Hello:
I want a simple control for a small DC motor.

What I want to do is start the motor, have it run for about 2 seconds then stop. The thing is I want to stop it as acturately as possibly using a switch to cut off the voltage to the motor. But then I want to be able to start the motor, have it run in the same direction as before and then stop at the same switch.


I could use a timer and run the motor for 2 seconds and then stop when the timer "times out" but I'm afraid the motor will not stop at the same place each time and thus "drift" as it is started and stopped repeatedly.

Any thoughts are much appreciated.
Thanks in advance,

Richard

 

A flip-flop will do the job.
The attached diagram shows two switches. The START switch is a momentary switch to start the motor.
The STOP switch is being closed when the motor arrives at the desired position, thus resetting the FF.

Q1 and D4 need to be rated according to your motor ratings.

There are more accurate options but they require a different motor.

 

If you cut the power fed to an electric motor then it still spins as it slows down.
But if you apply brakes to a DC electric motor by shorting it then it stops almost immediately.

 

The thing is I want to stop it as acturately as possibly

You need to do it with stepper motor...simple solution but complex design.

http://www.allaboutcircuits.com/vol_2/chpt_13/5.html
http://en.wikipedia.org/wiki/Stepper_motor

Good Luck

 

If you cut the power fed to an electric motor then it still spins as it slows down.
But if you apply brakes to a DC electric motor by shorting it then it stops almost immediately.

Yes, that's true. However, if he uses a constant speed (and motor load) the distance travelled from the moment where the STOP switch is hit to the moment when it stops will be always the same. And if there is load connected to the rotor it'll stop quite fast.

 

We need more information about the motor. What type of a load is it driving? How long is the shaft? Is the load always the same? Why is position critical?

To get a positional stop on the motor 2 mini super magnets opposite each other can be attached to the shaft for balance and a quick 0.25 second pulse to 2 electromagnets set to align the magnets to the correct stop position after the motor stops would be satisfactory.

Secondly, a disk with a hole can be placed on the shaft. When the motor "stops" the current is greatly reduced so that the motor creeps until the disk hole lines up with a infrared transmitter and receiver to trigger a device to cut out all power to the motor.

Thirdly, a burst of reverse polarity pulses will also slow it dramatically. The power of the negative pulse, pulse widths and the total time of the pulses can be determined by a Dual 555 Timer, which can be controlled by pots.

The question is what is the tolerance of accuracy that we are looking for.
What we need to know is how far, or how may revolutions the motor travels after power is cut to determine the best method of stoppage and alignment.

Danny

 

Hello:
Thanks for all the excellent comments.
I'm a retired engineer DIY guy with some electronic experience so I follow what's being said.
I am familiar with stepper motors but don't want to use a stepper because of the digital control cost involved.
The motor is a small dc gear motor with 40 rpm output. The gear friction may cause it to stop repeatably but I am waiting for some parts before I can actually see how repeatable it will be. The will be a cam on the motor shaft to actuate a stop switch.
Praondevou: I think what you suggest is what I want but a few questions:
1. I want to understand the operation: Start switch closes, FF outputs a signal to the LOAD (the motor, which starts to turn), at some point the STOP switch is actuated by a cam on the motor shaft.resetting the FF, Load is disconnected.
To restart the START switch must be pressed again. is this correct?
2. After the first sequence the STOP switch is closed but because the FF has been reset depressing the START switch will start a new sequence. Is this correct?
3. I assume 4013BT_5V is a FF, yes?
4 What is Q1?
5. What does "C4 near U1A mean"?
6. V1 will be about 6v from a battery, OK?

Thank you very much
Richard

 

1. I want to understand the operation: Start switch closes, FF outputs a signal to the LOAD (the motor, which starts to turn), at some point the STOP switch is actuated by a cam on the motor shaft.resetting the FF, Load is disconnected.
To restart the START switch must be pressed again. is this correct?

Yes. It wasn't quite clear to me how you wanted to start the whole sequence, that's why I chose a momentary push button.

