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Easy and Cheap Stepper Motor Controller

Do not contact (N4ZOU) on January 9, 2004
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Easy and cheap Stepper motor controller

You know how nice Stepper motors make driving a capacitor for remote controlling easy but the stepper motor electronics for controlling it can get expensive and hard to build yourself. Sure you can use the printer port on your computer with some transistors but then you need to drag it around with you if you go portable or even mobile!

Here is the cheap and easy answer to that problem. First of all you will need a suitable stepper motor. Stepper motors come in two types, Unipolar and Bipolar. As we are not going to control our motor with any electronics we must use a unipolar motor, which will have 5, 6, or 8 leads. Bipolar motors only have 4 leads. The controller for our stepper motor is a 12 position rotary switch, a fuse suitable for the motor you use and a single pole single throw on/off switch. Yes, it's that simple! Below is a diagram for a unipolar motor.

0x01 graphic

To turn the motor you apply power to one of the coils to pull a magnet turning the shaft one step. Switch that coil off and the next coil on pulls the magnet over another step. Leaving the power on locks the shaft in place. As we are driving a capacitor with our motor it would not be required to lock the capacitor in place and so an on/off switch would allow turning off the power and saving your battery pack if so equipped. As with any circuit a fuse should be provided to protect all the components. The 12 position rotary switch provides a way to supply power to each coil one at a time and in the proper sequence. The 12 position rotary switch provides 3 segments of 4 switches required to step the motor. Each segment will consist of switches labeled A B C and D in that order going around the switch. All switches labeled A will be common to each other, as will B C and D. The common connection point for the switch will connect to the on/off switch, the fuse, and then the positive side of the power supply or battery. The terminals marked 1 and 2 in the drawing above connect to negative of the power supply or battery. On 5 wire motors terminals 1 and 2 are common inside the motor. On 6 wire motors terminals 1 and 2 are as shown in the drawing. On motors with 8 wires each coil has it's on pair of leads coming out of the motor as shown below.

0x01 graphic

The complete drawing of the circuit is shown below.

0x01 graphic

All letters are common. Switch is shown in B position.

When you start to wire up the switch to the motor you will need to identify all the wires coming from the motor. As an example if you measure from A to B and then from A to 1 the ohmmeter will read half the resistance. Also note that even when you have a 6-wire motor leads 1 and 2 can also be common inside the motor. After you figure out which wire is connected to which coil set up a battery with a clip connected to negative and leads 1 and 2 and another probe connected to positive with a properly rated fuse. You will also need to stick a peace of tape on the end of the motor shaft so you can see it move and in which direction. Now label a wire A with tape or wire markers and touch the probe to it. You should see it jump a small amount and stop. Now touch another wire and note which direction it moves and how much it moves. If it jumps a single step clockwise or counterclockwise label it B and continue to another wire. If it jumps the same direction label it C, if it reverses label it D. Now you should be able to rotate the motor one step at a time and in the same direction by touching wires A B C and D in that order. If everything is working properly you can now move on to getting the 12 position rotary switch wired up. Use three jumper wires for each switch terminal listed as A as per the circuit drawing. Then B and so on for C and D. Below is a drawing showing the switch wired up.

0x01 graphic

Now build a box to contain your on/off switch, fuse, and 12 position rotary switch. If you like you can add a power on light or LED, a battery holder so the control box can be self contained or a power jack for external power. Also add a terminal connector to the back for hooking up the control cable used between the control box and stepper motor. You may also add a turns counter to the rotary switch so you can simply dial up a position later. To calibrate your turns counter simply rotate the turns counter until it reads 0 with the power off. Now rotate the capacitor by hand until it is completely unmeshed and at minimum capacitance. Later when you have the capacitor connected to an antenna you can recalibrate by noting where the counter should read at a chosen frequency. If it's off simply turn the power off and turn the counter where you would like it to read after setting the capacitor for a minimum SWR reading at the frequency of choice. You might have noted there is no reverse switch in the circuit. It's not required! The motor will rotate one way when turning the switch clockwise and reverse when turning the switch counterclockwise.

The box for the motor and capacitor should be weatherproof and provide a terminal for hooking up the control cable. You must also supply mounts and terminals for attaching leads for the capacitor to the antenna elements. You will also need a cable that will need a minimum of 5 wires used between the control box and motor box. Also be aware that some stepper motors might make a step that's too much for getting the SWR low enough for some antenna types. I use a 400 step per revolution motor with a direct drive to my capacitor with an insulator to electrically separate the motor from the capacitor and this is fine when used on a Butternut HF2V vertical even when used with the TBR-160 coil kit for 160 meters. With a small loop antenna or other very narrow bandwidth antenna you will need a gear drive between the motor and capacitor. There are stepper motors with a built in gear reduction drive if you want to spend the money for them. You could also use a belt and pulley drive or even find a toy gear drive that would work, after all we are trying to keep the cost at a minimum! I did see one setup that used some all thread connected to the stepper motor shaft and a nut connected to a plastic lever on the capacitor. Stops prevented over driving and damaging the setup. It would not move the capacitor a full 180 degrees but it was close enough to work with a small loop antenna.

Finding a stepper motor is easy! They're used in printers, fax machines, copiers, and anything that needs to precisely move something. Do a search on Stepper motors and you will find plenty of sites that sell them both new and used. Here are some interesting links you can try. Plenty of good information is contained in them.

Member Comments:
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Easy and Cheap Stepper Motor Controller  
by N6AJR on January 9, 2004 Mail this to a friend!
great article, and simple to do, if not a bit tedious.. thanks for the ideas.
RE: Easy and Cheap Stepper Motor Controller  
by AC5E on January 9, 2004 Mail this to a friend!
Well, you can do it that way - or you can use most any small 12 V DC motor that you can couple to a shaft and a "proportional controller." These apply a very low DC voltage plus a DC spike to the motor, giving very smooth low speed operation from high speed DC motors.

The model train set use them to bring their trains down to scale speeds - instead of rolling through the station at 220 mph. Most hobby shops specializing in model trains have these, at very reasonable prices.

I picked one up for about $30 and at dead slow takes about an hour to turn a capacitor from full open plates to full closed. Turn it up and you can get just about any speed you want - including a blur!

73 Pete Allen AC5E
Easy and Cheap Stepper Motor Controller  
by NE1RD on January 9, 2004 Mail this to a friend!
Nicely done. I might have considered putting in the ability
to have a means of applying a "holding current" once I got
that positioned... perhaps with a resistor in-line. So, you'd
step with full power and hold with the current-limited power.
[I don't know if I'm being clear... sorry.]

Anyway, I'm looking forward to the next installment:
the encoder for position reporting! Thanks again for the
effort and the nice article.

-- Scott (NE1RD)
Easy and Cheap Stepper Motor Controller  
by KZ1X on January 12, 2004 Mail this to a friend!

Thanks for the timely article.

I would have liked it even more had you included the sources for your 12-position switch and the 400-step-per-rev motor.

If it's the one at All Electronics, good ... if not, please tell more about the specifics of what you used, including power source, fuse type and rating, RF decoupling, connectorization and type of cable used, cabinet, etc.

73 Steve KZ1X/4
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