# Normally closed contact switch and Garage door

M

#### Moloney

Jan 1, 1970
0
I am trying to detect whether or not my garage door is open. I purchased a
normally closed contact switch to place on the bottom of the railing and
bottom of the door. Being new to electronics I only understand the basics,
but would like to try and use the N/C contact to send some type of signal to
me that my garage door is open.

Could I hook this N/C contact to a breadboard and have an LED light up when
the door is open? What would I need to detect if the circuit is opened or
closed?

B

#### Byron A Jeff

Jan 1, 1970
0
-I am trying to detect whether or not my garage door is open. I purchased a
-normally closed contact switch to place on the bottom of the railing and
-bottom of the door. Being new to electronics I only understand the basics,
-but would like to try and use the N/C contact to send some type of signal to
-me that my garage door is open.

Sounds simple enough.

-
-Could I hook this N/C contact to a breadboard and have an LED light up when
-the door is open?

Sure.

- What would I need to detect if the circuit is opened or closed?

You already have the detector, the NC contact. The only problem is how to hook
it up so that the LED lights up when the door is opened. The simplest way is
to utilize the fact that an LED must have a current limiting resistor so that
it won't burn up. So let's start with a simple LED circuit:

V----R----A----LED-----GND

V is a positive voltage source, presumably a DC wall wart. R is the current
limiting resistor (more on that in a bit), A is just a mark point which I'll
get to in a second, just note there's no physical device at point A yet, LED
is the LED (duh?!) and GND is the ground end of the wall wart. If you wire
this on a breadboard, current will flow through the resistor and the LED and
the LED will light. To pick a resistor value divide the voltage by 10
milliamps (.010A) an use a resistor that's close to that value. So
for example if you have a 9 volt wall wart, divide 9 by 0.01 giving 900. So
pick a resistor around 900 ohms.

Now on to the dectector. It's normally closed which means that it's an unbroken
circuit when the door is closed. It wouldn't help to wire the detector between
point A and the LED because then the LED would be on when the door is closed.
But what you can do is use the detector as a bypass around the LED. So if you
wire it between point A and ground, when the door is closed then the current
will flow from point A through the detector and into ground (or vice versa ;-)
bypassing the LED. So the LED will go out because it doesn't have current
running through it. However when the door opens and the detector opens, then
current can no longer flow directly between point A and GND, so it must then
flow through the LED, lighting it.

Hope this helps,

BAJ

B

#### Byron A Jeff

Jan 1, 1970
0
-In article <[email protected]>,
--I am trying to detect whether or not my garage door is open. I purchased a
--normally closed contact switch to place on the bottom of the railing and
--bottom of the door. Being new to electronics I only understand the basics,
--but would like to try and use the N/C contact to send some type of signal to
--me that my garage door is open.

-- What would I need to detect if the circuit is opened or closed?
-
-You already have the detector, the NC contact. The only problem is how to hook
-it up so that the LED lights up when the door is opened. The simplest way is
-to utilize the fact that an LED must have a current limiting resistor so that
-it won't burn up.

-
-V----R----A----LED-----GND
-

One last point I forgot. Point A has to between the resistor and the LED. The
reason for this is so that the resistor can limit the amount of current drawn
from the wall wart. If point A were between V and R, then the closed circuit
would be a dead short on the wall wart, which leads to all kinds of problems.

That's why the fact that the LED requires a current limiting resistor is
important.

BAJ

J

#### John Fields

Jan 1, 1970
0
I am trying to detect whether or not my garage door is open. I purchased a
normally closed contact switch to place on the bottom of the railing and
bottom of the door. Being new to electronics I only understand the basics,
but would like to try and use the N/C contact to send some type of signal to
me that my garage door is open.

Could I hook this N/C contact to a breadboard and have an LED light up when
the door is open? What would I need to detect if the circuit is opened or
closed?

---
Being new to electronics, the distinction between a normally closed and
a normally open switch may not be clear to you. A normally open switch
is one in which the contacts would be open when not being actuated by
the garage door, and a normally closed switch would be one where the
contacts would only be open when the garage door forced them open.

Viewing the following with a fixed pitch font like Courier,

If you wanted to use a normally open switch you could do this:

+V
|
[R]
|
+--[<LED]--O |
| |---DOOR OPEN
+----------O |
|
|
GND

If, on the other hand, you wanted to use a normally closed switch you
could do this:

+V
|
[R]
|
|A
[LED]
|
O|
|---DOOR OPEN
O|
|
|
GND

For a 20mA red LED and a 5V supply the value of the resistor would be
about 150 ohms and it would dissipate about 167mW of power in the first
example (with the door closed), which would be worst case, so you could
use a 150 ohm 1/4 watt resistor.

If your switch is really normally closed then the circuit would only be
drawing current when the door is open, as opposed to all the time if you
used a normally open switch.

S

#### Stephen Kurzban

Jan 1, 1970
0
John said:
If, on the other hand, you wanted to use a normally closed switch you
could do this:

+V
|
[R]
|
|A
[LED]
|
O|
|---DOOR OPEN
O|
|
|
GND

John,

Did you mean:

+V
|
[R]
|
|A
[LED]
|
|O
|-----DOOR OPENS CONTACTS
|O
|
|
GND

J

#### John Fields

Jan 1, 1970
0
John said:
If, on the other hand, you wanted to use a normally closed switch you
could do this:

+V
|
[R]
|
|A
[LED]
|
O|
|---DOOR OPEN
O|
|
|
GND

John,

Did you mean:

+V
|
[R]
|
|A
[LED]
|
|O
|-----DOOR OPENS CONTACTS
|O
|
|
GND

---
No, I meant that the contacts are closed when the door is open.

Perhaps this would have made it clearer:

+V
|
[R]
|
|A
[LED]
|
O|
|<---CLOSED WITH DOOR OPEN
O|
|
|
GND

What the OP was looking for was an LED ON when the door was open, and
your way will turn the LED OFF when the door opens

S

#### Stephen Kurzban

Jan 1, 1970
0
I reckon the title "Normally closed contact switch" let me
to believe the objective was to show the OP a way to wire NC
as well as alternative NO. I should have read the text of
the message instead of assuming anything...

Best,

SBK

John said:
John said:
If, on the other hand, you wanted to use a normally closed switch you
could do this:

+V
|
[R]
|
|A
[LED]
|
O|
|---DOOR OPEN
O|
|
|
GND

John,

Did you mean:

+V
|
[R]
|
|A
[LED]
|
|O
|-----DOOR OPENS CONTACTS
|O
|
|
GND

---
No, I meant that the contacts are closed when the door is open.

Perhaps this would have made it clearer:

+V
|
[R]
|
|A
[LED]
|
O|
|<---CLOSED WITH DOOR OPEN
O|
|
|
GND

What the OP was looking for was an LED ON when the door was open, and
your way will turn the LED OFF when the door opens

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