Reversing motor direction, help needed

Thread Starter

KiwiKid

Joined May 12, 2011
16
Hi,
For a school assignment we must make a circuit that can raise and lower a barrier arm using a voltage drop that includes a LDR, a Picax 08M chip, a motor and a transistor (or 2) used as a switch. I know how to set it up so it can raise the barrier arm but I don't know how to reverse the motor without having to rewire it every time or using a switch. I have this idea
http://s1195.photobucket.com/albums/aa391/pieterthegreat/?action=view&current=circuit2.jpg
but i'm not sure it will work. If it will not can someone please send in a design of a circuit I can use.
Thanks
KiwiKid
 

R!f@@

Joined Apr 2, 2009
9,747
Welcome to the Forum.

That circuit is likely to end up in smoke.
To reverse motor effectively use a H bridge or Relays.
Or a chip designed for that purpose.

It will give u less trouble and easy PCB making
 

CDRIVE

Joined Jul 1, 2008
2,219
Are you saying that your instructor has limited you to two transistors max?
If so, the suggestion by dpeterson3 would be about the only way you could go. However, you would still need limit switching.
 

John P

Joined Oct 14, 2008
1,892
Hold on a second. For all we know, the motor is a very low-power type that can run on the outputs of the processor, no transistor needed. Or if it's legal to use a couple of extra resistors, two transistors are enough to build a reversing circuit. It won't be incredibly efficient, but for intermittent action it might be OK. You gotta think outside the box.
 

CDRIVE

Joined Jul 1, 2008
2,219
Hold on a second. For all we know, the motor is a very low-power type that can run on the outputs of the processor, no transistor needed. Or if it's legal to use a couple of extra resistors, two transistors are enough to build a reversing circuit. It won't be incredibly efficient, but for intermittent action it might be OK. You gotta think outside the box.
First of all the 08M is not going to directly drive ANY motor of any type or size. The output pin capabilities of a 08M barely suffices to drive a LED. It's a CMOS uC that requires drivers to do any power work.

Secondly, I'm sure there are many of us that are waiting with anticipation for you to post a schematic of what you have in mind. If not, I certainly am! :rolleyes:

Keep in mind that bold statements like this will invariably be challenged. When you make statement like this you should fully expect a "Show me"!
 

John P

Joined Oct 14, 2008
1,892
O what a world of skeptics we live in.

I went and looked at the specs for the PicAxe to see if they're different from PIC processors generally, and they're just the same. 20mA capacity source or sink. If you think that's not enough to run a motor, see here:
http://www.picaxeforum.co.uk/showthread.php?t=6920

And here's the two transistors plus resistors reversible motor drive. I didn't say it was efficient, but there's no reason why it shouldn't work. Did I not say, you gotta think outside the box?

 

CDRIVE

Joined Jul 1, 2008
2,219
John, I read the very surprising motor data in your Picaxe forum link and then I stared at your circuit thinking of something clever to say... How about "I'm Eating Crow"? :eek:

I confess that I had no idea that little milli-current motors like that existed.... Touché!!
 

John P

Joined Oct 14, 2008
1,892
Cdrive--spoken like a gentleman, sir.

I'm puzzled by "have to use a voltage drop" too.

Can't they at least give you some PNP transistors?
 

CDRIVE

Joined Jul 1, 2008
2,219
Your link is bad. We still don't know what voltage drop you're referring to and you didn't specify why you can't use John's circuit.
 

CDRIVE

Joined Jul 1, 2008
2,219
Your link is now working but I can't make sense of your schematics and still don't know what voltage drop you're talking about. Are you a verbal minimalist?
 

Thread Starter

KiwiKid

Joined May 12, 2011
16
The voltage drop I am reffering to is the circuit with the LDR in it. I will use that to get infomation on light levels, but that is unimportant as I already know how to do that, what I am concerned about is reversing the motor direction.

What about my diagram is confusing? When I turn a pin on it will 'open' the two transistors connected to it. The current will then flow through a transistor through the motor and down through the other transistor.

Will Johns circuit work? I am confused on how it will so? But if you can explain that would be great.
 

CDRIVE

Joined Jul 1, 2008
2,219
The voltage drop I am reffering to is the circuit with the LDR in it. I will use that to get infomation on light levels, but that is unimportant as I already know how to do that, what I am concerned about is reversing the motor direction.
Thanks for explanation.

What about my diagram is confusing?
You have conductors that appear to make connections to air. Since there's no antennas in your project this is not acceptable.

Will Johns circuit work?
If you use the same or same spec motor.. yes.
I am confused on how it will so? But if you can explain that would be great.
Since your working with microcontrollers in your class I have to assume that you've completed DC and AC theory. You should know this stuff by now. Think Ohms Law and re-read John's posts. He provided links that discussed a very low current motor that WILL work in his circuit.

I was going to post a schematic for you but my circuit is more complex than John's. If you can't grasp how his works how are you going to follow mine?
 
Top