Serial Controlled Variable Speed Motor

Control the speed of a small DC motor with nothing but the serial port on your computer, a single MOSFET, and some trivial software. (The MOSFET and the serial port make up the "speed control"; you'll still need a motor and an appropriate power supply for that motor; while the serial port can provide the voltage to turn a mosfet on and off, it can't supply the current needed by a typical motor.)

Step 1 Look at the circuit

We're going to do Pulse Width Modulation using a generic N-channel power MOSFET connected to the Transmit data pin from the computer's rs232 port. When the serial port is idle, the pin will sit at the "1" state, which by the time it's translated to rs232, is something like -12V (depending on drivers, it might be closer to -9V or -5V), and the transistor will be quite OFF. When we transmit "0" bits on the serial port, the rs232 pin will go to +12V or so, which is enough to turn on most mosfets pretty well.

If we transmit a lot of "0" bis in a row, the motor will be close to fully ON and the motor will run fast. If we transmit mostly "1" bits, the motor will run more slowly.

Comments

[cylove]

can i have the codes for this?thank you!

[westfw]

They should be right there in step 5 in the ComSpeed.zip file...

[profpat]

 greetings,

i tried this design using a desktop instead, an IBM P3 model 300PL, it didnt work, whenever i type copy 0.pwm com1:, the dos prompts writes " WRITE FAULT ERROR WRITING DEVICE COM1" 
abort, retry, ignore, fail?

in order it to work, i wired the 9pin dsub as  a null modem...now it worked..

to make a 9pin null modem..
connect pins 3 to 2, pins 7 to 8, pins 1,4,6 and 9...

now connect gate of mosfet to pin 2 and 3, and ground wire to pin 5...

it works great...

have a nice experiments..

[profpat]

 great! will try this, hope it will work on my laptop using a usb to serial port cable..

[MACKattacksnipe]

i used this for my pc tube light and giant cooling fan

[HADJISTYLLIS]

I founded one on old graphics card!

[geeklord]

Will one of these work? I found them on an old mother board.

[westfw]

http://www.datasheetcatalog.org/datasheet/cet/CEB7030L.pdf Should work fine...

[geeklord]

Thanks, what kind of voltage/current is at the gate of the MOSFET when its on?

[westfw]

See "step 1"

[TOCO]

I have downloaded your link. How do I use it? If I click on it, it will say illegal command or somthing like that. Do I need to hook up the motor first or can it be opened without pluging it in?

[westfw]

It doesn't need to have the motor first, but you may need to get visual basic studio and compile with the proper com port defined? I really don't remember exactly what's in there; the instructable was initially published over two years ago...

[westfw]

(I'll try to take a look at it if I get a chance. But I'm pretty behind on a lot of things right now...)

[TOCO]

Could you send me all of the software or links for the use of your serial port controlled motor? Please! I think that you should probably put the links onto the instructable so people can use it easier. I mean so they can build it and use it. And also, I have a few differend mosofets and they say different things on them. Can I use them or do I have to buy a new one?

[raj007]

yaa pls send me a sotware to control multiplestepper motors

[cupnoodles2]

Can this sistem controll 2 motors? i mean is there anyway? thanks my email is [e-mail] Thanks

[westfw]

Not really. (you could use two com ports simultaneously, I guess.) It's pretty much just a cute hack; probably not useful for anything real except to demonstrate the principle. For real PWM apps, you ought to go and put a $5 microcontroller in there... (alas, by the time you have the $5 microcontroller on a PCB with the extra electronics that will let it drive MOSFETs and talk to your computer, you're usually talking about something a lot more than $5. Which is one of the things that makes this particular hack so neat.)

[rab63]

thanks for prompt reply westfw. will look at getting a usb to serial adaptor. thanks again.

[moordere]

yes

[rab63]

hi, all How can i do the same using pinter port.

[westfw]

significantly harder, since the parallel port isn't "self-timed" the way serial is.

[Metaferia]

interesting ... just a quastion, how many volts can a serial port can give us? what about a USB port? thanks

[combomatrix]

Really nice thing to work on. Controlled by a program.... Is there any way to do this using an USB cable attached to a similar circuit from a USB port?

