Re: parallel port signal to relay

--- In CAD_CAM_EDM_DRO@y..., "turbulatordude" <davemucha@j...> wrote:

> Hi all,
>
> I am looking for a simple circuit to turn on my motor (spindle)

and

> a solenoid for compressed air for a mister.
>
> I was looking to run it from the parallel port directly, so

isolation

> may be beneficial.
>

You should strive to keep your PC and machinery isolated (different
grounds and power). The cheapest way to do that with low frequecy
signals is to use an optoisolator. It is essentially an LED and a
light sensitive transistor that conducts current when light is shined
on it. It's enclosed and all setup so all you have to do is hook
your signal to the LED side and use the transistor on the other side
to turn on a Darlington or a small relay that can then drive a larger
relay or contactor.

Here's what you need. Steal +5 from your PC game port (pin 1) and
put a 330 ohm (org-org-brn) 1/4W res in series with the ANODE of the
Opto input pins (Anode & Cathode). You can put the resistor on either
pin but for this explaination use the ANODE. Connect the other end of
the resistor to the PC +5. The CATHODE connects back to the pin that
you are going to use to turn on your equipment. The LED will turn on
when you take the pin low (sink current). A parallel port on a
computer is not made to "drive" (source) current and will only sink a
few milliamperes (about 14 ma) so the load presented needs to be
light. Try to find an Optoisolator that will be operate with 15 ma
or less of Input current.

For the other side of the opto you normally have an NPN transistor
with max collector current of 100ma. The older series of
optoisolators had a net current gain of less than 100% which means
that if you run the input at 15ma you should not expect the output
transistor to provide but about 5ma. Newer darlington optos have
current transfer ratios that are > 100%. The NPN should be used to
turn on another transistor that will give you enough current gain to
handle larger loads. You have to provide the DC voltages for the
output circuitry (+12 to + 32VDC) from a source tied to the machinery
side of the circuit.

Now for the practical answer: Use a SSR (solid state relay). They
are relatively cheap and available on the surplus market. They have
isolated inputs, they handle AC at high currents (motors,
solenoids,etc) and you can drive most of them from the parallel
port. I bought several (3-30volts DC input; 10A 240VAC output) for
less than 8 bucks each. They use about 5ma at 5 volts on the input to
control 2.4KW! They will NOT switch DC voltages and loads. I built a
Single to 3 phase converter using a 5 HP 3P surplus motor and some
caps. I designed a power on one-shot that used a 555 to drive an SSR
module to automatically switch in and out the motor starting cap. I
work pretty cheap but it would take several hours to wire up and test
a descrete optoisolater circuit and the transistors, relays, etc.
Last time I made 2.00/hr I was in the Army. I would rather use my
time to do work on the mechanical stuff ;-)