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This photo shows the stepper motor PC board teamed with a small stepper which could be used for a variety of tasks. Note: D3 & a 10µF capacitor have been added since this photograph was taken. This circuit will drive a stepper motor in one direction or the other for a fixed time. It has a variety of applica­ tions & could be used to power a model railway boom gate or give slow motion operation of points. Manual control circuit for a stepper motor By RICK WALTERS Typical stepper motor applications generally involve a drive circuit under the control of a computer or microprocessor. By contrast, this circuit has been produced as a self-contained PC board designed specifically to suit small stepper motors which draw just a few tens of milliamps at 5V. When the actuate button (S1) is pressed, the stepper motor will run in one direction for a fixed time. When the actuate button is pressed again, 62  Silicon Chip the stepper motor will run in the other direction for the same fixed time. You can use two buttons (S2 & S3) to preset the forward and reverse directions of the motor before the actuate button is pressed. While the speed at which the stepper motor runs is fixed, you can set the stepping rate by changing a resistor or capacitor in the circuit. Note, however, that this circuit does not allow an exact number of steps to be specified, just the speed, duration and direction. Model railway application There are still many places in Australia where level cross­ings are controlled by boom gates. Wouldn’t it be nice to have a level crossing with motorised boom gates on a model railway layout? This stepper motor drive circuit could be used to provide the motive power. In practice, the actuate switch (S1) could be a reed switch operated by the model locomotive as it approaches the crossing. This would cause the boom arm to lower. A second reed switch, wired in parallel with the first, is placed after the crossing, so that the locomotive operates it to raise the boom arm. Circuit operation The full circuit of the stepper motor controller is shown in Fig.1. It can be divided into three sections: one controlling the duration of operation, one controlling the speed and direc­tion of stepping, and the third controlling the stepper motor drivers. The first section involves IC1, a 555 timer connected as a monostable. When pushbutton switch S1 is closed, pin 2 of IC1 is pulled to 0V and its output at pin 3 goes high for about 10 seconds. This will turn PNP transistor Q1 off and its collector voltage will fall from +5V to 0V. This has two outcomes. First, D1, which held the 0.1µF capacitor at pins 1 & 2 of Schmitt NAND gate IC5a at +4.4V, is no longer conducting and therefore IC5a works as an oscillator. Its output at pin 3 will be a square wave with a frequency of about 100Hz. This signal is fed to the clock input of a decade counter, IC2. When this input is clocked each of the 10 outputs of IC2 will change from low to high in sequence. The second outcome is that the collector of Q1 – which held IC2 reset via diode D2, IC4a reset at pin 4 and IC4b reset at pin 12 – is no longer high and so these ICs are now enabled and can be clocked. Q1’s collector is also connected to the clock input of IC3a, but as this IC needs a low to high transition to toggle the output, this change has no effect. Bridge drivers Before we describe the logic operations any further, let’s look at the stepper motor drivers. The type of stepper motor specified consists of two centre-tapped windings MA and Fig.1 (right): this motor driver circuit is suitable for driving low current stepper motors. It drives the stepper motor in one direc­tion or the other each time switch S1 is closed. June 1997  63 Fig.2: follow this parts layout diagram when installing the parts on the PC board and be careful not to mix up the transistor types. output high for around 10 seconds, as already noted. During this time IC5a will clock IC2 at 100Hz. This means that the output of IC2, pin 3, will go high for 10ms then low as pin 2 goes high for the same time. Pins 4 and 7 will follow this sequence but when pin 10 goes high it will immediately reset IC2 through D3, causing pin 3 to go high again. Then the sequence will repeat. Each time pin 4 of IC2 goes high, it clocks IC4a and re­verses the direction of the current through MA. IC4b is clocked either by pin 2 or pin 7 of IC2, depending upon the state of the outputs of IC3a. If pin 1 of IC3a is high, gate IC5c is enabled and pin 7 of IC2 will clock IC4b. If pin 2 of IC3a is high, gate IC5b is enabled and pin 2 of IC2 clocks IC4b. In the latter case, IC4b is clocked before IC4a and the motor will step in one direction. If IC3a is toggled then IC4a will be clocked by pin 4 of IC2 before IC4b will be clocked by pin 7. Therefore, as explained previously, the motor will now rotate in the opposite direction. Turn off Fig.3: this is the full-size etching pattern for the PC board. Check your board carefully before installing any parts. MB, the centre taps of which are not used. Each winding is connected across a bridge of four transistors, Q2-Q5 and Q6-Q9. We will first describe how winding MA is driven, as the drive to MB is identical. Assume pin 1 of IC4a is high, and therefore its complement, pin 2, will be low. Pin 1 will turn Q2 off and Q3 on. Pin 2 will turn Q4 on and Q5 off. As both Q3 and Q4 are turned on, current will flow through winding MA from right to left. If IC4a is now clocked, its outputs toggle and so pin 1 goes low and pin 2 64  Silicon Chip goes high. If you trace it out, you will see that Q2 and Q5 are now turned on and the current flow in MA is from left to right. Therefore, by clocking IC4a we reverse the direction of the current in MA. A similar reversal occurs for IC4b and winding MB. To make the motor rotate (in either direction) we have to delay the phase of the current in MA relative to MB. To rotate it in the opposite direction we must delay MB relative to MA. Now that we know what we have to