Motor Controller

[Hank Fletcher]

Get it, you want it, trust me. It'll work great with your PIC and your current motors. You might want a capacitor to smooth out the output voltage pulse to the motors, but that's about all else you might need. You're gonna have a blast playing with it either way, because one SN754410 equals four 1A relays in a compact space, which makes it really easy to add cool, high powered peripherals to a small 'bot.

 

[Omar.M]

So pretty much, get the chip, stick some caps on the motor lines and be happy? What's the chance of me being able to get them from Counterparts?


Thanks a lot for your help, I'll let you know how things turn out (hopefully very soon!)
Omar

 

[Hank Fletcher]

I think your chances with Counterparts are really good, especially if dad comes through with the credit card (come on, dad, it'll cost less than $10!). The only catch is their store hours - check it out, they're only open 9-5 weekdays, and for a short time on Saturdays. Toll free phone for Ontario residents, though!

By the way, if you find the idea of getting PWM from your PIC daunting, I thought of another way around it. If you have a 555 lying around (or want to get one from Counterparts or the Source) you could set up a pulse to the SN754410 to get the voltage limited to where your motor will run best. Make the calculations for the RC circuit, add your supply to the SN754410, and check the SN754410 outputs. The 555 pulse would go to the EN pin on the SN754410. The other input pins of the SN754410 would be connected to the output pins of your PIC. This is without a doubt the easiest solution (no PWM programming required). The only downside is that your robot wouldn't have autonomous control over motor speed, but it would over starting, stopping, and direction. You could use a trim pot in the RC circuit, though, to make it easy to adjust manually the motor speed. Get the SN754410, and then we'll talk about how to hook the thing up. If you want to be really sure that what's coming out of the SN754410 is safe for your motors, don't use trial and error. Check that the voltage is within the limits with your multimeter before connecting the motors!

 

[Omar.M]

Alright. Thank you very much. I'll beg and pester my father (it seems it is the #1 policy for anything!) and see where I can get with my conversation skills for the drivers.

Hopefully everything will turn out well, or don't you worry-- I'll be back here again .

Thanks a lot once again, your help was VERY much appreciated. I'll let you know how things are turning out.

 

[Hank Fletcher]

Absolutely my pleasure, but I want the payoff. Maybe you can make a vid of your final robot.

 

[Omar.M]

That I will do for you, sir. That I shall.

 

[Hank Fletcher]

I was just looking over the datasheet for the SN754410, and the extreme ratings for this IC. I can't see it having a problem with a 1.5V supply. It even seems that it's not too fussy about the logic voltage supply, either, but I don't recommend using anything lower than normal logic levels for that, just to make sure your data's always getting transmitted properly. Before trying any of the above work arounds, put 1.5V (your desired voltage) on the SN754410 supply, and see what happens at the SN754410 outputs with your multimeter. In fact, I think I'll go try that right now...

 

[Omar.M]

Thank you sir!
Tell me what happens.

Today's the big day I call them. I'll do that right after school.
Good day,
Omar

 

[Hank Fletcher]

Before trying any of the above work arounds, put 1.5V (your desired voltage) on the SN754410 supply, and see what happens at the SN754410 outputs with your multimeter. In fact, I think I'll go try that right now...

Well, I just gave it a try. Although it consistently delivers 1.5V if you use a single AA battery for the supply, I had trouble getting any current to drive a motor. I used a 5V regulated PSU to power the logic inputs. An LED lit up dimly, but there wasn't enough current to even budge the motor from my nose-hair clippers (the only motor I had around that uses a single AA battery). I don't know why that happens, but I guess we're getting into the grey area of using a component outside its normal specs. Pulsing the SN754410 for the desired voltage with a 555, however, would be within the h-bridge's specs. I'll try that in the next couple of days, pulsing the 5V regulated supply through the h-bridge to get 1.5V on the outputs.

 

[Omar.M]

-sigh-
I called them today, many many times. No one picked up on the toll-free and normal number.
Is there a chance this store is closed?

I hope they'll pick up next time.

 

[Hank Fletcher]

That's odd. Why not try Active since they're in Ottawa? They look like the same kind of store, and you'd be able to deal with them in person. Sorry, I know you asked about them before, but I've never used them so I can't tell you what to expect. They look like the kind of place that would order you that h-bridge, and there's no harm in asking. Start with the easiest solution first.

 

[Omar.M]

Hmm. I just called Active. The people were VERY nice, but they said they couldn't buy it for me because of the shipping charges being so high.

So I was talking and they said they did have a few H-Bridges /motor drivers around. Their website apparently is quite poor and not updated.

Anywhom, they seem to have L298N Motor-Controller Chip, which is used on the Solarbotics "Secret" driver kit. They also have that, as well as a few others. The L298N is $6.99 for one, which is not a bad price. What do you think, sir? Will it do?

