Re: parallel port relay control
- From: "scada" <scada@xxxxxxxxxxxxx>
- Date: Tue, 3 Jan 2006 17:26:55 -0500
"Bob Monsen" <rcsurname@xxxxxxxxxxx> wrote in message
news:pan.2006.01.03.20.37.24.384685@xxxxxxxxxxxxxx
> On Mon, 02 Jan 2006 12:25:00 -0800, jgershonw1 wrote:
>
> > I am trying to control a relay from a computer's parallel port using
> > the circuit labeled Fig B from this webpage
> > http://ourworld.compuserve.com/homepages/Bill_Bowden/page6.htm. The
> > circuit works fine and I am able to control it with my software. But, I
> > have no control while the computer is booting, where the the data lines
> > can be set high or low. I need the relay to remain off until my
> > program is running. I have read a suggestion that said to setup a
> > hardware tri-state condition and use a second data line to act as
> > control valve? Essentially, setup a unique state that is not normally
> > produced by the computer or OS during the boot process. Can somebody
> > suggest a way to modify the circuit in the above link. I am not a
> > electronics expert, so providing a simple schematic would be helpful.
> > Thanks!
>
> I'm assuming you have an external supply which powers the relay.
>
> Use a counter with an active low reset. The port will try to pull down to
> gnd when the computer is off (you probably want to have a pull down
> resistor of like 10k in case it doesn't). If the counter comes out of
> reset because the pin connecting it is high, it'll still be reset to its
> initial state.
>
> So, the process of turning on the relay is to bring the counter out of
> reset, and then to clock it using another data pin which is connected to
> the clock input. To add to the fun, you can have a feedback pin at the
> output, connected to a status pin, that tells you when the count is
> complete. Any binary 4 bit counter should work. You could use the Q3
> output, and then it would take 8 clocks to make it turn on. When you want
> it off, just reset the counter. A '163 should work.
>
> This will be safe unless some other program tries to flip bits on the port
> before you get to it.
>
> --
> Regards,
> Bob Monsen
>
> "Nothing before had ever made me thoroughly realise, though I had read
> various scientific books, that science consists in grouping facts so
> that general laws or conclusions may be drawn from them."
> -- Charles Darwin
I posted a schematic to Alt.binaries.schematics.electronics, for your
review.
Good Luck!
.
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- From: jgershonw1
- Re: parallel port relay control
- From: Bob Monsen
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