do to run the motor, let’s look at how it happens. When IC1 is triggered it will hold its Ten seconds after switch S1 was closed, the pin 3 output of IC1 will go low and Q1 will turn on again. This resets all the counters and the motor is stopped. At the same time, this low to high transition by Q1’s collector will clock IC3a, thereby ensur­ing the motor will rotate in the opposite direction next time it is powered. At power on, the 0.1µF capacitor connected to pin 6 of IC3a ensures that this pin is briefly pulled high. This sets IC3a so that its pin 1 is high. Thus, the motor will always rotate in the same direction each time the power is first applied. Forward/reverse, up/down Provision has also been made for two switches (UP & DOWN) to change the direction of the motor. These are on the set and reset pins of IC3a. These switches should only be used when the motor is stopped. The motor may not reverse its direction if they are used while it is running, as it depends on the actual phase of the drive waveforms. PC board assembly Begin as usual by checking the PC board against the artwork of Fig.3. Check for undrilled holes, shorts between tracks, especially where the tracks run between the pads on IC4 and IC5, and open circuit tracks. Make any necessary repairs before pro­ceeding. Use the component overlay diagram of Fig.2 as a guide when inserting components into the PC board. Begin the assembly by fitting and soldering the seven links, followed by the resistors and IC sockets, if used. To give the PC board that professional look, make sure that all the resistors have their colour codes running the same way, vertically and horizontally (this also makes them easier to check later on). The MKT and monolithic ceramic capacitors are fitted next and their markings should be similarly aligned. Lastly, fit the two electrolytics, three diodes and nine transistors, making sure that all are correctly orientated. Once you have finished, check your soldering, making sure that all the joints are nice and shiny and that there are no bridged tracks. A dull joint is a sign of potential trouble. Finally, insert and solder the ICs, or plug them into the sockets. Make sure they are inserted correctly. Testing the controller The specified stepper motor’s leads can be removed from the plug by pulling the wire gently while pressing the retaining lug on one side of the socket with a jeweller’s screwdriver or a small nail. Leave the green wires in the plug at this stage. Solder the pins into the PC board with the colours as shown in Fig.2. The motor should turn reasonably freely but when the power is applied the circuit should draw around 50mA and the motor will “lock” and be much harder to turn. Briefly short pin 2 of IC1 to pin 1 and the motor should begin turning. After 10 seconds or so it will stop. If pin 2 is shorted again the motor should run again but in the opposite direction. Once you trigger IC1, the motor will run for about 10 sec­onds in either direction. If you need to run the motor for a longer time, increase the 1MΩ resistor at pin 7 of IC1. The run time is directly proportional to the value of the resistor. Increasing it to 1.2MΩ will run the motor 20% longer. Conversely, if the motor runs for too long, reduce the resistor value. You can also change the speed at which the motor steps by varying the 100kΩ resistor or 0.1µF capacitor at pins 1 & 2 of IC5a although there are limits. If you try to run the stepper too fast it will merely stall. As a guide though, you could double the speed of stepping by halving the 100kΩ resistor between pins 1 & 3 of IC5a. Or if you wish to run the motor at half the speed, double the resistor value between pins 1 & 3 of IC5a. It doesn’t work! The first step is to check your work against the PC board overlay of Fig.3. A tiny solder bridge is all it takes to stop the unit from operating. Next, set your meter to the 10V DC range and connect its negative lead to the DC negative input. Connect its positive lead to D1’s anode. The meter should read 5V ±10% (due to the tolerance on REG1). Momentarily short pin 2 of IC1 to pin 1 and the meter should read 0V for about 10 seconds then return to the previous reading. Check that this occurs at pins 4 and 12 of IC4 and pin 3 of IC3. Each time the anode of D1 goes high it should clock IC3a. Make sure pin 1 of IC3 alternates (+5V or 0V) each time you trigger IC1. While IC1 is triggered, the outputs of IC2 (pins 2, 4 & 7) should be cycling. If you put an analog multi­meter on each pin it should read around 1.3V. A digital meter will jump around PARTS LIST 1 PC board, code 09106971, 76 x 97mm 1 stepper motor, Oatley Electronics M17 or equivalent 1 8-pin IC socket 2 14-pin IC socket 2 16-pin IC socket Semiconductors 1 555 or 7555 timer (IC1) 1 4017 decade counter (IC2) 1 4013 dual-D flipflop (IC3) 1 4027 dual-JK flipflop (IC4) 1 4093 quad NAND Schmitt trigger (IC5) 5 BC558 or BC328 PNP transistors (Q1,Q2,Q4,Q6,Q8) 4 BC548 or BC338 NPN transistors (Q3,Q5,Q7,Q9) 3 1N914 small signal diodes (D1-D3) 1 1N4004 1A diode (D4) Capacitors 1 100µF 16VW PC electrolytic 1 10µF 16VW tantalum or low leakage electrolytic 1 10µF 25VW PC electrolytic 5 0.1µF 100VW MKT polyester or monolithic ceramic 1 .01µF 100VW MKT polyester Resistors (0.25W, 1%) 1 1MΩ 10 10kΩ 2 100kΩ 1 4.7kΩ 1 22kΩ 1 3.3kΩ Miscellaneous Tinned copper wire, red & black hook-up wire, solder with readings varying between 1.2V and 1.4V. Once you locate the area where the problem exists you will have to check for incorrect component values or solder bridges and the PC board etching SC for shorts or open circuits. RESISTOR COLOUR CODES  No.    1    2    1  10    1    1 Value 1MΩ 100kΩ 22kΩ 10kΩ 4.7kΩ 3.3kΩ 4-Band Code (1%) brown black green brown brown black yellow brown red red orange brown brown black orange brown yellow violet red brown orange orange red brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown red red black red brown brown black black red brown yellow violet black brown brown orange orange black brown brown June 1997  65