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[Solved] was: Need electrical circuit help (Rotating Monitor w. Parallel Port)
Blanka:
What is really weird in your scheme is that the power-side of the transistor is connected to the parallel port. Normally you connect the the base to the signal pin, with a 10K resistor from the base to the ground, and the motor in the collector/emitter line. Also I would pick a MOSFET like an IRF 520 for it, as it is better in fast switching, allows high current (9 amps) and used less power itself.
MonMotha:
The transistors are being used as pass elements, not the normal "current amplifier" that you're probably used to seeing. In this design, if some current is allowed to flow from the base (these are PNPs, remember), then much more current will be allowed to flow from the parport into motor controller when the parport output is high (i.e. willing to source current). This base current is controlled by the third transistor which is in turn controlled by another parport output. If the parport output is low or the base current path is cut off, no current will flow to the motor controller.
I suspect the motor controller just has transistor inputs, so this is cool. Attempting to drive push-pull TTL inputs or, even worse, CMOS inputs with this scheme would not likely meet with total success as the setup cannot sink current. Using a digital buffer here such as a 74LS244 would solve that problem.
Using MOSFETs here might meet with several challenges. Those large power MOSFETs like the IRF520s you mentioned (which there's no reason to use here as this is not a high-power application) often have a Vth of around 3V. Many modern PCs that still have parallel ports only have 3.3V logic levels on them and therefore will not be able to get such a MOSFET into the ohmic region. Low power is also not likely a concern (and you're only saving ~2mW in power used to drive the base anyway). On-state conduction losses are not a concern here.
MOSFETs are not a panacea. They have advantages, but they also have disadvantages. While it's often possible to design either into an application, I find that, for novices, BJTs are usually a little easier to understand and also easier to acquire. BJTs are also not generally ESD sensitive, so that's a bonus for novice hobbyists.
csa3d:
To keep this thread up to date as well, I'm attaching the final wiring I used in the circuit, now that it's up and running. Comments are welcome.
click image to enlarge.
-csa