Omar

 

[Hank Fletcher]

The L298N will do nicely as a substitute, and it's a good deal for their price. It's a bit beefier, and I saw that they had the L298N and was thinking of recommending it, but I was holding off because of the extra components it requires. You'll need some diodes and a capacitor for the L298N (whereas you wouldn't for the SN754410, because they're intergrated on the chip), but they'll only cost a couple bucks extra, tops. Read through the datasheet for the L298N, and take a close look at figure 6. It shows you how to hook up a DC motor, and tells you the extra components you'll need. Ask me any questions about that if you find the datasheet confusing.

Even though the L298N will operate (according to the datasheet) with a lower motor supply voltage than the SN754410, it still doesn't go down to your desired 1.5V. That's okay, because we can use our plan to pulse the SN754410 using a 555 with the L298N instead, and it should work pretty much the same. Also, the L298N is more fussy about voltages for logic highs and lows. Should I presume you're using a regulated 5V supply from a battery pack for your logic and motor power? Let me know what you're planning to use. Also, I'm curious to know what you're going to use to look at the line your robot will follow.

For the record, I think it's bad form for them not to take the time to order the SN754410. It's just bad business sense - they're in the business of getting customers what they want, not making excuses. Still, if that's the best they can do it's not a bad compromise.

 

[Omar.M]

Shoot. I wanted to mention that I want to hold off the pulsing for a while. I want to keep it to a minimum, the chips-- so I won't use PWM just yet.
So anything I do right now should be without the PWM in mind.

Argh. Nothing seems to be working right now. I want to use the least amount of components and such. Grrraaaaah. Can you have a look on the active site:
**broken link removed**
and tell me if they have any good drivers?

If not, the company seems to be good with NTE and NTE makes a lot of drivers. Would you know any good NTE drivers?

I am using bright LED as the emitter, and a Cds photocell with a resistor as the sensor. It seems to distinguish between the line and the ground pretty well (took some sensor readings from the multimeter) so it should be alright.

Of course I am using a 5V regulated supply. Breadboarding everything with lots of capacitors.

 

[Omar.M]

Just wanted to mention, they apparently have a lot of stock that is not shown on the website-- so if you have any good ones I can probably run it off them to see if they have it.

I agree about the business. What can I do-- I live in Ottawa. They are right to say that it'll cost them about 20 bucks for my order and they'll probably lose more money in the process. Well, they told me about losing 20 dollars, but not losing the money. That'd be pretty bad xP

Edit: By the way, if I get a good motor driver I will use separate power supplies then. Probably 5V regulated from 9V battery (for the chips and stuff) and a separate 3-5V setup that's not regulated for the motors.

 

[Hank Fletcher]

You're getting too obsessed with conserving breadboard real estate, unless there's some kind of prize for that for whatever competition you want to enter your robot in. I was wrong about the L298's minimum motor supply voltage, too - you'll need at least 4.8V for the supply voltage, and at least 7.5V if you're using 5V logic highs.

I'm not confident you're going to find anything better than the SN754410 pulsed with a 555 for flexibility in voltage output and conservation of space. If the folks at Active can't think of a better product for what you need that they can get for you, it's unlikely I'll be able to. Try to get in touch with Counterparts again, or try another Canadian supplier, or worst case scenario, order the SN754410 from the U.S.

Breadboarding everything with lots of capacitors.

Don't use anymore than you have to. That'll save some space.

 

[Omar.M]

Shoot. That really stinks. -sigh-

There's no way out this time.

 

[Hank Fletcher]

They are right to say that it'll cost them about 20 bucks for my order and they'll probably lose more money in the process.

No they're not! That's a bold-faced lie they're telling you because they're too lazy to do their job properly. That kind of baloney is reason enough for you to take your business elsewhere. On the other hand, if you're using a 9V battery, you could use that to supply the L298N motor power, and the regulated 5V for the logic part of your circuit. That'll work, but you'll need to drop a lot of voltage with the 555.

 

[Omar.M]

Well it seems any where I look the drivers are 4.5 V and 1A. This seems to be the standard. Looking at other forum posts and tutorials (at SparkFunElectronics) it seems people have used most of the chips properly without any extra circuitry.

Right now I am so confused, I am not even sure what to do. Active won't get the chip for me, grah. Not sure what else to do-- if it is even worth the hobby if I keep getting stuck like this.

I'm not sure, maybe active has some transistors so I can maybe build my own h-bridge like I started out to do. Gah, I don't even know anymore.

 

[Hank Fletcher]

Take a look at the SN754410 datasheet again before you choose to make your own h-bridge. Look at the internal workings of the IC and what you would need in terms of transisitors, diodes, and resistors just to make one input and one output (and that's just half an h-bridge, you'd have to build it twice if you wanted a full h-bridge). The SN754410 has two full h-bridges and no need for external circuitry, with the exception of your case because you want a compact h-bridge and low voltage. Don't look a gift horse in the mouth. You can get what you want with a voltage divider circuit, or with a 555, or use the L298N instead, or forget about trying to reduce the voltage and just get bigger motors. I think you know by now that my preference is for a 555 with the SN754410, and I think that's a pretty clever solution.

I really don't understand how you could be stuck at this point. I've given you plenty of solutions, you just have to pick one and do it. If you think someone's come up with a better solution on some other forum for what you're trying to do (low voltage h-bridge), I'd like to see it.