1 GLOLABTwo Wire Stepper motor Positioner1 Introduction_____A simple and inexpensive way to remotelyrotate a display or object is with a positionerthat uses a stepper motor to rotate it. Themotor is driven by a circuit mounted nearthe motor and by a control circuit at aremote location. Power for the motor and itsdriver circuit and for the signals that controlthe speed and direction of the motor are allcarried over a single two conductor is a device that will remotely positionan object on command to any desiredrotation at an adjustable speed in smallsteps. The object can then be left in thatposition until a different rotation is desiredor it may be continuously rotated in it works_____MotorsAlmost any two phase (sometimes calledfour phase) unipolar stepper motor with avoltage rating of from 9 to 24 volts and acurrent rating of 900 milliamps or less maybe used.
2 A unipolar motor has two centertapped windings with six leads and has itsvoltage and either its current or resistancemarked on the nameplate. Another motorcharacteristic is its stepping angle which isalso marked on the nameplate. An angle degrees or less is preferred becauseeach step is smaller but degrees or evenmore can be used. Although many surplusmotors come without a wiring diagram, youcan easily find the correct connections withan ohmmeter. Figure 1 is a diagram of atypical stepper motor . The windingresistance will be a few hundred ohms orless. To find the center tap first measurebetween any two leads. If you measure anopen circuit try again until you get a readingand then record its value.
3 Number theseleads 1 and 2. Connect the meter to lead 1and a lead other than 2 until you get anotherreading and then number it 3. If this value isthe same as the 1-2 value then lead number1 is the center tap. If it is twice the 1-2 valuethen lead number 2 is the center tap. Makea note of which lead is the center tap as thislead will be connected to the +V the above procedure for theremaining leads numbering them 4, 5 and 6to find the center tap of the second motor voltage is not very critical. Alower than rated voltage may be used with aresulting lower torque. You should expectyour stepper motors to run very warm oreven hot because power is applied to twowindings at all times. You may have seenspecifications or applications that use aresistor in series with the center tap of eachwinding.
4 Their purpose is to maintain amore constant torque at high speeds andare not necessary for low speed resistors are used, a higher thanrated voltage is applied and the excessvoltage is dropped across the resistor. Athigh speed as the motor impedanceincreases more voltage drops across themotor and less across the resistor tomaintain a more constant torque. A constantcurrent driver circuit would do the circuitFigure 2 is a diagram of the control circuitbuilt on a X inch PC board. IC1 isan LMC555 CMOS timer that generates a200 microsecond wide clock pulses to stepthe motor and control its speed. The speedcan be varied by changing the pulserepetition rate with R1.
5 The negative goingclock pulses at pin 3 of IC1 drive the gateof Q1, an IRL530N power FET thatmomentarily turns OFF and disconnects thedriver board from ground. These powerinterruption sends pulses to the motor drivercircuits that cause the motor to step. Motorspeed is controlled by the rate of theinterruptions and direction is controlled bythe polarity of the voltage applied to thedriver circuit through interconnect lines L1 GLOLABTwo Wire Stepper motor Positioner2and L2. Bipolar MPSA05 NPN transistor Q2and MPSA55 PNP transistors Q3 and Q4invert the pulse from pin 3 and pull the drainof Q1 UP when it is OFF. Pushbutton S2starts and stops the motor by turning theclock on and off. Toggle switch S1 sets itsdirection by switching polarity.
6 Power for themotor is provided by DC wall transformer T1and filter capacitor C2. Five volt regulatorIC2 and filter capacitor C3 supply power circuitFigure 3 is a diagram of the motor drivercircuit built on a X inch PC motor is driven by four IRL530N powerFETs having very low ON resistance of resulting in very little voltage drop andalmost no heat. These FETs have a logiclevel gate threshold making them ideal foruse in circuits powered by 5 volts. Theywere also chosen for their specifications of60 volts at 15 amperes. FETs Q2, Q3, Q4and Q5 are driven by CD4013 dual D typeflip flop IC3A and IC3B, each having dualphase outputs that latch either Q2 or Q3 ONand either Q4 or Q5 ON, depending on thestate of the flip flops.
7 Current flows throughone half of each motor winding at all flip flop has its data input crosscoupled to the other flip flop s outputthrough a CD4070 exclusive OR gate inIC4. The cross coupling between flip flopscauses them to change state alternatelywhen simultaneously triggered. Thesequence of change, IC3A before IC3B orIC3B before IC3A, is controlled by theexclusive OR circuits that feed data intopins 5 and 9 of IC3. This sequence changecontrols the direction of the motor . A DOWNor UP level passing from L1 through D3 todirection control pins 2 and 6 of IC4determines weather it will invert or non-invert the data applied to its input pins 1 and5. Both the clock and the direction controldata are carried to the driver board over thesame wires that power the motor .
8 If L1,figure 3 is positive then the motor will rotateleft when IC3 is triggered by a clock pulse. IfL2 is positive then the motor will rotate motor steps when the voltage across L1and L2 goes DOWN and then goes UPagain producing a clock pulse. The UPtransition feeds through D4 or D5 and R8,depending on the polarity of L1, L2, intoclock input pins 3 and 11 of IC3. Zenerdiodes D4 and D5 serve the dual purpose ofcoupling the clock pulses into IC3 andacting as a transient voltage voltage spikes caused by switching,that may appear across L1, L2 will passthrough the forward conduction of one ofthese diodes and through the reverse zenerbreakdown of the other, thus clamping theL1, L2 voltage to a maximum of about 33volts.
9 C4 delays the rise of the clock pulseso it arrives later than the direction controlpulse at IC4. D2 improves the discharge ofC4 through R9. Power is supplied to themotor and the circuits through bridgerectifier BR1 which rectifies the L1, L2 inputinto a positive output. Power for IC3 and IC4also passes through 5 volt regulator IC5 andis further filtered by C6. D6 clampsinductive spikes from the unpredictable motion may occurwhen the direction is changed. This iscaused by a pulse or pulses that aregenerated when the polarity is pulses appear to the circuits as clockpulses. This is usually not objectionablewhen the positioner is used to rotate adisplay or similar BoardInstall all resistors, D1, IC2 and allcapacitors except C2.
10 Install a jumper madefrom an excess resistor lead, the IC socket,terminal block, Q1 - Q4 and IC2. Install C2,the wires for R1, S1, S2 and T1. Connectthe wires to R1, S1 and S2. Solder the leadsfrom a DC wall transformer to the PC boardholes marked +V and -V. Use a transformerwith a voltage rating similar to that of thestepper motor and a current rating a littlemore than twice the motor current IC1 into its socket being careful tohandle it as a static sensitive device andyour control board is complete. The controlGLOLABTwo Wire Stepper motor Positioner3board may be mounted in a suitableenclosure with R1, S1 and S2 accessiblefrom the top or BoardInstall all the resistors, capacitors, diodesand IC5, then two jumpers made fromexcess resistor leads.