2. After the first sequence the STOP switch is closed but because the FF has been reset depressing the START switch will start a new sequence. Is this correct?

Yes. Actually, the position of the STOP switch doesn't matter in order to restart. Both switches give only a short pulse to the FlipFlop.

3. I assume 4013BT_5V is a FF, yes?
4 What is Q1?
5. What does "C4 near U1A mean"?
6. V1 will be about 6v from a battery, OK?

3. Yes it's a FlipFlop. Any FF with set/reset inputs will do it. I chose a CMOS IC because they have a wide supply voltage range.
4. Q1 is a MOSFET. I'd chose one in a TO220 package, if ever needed you can mount it to a heatsink. A common type is the IRF540, but there are many others. You need to check what's the easiest and cheapest available.
5. C4 needs to be mounted near the supply voltage pins of the IC. It's a bypass capacitor. Also, in the IC there is one more FF, it's inputs need to be tied to GND.
6. 6V is ok, but then you really need the MOSFET.

 

Praondevou: thank you and all the others that replied too.
The application is a simple one. I am making a diorama and have made a prototype of the part that requires positioning a figure. The mechanics and power required have all been worked out. The important thing is for the figure to move and them return to the same location. Accuracy of +/-1/16 of an inch is OK. My prototype tells me what I want to do works but as I said I have not yet run the motor to see how repeatable the figure will position.
My original thought was to us a 555 Timer and just let it run for a specific length of time. My experience tells me you don't obtain accurate position on a time basis, therefore, I was looking for a means which incorporated a switch for position but didn't know how to restart the motor in the same direction once it stopped.

Your answer is exactly what I was looking for.

This is a great site!! Keep contributing.

Thanks again,

Regards,
Richard

 

Praondevou:
Your solution got me to thinking about using a 555 Timer because I happen to have one.
Can I do as your solution does with the 555 and the STOP switch resetting it?
Regrds,
Richard

 

Can I do as your solution does with the 555 and the STOP switch resetting it?

I don't understand what would be the advantage. Do you want to start the motor by pressing a switch or after a certain time? Please explain in detail how you want it to function.

 

Using a small gear-head motor like Electronic Goldmine G18213 @ 3V about one rev. in 2 sec., with object at a radius of 1.5 in. would stop in 1/16 in if motor was dead shorted at stop signal; a diode short made little difference but is required if driven by 555. A 555 can be used as FF; momentary start SW gnd to pin 2 tied high thru 10k to +V, momentary cam driven SW NO from +V, capacitivly coupled, .1 μF, to pin 6 which is tied low with 10k to gnd. Output drives small relay with SPDT contacts. Motor to common, pin3 to NO, NC to ground. A short press of start SW, motor runs one rev. when cam hits stop SW,re setting 555, stopping motor dead in its tracks. Differentiated stop pulse keeps 555 from hanging up on cam SW.

 

Also look at Project post by meet, 555 timer project & esp reply by JingleJoe, edge triggering 555. Looks like I left out capacitor discharge resistor to ground, with out discharging the cap would remain charged after first operation, eleminating any suceeding stop signals.

 

Hello:
I was able to answer my own question.
A 555 Timer can drive a load up to 200ma. My small motor uses only about 100ma so I connected it to the output of the 555 that I have and it works just fine. I can reset it too with no problem.

If it had not been for this formum and Praondevou in particular I never would have thought of using the 555.

Thanks everyone!

Regards,
richard

 

Hello:
I posted my previous reply before I saw Bernard's and Praondeou's reply. I did ehat Bernard suggested except becasue I was using the 555 I had and it was set as a 5 sec. timer I used pin 4, Reset, to stop the motor. This allows me to go more than one revolution if subsequent events dictate that more than one is needed.
Praondeou: What I wanted to do was run the motor 1, 2, 3 revolutions, stop it and then some time later, maybe one to 10 minutes, restart it going in the same direction. The decision as to how long the moter should run will depend on finalizing my prototype.

Regards,
Richard