,

I'm mick from australia. I'm a software guy who loves electronics - I especially love controlling motors etc. through software (I write software for a living)

I've got a little project going, and am pulling my hair out due to my total noobness on the electronics front. I'm hoping one of you kind electronics people will be able to give me a hand

I put together a K8055N (usb experiment board with open collector outputs) and I'm trying to make it control 2 dc motors and their directions.

The K8055N works great, and I have no problem switching the open collector outputs on/off through software. But I'm at a loss as to hook it up to my 2 DC motors so I can control their direction.

I have a h-bridge chip that I tried to hook up with no success (the outputs on the board are open collector, and I'm at a loss as to how to supply power and send a signal to the h-bridge chip).

I also have some diodes, and was hoping to find a circuit that will let me control 2 DC motors (forwards and backwards) with 4 open collector outputs and just the diodes. After hours of googling I'm about to give up, but thought i'd come here and see if anyone can help.

Is it even possible to control a dc motor without a h-bridge? I was hoping to just arrange the diodes in a certain way so I can use output 1 to make one motor go fwd, output 2 to make it go backwards, and the diodes somehow (magic?) let this happen.

If it's possible I'd like to make it work with as simple of a circuit as possible (just the power diodes?)

Please forgive me if this is a stupid question - I know alittlebit about electronics, but not that much (obviously). Any help and advice would be very much appreciated. Even if it's just "it's impossible to control a dc motor direction without a h-bridge".

In case your wondering - I'm building an internet controlled tracked robot that I can drive around my back yard and cut the grass (very slowly, but from the comfort of my PC or TV)

Cheers,
Michael

 

[BobK]

You need an H-bridge.

The trick with open-collector outputs is that they can only make something low. When they are set to high, it is like and open connection (hence the name). So if you want to a signal to go high and low, you need a resistor from a positive voltage connected to the open collector out. Then, when the open collector out is high, your signal line sees V+ through the resistor. When it is low, it sees ground through the open collector out. So use a resistor this is capable of holding your H-bridge input high on each open collector out connected to the H-bridge.

What H-bridge chip are you using?

Bob

 

[mikgol]

Hi Bob,

Thanks so much for your explanation. The h-bridge I'm going to use is L293LE.

I managed to get it working without the h-bridge using just power diodes, 2 STSP relays, and 3 separate power sources. Using 4 of the open collector outputs on my interface card I can now control each of the 2 tank tracks independently (backwards and forwards).

It's a pretty hacky solution I guess, given that I need 3 separate battery packs, so I'll make the effort to replace this with the h-bridge and rig it up like you say Thanks again for your explanation - I appreciate you taking the time to help

Cheers,
Michael

 

[BobK]

3 supplies is a little extravagant, 2 should have done it. With dual supplies + and -, you can use a half-bridge in place of an H-bridge, which is what I expect you built with your 2 relays. An H-bridge allows you to send either polarity to the motor with a single supply.

Bob

 

[mikgol]

Hey mate,

Thanks for your suggestion. I re-did it using just 2 power supplies as per your email, and 2 solid state relays.

Quick question - I'm running 2 engines directly from the open collector outputs and have put a power diode in there to stop problems as the motors turn off.

This seems to work fine, but is there a risk of damaging the USB board this way? Or is it fine to use open collectors like this?

I don't have any problems running the tank tracks like this, but hooking up another dc motor in this way (for a spinning blade) seems to stay switched on even if i turn the open collector off (I've got a diode for this one too - should I use relays all the time to drive dc motors like this?)

My robot is almost built - I'll share a link to a video once it's done

EDIT: I forgot to ask - what's the best way to connect a servo to this board (K8055N)? It has 2 PMW outputs