Stepper motor (3)

I'd say that if the stepper coils can handle 12 volts without cooking they will probably be high inductance and the transistors will explode as soon as you switch them from on to off and the volts go through the roof. Some form of eye protection could be advisable :o)

Just my .02p

Robin G Hewitt

The PC port was orginally driven by 74LS374 octal latch, which can source 2.6 mA and sink 24 mA. If the load pulls these lines to ground, excessive current may easily be sourced. Today, ports /should/ provide at least that or more, however some PC printer (parallel) ports only put out 3.5v when at ogic "high".

I would really suggest you look carefully at figure 3.7 at

Try an old, disposable PC first.

Use small MOSFETs for the transistors (e.g. 2N7000 series), or add base resistors to the trannies (and ensure they're high-gain types). Put pull-up resistors (around 4k7 should do) on the LPT port outputs. Put a reverse-biased diode (e.g. 1N4001 through 1N4005) across each stepper coil. Read up on how to drive steppers (phase and timing).

Steve