[westfw]

It should work fine connected to a USB->serial adaptor. I don't think there's a simple circuit that would connect directly to USB. There's too much software negotiation that has to go on.

[Derin]

the only way is a ftdt chip

[adi12104]

sorry about my previous comments i justy realize that i havent install the vb.net program so i cannot run the execute file and ask for some serial number by the way sir i still cannot find the 0.pwm 1.pwm 3.pwm 7.pwm 15.pwm 31.pwm 63.pwm 127.pwm in your zipped file

[westfw]

Ah. I hadn't realized that you had to install the shared libraries (or whatever) before it would work; they must have been present on the several systems I tried it out on (I think.) The 0.pwm and etc were examples; they should be easy for anyone to create from scratch. Although putting them in the zip file is a fine idea and I'll see what I can do.

[adi12104]

pardon me somebody please give me the file of 0.pwm 1.pwm 3.pwm 7.pwm 15.pwm 31.pwm 63.pwm 127.pwm by the way can any one tell me ho to use the com speed zip file because when i opened it it want the pass

[westfw]

That's odd; I just downloaded the zip to a different system, and it didn't ask for any password...

[Sonic]

how risky is this project? I mean there is some obvious risk of damaging the mobo or killing the computer... isnt it? ..or is it safe?

[westfw]

I would think that it is relatively safe. The biggest danger would be that you could somehow mis-wire things, and get the motor power supply connected (wrongly) to your serial port. Even then, serial ports are relatively "robust", and I don't think you should burn out more than the rs232 driver chip (which is before the more important functions of the motherboard.) I supposed that will really beefy motors and really beefy motor power supplies, there might be some danger of getting the backEMF spikes onto the motherboard, causing problems, so ... don't do that. Why do you think there is "obvious risk"? I could be missing something... A separate serial card or USB/serial converter would provide additional isolation, but sorta defeats the "simple and cheap" aspect of the project.

[Silence]

Use diodes... will isolate the wires from feedback.

[robin4286]

Is there a way I can use the serial port to turn a single LED on or off using similar methods? Thanks!

[robin4286]

hmmm okay so it would seem I have a problem: So when I copy a file (made in notepad, using alt+01 repeatedly, it is a .txt format) of 5000 bytes, in windows xp to com4, which is a USB-to-db9 serial adapter (by cables to go, if it helps), it sits there and does absolutely nothing, no errors, but also no confirmation of sending. I checked the settings multiple times and had them set properly. I used a multimeter to check charge and it stayed at about -6.10 volts(Or maybe positive, but I didn't think so). The multimeter(and an LED) were connected from pin 3 to pin 5. I think the actual problem was in the copy in dos, as it gave no confirmation as it did when I tried a parallel port, and basically locked up, so I had to close the window. Could it be that this does not work in windows XP? Any Ideas on what the problem would be? Thanks!

[westfw]

Did you use "mode" to set the serial parameters first? I've had similar problems with a usb->serial adaptor on a Mac; I wonder if the USB converter loses some initialization information in between "open" calls (you wouldn't think so, but...) Do you have a dumb ascii upload utility you can try instead? Someone that you know successfully talks to the serial port via USB (hyperterm?)

[robin4286]

Oh wow I feel like an idiot now... This feels like Java all over again (or javascript, or C#). 3 years of programming and I still forget the Semi/colons I forgot to put the colon at the end of the copy statement... Really sorry to trouble you!

[westfw]

You can probably get enough current from a serial port output pin to drive a (small) LED directly, and there are at least three pins you can use for output (Data, DTR, RTS.) After all, I made this "breakout box", and similar commercial devices used to be very common (and very necessary!) back when serial ports were common.

[foxwestjava]

Does anyone know the most universal mosfet series i use for that ? Thank's

[Sonic]

I still haven't tried this yet, but does it go backwards also? if not, can it be done with reprogramming? or do you have to use a different circuit?

[westfw]

No; you could probably add backwards with a DPDT relay driven from one of the modem control signals. It would have to be a relatively low current relay, though.