Circuitry

the complete hand paddle and stepper motor driver circuit...

The TTL 7404 hex inverters can source 40 milliamps.  At 5 volts, this current draw from each pin of the hex inverters is limited by the 470 ohm resistors to 10 milliamps.  The transistors will amplify this current 800-1200 times.  So motors up to 10 amps can be used.  Most traces on circuit boards and point to point wiring will limit this figure closer to 3 amps.

a circuit diagram of the optional field rotator drive unit...

a picture of the electronics box...

I added a voltmeter and ammeter to gauge battery voltage and motor current draw. I recommend placing all outside cabling sockets and terminals on the front or side of the box. The circuit is simple enough to use point to point wiring. Double and triple check all connections, tracing out all wires by hand, and use a ohmmeter to check for shorts. The power diode/ zener diode network is essential to prevent destruction of the transitors due to back e.m.f. from the motors. These voltage spikes will also make short work of the parallel port. Use a 6 volt battery for initial testing so that if there should be a wiring problem, the parallel port will not be risked.

Considering the relatively low 6 to 12 volts DC involved, and the simple drive circuit, isolation of the laptop/PC from the drive circuit with opto-isolators is not strictly called for. If you power the steppers with 24 volts or higher, you should consider including opto-isolators. You can use the following circuit, making sure that you set the variable InvertOutput to zero in the Config.Dat file...

The PC generates voltage waveforms for the four stepper windings and outputs them via the PC parallel port resulting in a very simple drive circuit. 74LS04 hex inverters receive the output from the parallel port and provide adequate current to drive the power transistors. The power transistors should be heat-sunk.

The handpad uses the 4 bits of parallel port input from parallel port base +1, and an external connection of +5 volts DC for a total of 5 lines. A long 9-pin PC serial cable is used. The normally open momentary push buttons are used for directional control. The 3-way switch is used to mark initializations, and to start and stop advanced functions, while the 2-way switch set the speed: either slow microstepping or fast halfstepping. All lines tie to ground via 220 ohm resistors so that when no button or switch is activated, the handpad outputs go to ground, or logical low. When pushing a button or moving a switch, + 5 volts DC is applied to the appropriate bit(s) of the parallel port. Some of the buttons and switches are tied to more than one bit. Diodes are placed on the buttons and switches outputs to insure that only the bits desired are activated. The remaining parallel port pins of 18 through 25 are grounds.

parallel port 25 pin connector pin-out:
2 altitude stepper motor - red
3 altitude stepper motor - green
4 altitude stepper motor - red/white
5 altitude stepper motor - green/white
6 azimuth stepper motor - red
7 azimuth stepper motor - green
8 azimuth stepper motor - red/white
9 azimuth stepper motor - green/white
1 field rotation motor pulse
14 field rotation motor direction
13 handpad input (pulled to ground via 220 ohm resistor)
12 handpad input (pulled to ground via 220 ohm resistor)
10 handpad input (pulled to ground via 220 ohm resistor)
11 handpad input (pulled to ground via 220 ohm resistor)
18-25 ground

handpad:
3-way switch left, for initializing, and doing other tasks, activates pins 13,10
3-way switch middle, neutral - does nothing, activates nothing
3-way switch right, also for initializing, and doing other tasks, activates pins 12,10
2-way switch left, slow microstepping, activates line 11
2-way switch right, fast halfstepping, activates nothing
upper button, momentary on = Up, activates pin 13
lower button, momentary on = Down, activates pin 12
left button, momentary on = CCW, activates pin 10
right button, momentary on = CW, activates pins 13, 12

Chris Dunn <cdunn@fastrac.net.au> 's circuit layout on perfboard


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...by Mel Bartels