well i got this problem
i want to control a motor from my computer..... from printer port and i need
a realy that can be controlled from that... or a transistor solution...
the current out from printerport is about 0,5mA and the voltage 5V.
can someone help me?
Bjørn Tore Hovda
Use a Solid State Relay (Hosfelt.com or similar)
I would still buffer that with a small silicon transistor in emitter follower
configuration. 2N2222 or a generic will work fine.
The SSR works from +3v - +30vDC so it is real easy to interface and they will
control a motor up to several HP if properly selected.
If you are not familiar with the transistor arrangement I am talking about...
Simply connect the 5VDC from the power supply to the collector, connect the
parallel port you are using to the base and connect the emitter to the + side
of the SSR. Minus on the SSR to power supply ground.
The AC terminals of the SSR are in series with the load. Bear in mind the SSR
will probably need a heat sink.
Solid state relays are pretty expensive, but are the only ones that will
work OK with the setup you just described. A coil relay would be much
cheaper, but you really should sink the coil current instead of sourcing it.
I always use optoisolators, too, after I managed to burn out a parallel port
during an accidental short a long time ago. The following site has a couple
circuits that should work for both isolated and non-isolated setups:
also be interested in his section titled "bad circuit example" -- though
with the solid-state relay it *probably* wouldn't be as much of an issue.)
You can drive an SSR directly from CMOS but I usually use the good old 2N2222.
The advantage is they are optoisolated from the load and from a guy like
Hosfelt they are in the $12 range for 10 amp and $25 for 50 amps. You also
avoid the dust problems and the inductive "kick" from a coil.
I use the hell out of them. My 4xxx CMOS spa controller uses SSRs to run both
pumps (3/4 and 2.5HP) and an 11kw heater. I also use them for lighting controls
all around the house.
I looked at the site and I see what you are talking about. It is somewhat moot
with the SSR since they "spec" at 3v and actually work a bit below that but I
can see where a regular relay driver could be a problem.
I agree the optoisolator is the safest choice and they are pretty cheap too if
you mail order. I got a bag of them for about a dime each.
Regular relays are isolated from the load too, it's the external power
supply that powers your 2n2222 that is the problem. Your relays already have
an optoisolator in them with a 1.5K resistance (I checked the datasheets to
see how it's hooked up). This would be great for hooking directly to the
parallel port. Basically, by introducing your transistor, you've nullified
all the advantages of the optoisolation, and it's much more likely to damage
the parallel port.
I'm not talking about damaging the PC power supply, I'm talking about your
parallel port no longer being optoisolated. The parallel port can supply
enough current to drive an optoisolator LED with a 1.5K resistor. This is
good because the parallel port is only connected to the LED and resistor,
and nothing else...in other words, isolated. When you have an additional
supply electrically connected to the parallel port pin, even through a
transistor, there is a greater risk of something going wrong and current
going where it's not supposed to. It's how I blew out my first parallel
You can take a calculated risk and consider it not worth the effort to
optoisolate; but in this case, your relays are already set up for
optoisolation. By introducing that transistor, you are throwing away a
feature of your relays that could enhance reliability and lower the risk of
damage to your computer.