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# Magnet movement -> com port type thing.

T

#### Tim

Jan 1, 1970
0
Hello.

I want to be able to detect when my door opens, (on my computer), so I
can do various fun things (switch workspaces

I've got a strong flat magnet, so I stuck (well, will stick) it to the
bottom of my door, and then I got some thin wire from a motor, and made a
small sensing coil, so that when to door opens, the magnet goes over the
coil, and induces a small current. And then somehow I get this to my
computer.

But how? I'm new to electronics... well, I know lots of physical law type
things (v=ir, etc), but not how to apply them to make actual complicated
ciruits. I'm guessing I use transistors or something, to amplify the
signal, and then convert it to a digital signal (how?)...

The current/voltage from the coil is very small...

+5V O--------,
|
__________|/ <--- transistor.
C 8 |\
O 8 |__________ COM data pin
I 8
L 8_______________________
|
0V O--------

That's what my inexperienced brain is thinking.

I'm guessing it wouldn't work though... Any ideas?

T

#### Tim

Jan 1, 1970
0
Also, IRC channel anywhere? If not why not?

C

#### Chris

Jan 1, 1970
0
Tim said:
Hello.

I want to be able to detect when my door opens, (on my computer), so I
can do various fun things (switch workspaces

I've got a strong flat magnet, so I stuck (well, will stick) it to the
bottom of my door, and then I got some thin wire from a motor, and made a
small sensing coil, so that when to door opens, the magnet goes over the
coil, and induces a small current. And then somehow I get this to my
computer.

But how? I'm new to electronics... well, I know lots of physical law type
things (v=ir, etc), but not how to apply them to make actual complicated
ciruits. I'm guessing I use transistors or something, to amplify the
signal, and then convert it to a digital signal (how?)...

The current/voltage from the coil is very small...

+5V O--------,
|
__________|/ <--- transistor.
C 8 |\
O 8 |__________ COM data pin
I 8
L 8_______________________
|
0V O--------

That's what my inexperienced brain is thinking.

I'm guessing it wouldn't work though... Any ideas?

Hi, Tim. Using a coil to drive a transistor probably won't work very
well, because the moving magnet only induces current when it's moving,
and it's a small current at that.

It doesn't look like you'll hurt anything trying your idea. However,
if you want something that works, you might want to use your magnet to
close a reed switch (think door security switch), and then read that
switch on an RS-232 port. If you've got a port to spare, you might
want to try this -- it's worked for me.

Reed Switch

_/ 7 (4)
.---o/ o----------oRTS
|
|
| 8 (5)
|------------------oCTS
|
|
| ___ 4 (20)
'---------|___|----oDTR

10K

created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de

The pin numbers are for a DB-9 connector, and the pin numbers in
parentheses are for a DB-25.

This QBASIC snip will work to read the switch, if you've got it
connected to COM1...

OUT &H3FC, 1
A% = INP(&H3FE)AND 16

If the footswitch is on, A% = 0
If it's off, A% = 16

It seems a little extravagant to use a whole COM port to read one
switch, but if you've got it to spare, go for it. Also, as an extra
added bonus, the RS-232 port pins are generally a little less
susceptible to ESD because of built-in protection on the ICs.

Good luck.
Chris

T

#### Tim

Jan 1, 1970
0
Hi, Tim. Using a coil to drive a transistor probably won't work very
well, because the moving magnet only induces current when it's moving,
and it's a small current at that.

It doesn't look like you'll hurt anything trying your idea. However, if
you want something that works, you might want to use your magnet to
close a reed switch (think door security switch), and then read that
switch on an RS-232 port. If you've got a port to spare, you might want
to try this -- it's worked for me.

Reed Switch

_/ 7 (4)
.---o/ o----------oRTS
|
|
| 8 (5)
|------------------oCTS
|
|
| ___ 4 (20)
'---------|___|----oDTR

10K

created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de

The pin numbers are for a DB-9 connector, and the pin numbers in
parentheses are for a DB-25.

This QBASIC snip will work to read the switch, if you've got it
connected to COM1...

OUT &H3FC, 1
A% = INP(&H3FE)AND 16

If the footswitch is on, A% = 0
If it's off, A% = 16

It seems a little extravagant to use a whole COM port to read one
switch, but if you've got it to spare, go for it. Also, as an extra
added bonus, the RS-232 port pins are generally a little less
susceptible to ESD because of built-in protection on the ICs.

Good luck.
Chris

Someone else also emailed me, suggesting a reed switch, so I've ordered
one (two actually) from maplin, and last night I spent ages trying to get
a switch to work via the parallel port.

I was trying to use D0->Gnd in reverse mode (as someone suggested on
IRC), but this morning I realised that wouldn't work (as D0 is 0V in
reverse mode ... I think) so shorting it to ground wouldn't change D0...
So this morning I changed it to what I was originally going to do...

D0 -> switch -> Ack. And then read ack, and it works like a charm.

I measured the current, and if ye old meter is right it is 1.4 mA ..
which means I dont need a resistor... right?

Anyway I wrote some code to execute programmes when it closes and when it
opens... Now I just have to wait for le reed switch...

Its great I tell you! I can switch workspaces from doing stuff I
shouldn't, to one full of work, just my closing a switch! Aye!

Can you see the applications? ))

C

#### CFoley1064

Jan 1, 1970
0
Can you see the applications? ))

It's a whole new world.

For interfacing through serial/parallel/USB ports, see Jan Axelson's Lakeview
Research website, and also Serial Port Complete/Parallel Port Complete/USB Port
Complete books.

http://www.lvr.com/

There's enough on the links on the website to keep you busy for a while.

Good luck.
Chris

T

#### Tim

Jan 1, 1970
0
It's a whole new world.

For interfacing through serial/parallel/USB ports, see Jan Axelson's
Lakeview Research website, and also Serial Port Complete/Parallel Port
Complete/USB Port Complete books.

http://www.lvr.com/

There's enough on the links on the website to keep you busy for a while.

Good luck.
Chris

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