N/C Switches to turn on stuff

[Andrew Howard]

My work wants me to whip up an alarm system for the back door, which
sometimes gets left open accidently.
I was planning on hooking a power supply to a magnet switch and a siren but
I have encountered a problem.

The switch is a N/C one but I want the alarm to sound when the door is open.
I originally thought of using the switch in parallel, to short the power
supply when the door is closed, but I don't want to constantly use the power
like this, in case I use a battery.

Can anyone offer a simple solution to this?

Thanks
Andrew Howard

 

[Dr. Anton T. Squeegee]

My work wants me to whip up an alarm system for the back door, which
sometimes gets left open accidently.
I was planning on hooking a power supply to a magnet switch and a siren but
I have encountered a problem.

Can anyone offer a simple solution to this?

Don't know how "simple" it would be, but... Would it not be a
wiser investment to install a real intrusion alarm system? The beauty of
a centralized system is that you can also hook in fire and smoke
sensors, and that it can report to a central station when tripped.

There are door-latching mechanisms available that have a built-in
alarm which sounds any time the door is opened. You may want to look at
such.

Happy hunting.


--
Dr. Anton T. Squeegee, Director, Dutch Surrealist Plumbing Institute.
(Known to some as Bruce Lane, ARS KC7GR,
kyrrin (a/t) bluefeathertech[d=o=t]calm -- www.bluefeathertech.com
"If Salvador Dali had owned a computer, would it have been equipped
with surreal ports?"

 

[Beau Schwabe]

Don't know how "simple" it would be, but... Would it not be a
wiser investment to install a real intrusion alarm system? The beauty of
a centralized system is that you can also hook in fire and smoke
sensors, and that it can report to a central station when tripped.

There are door-latching mechanisms available that have a built-in
alarm which sounds any time the door is opened. You may want to look at
such.

Happy hunting.

Those "fire and smoke sensors" might come in handy when he is shorting
his power supply - grin

Save yourself a headache and find a switch that is Normally Open!

-Beau Schwabe

 

[Peter Bennett]

My work wants me to whip up an alarm system for the back door, which
sometimes gets left open accidently.
I was planning on hooking a power supply to a magnet switch and a siren but
I have encountered a problem.

The switch is a N/C one but I want the alarm to sound when the door is open.
I originally thought of using the switch in parallel, to short the power
supply when the door is closed, but I don't want to constantly use the power
like this, in case I use a battery.

Can anyone offer a simple solution to this?

Thanks
Andrew Howard

Use the NC switch to operate a SPDT relay, then use the relay's NO
contact to operate the alarm.

 

[Robert C Monsen]

My work wants me to whip up an alarm system for the back door, which
sometimes gets left open accidently.
I was planning on hooking a power supply to a magnet switch and a siren but
I have encountered a problem.

The switch is a N/C one but I want the alarm to sound when the door is open.
I originally thought of using the switch in parallel, to short the power
supply when the door is closed, but I don't want to constantly use the power
like this, in case I use a battery.

Can anyone offer a simple solution to this?

Thanks
Andrew Howard

Andrew, in order to determine if the switch is still closed, you have to
push some current through it. Thus, with an n/c switch, you will end up
wasting some electrons.

However, if you waste, say, 1uA, it'll take years before your battery dies
(it'll die of old age before you drain it with 1uA.) Thus, use a transistor
like this:


(view with courier font, paste into notepad, choose format:font:courier)

VCC=9V or so
-------+-----------+
| |
.-. |
10 MEG| | .-.
| | ( X )
'-' '-'
| |
+----+ |
\ o | |
\ | ||-+D
\. | ||<- N-MOSFET
o +--G||-+S
| |
-------+-----------+
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


When the switch is closed (which is usually is) the current flows, and so
the voltage at the gate of the mosfet is equal to the voltage at the source
pin. Thus, the N-MOSFET is off. However, when the switch opens, the voltage
is (slowly) pulled up to VCC, and the mosfet is turned on.

['Slowly' means that the voltage will come up in 10ms, which means that if
you are sensing this voltage change using digital logic, you should use a
schmitt trigger, cause otherwise the slow transision could cause false
triggers.]

Use a mosfet that will turn on at < 5V. If you need to drive AC, use the
current through the mosfet to drive a relay.

This works because MOSFETs don't use current, only voltage, to turn on or
off, so that sensing current can be really small.

Regards,
Bob Monsen

 

[Outrider 141]

Shouldn't the N/C give you an output when it opens - use of that to
give you a ground for your ckt, essentially turning it on when the
door opens? Could be off on that, haven't used those much.

My work wants me to whip up an alarm system for the back door, which
sometimes gets left open accidently.
I was planning on hooking a power supply to a magnet switch and a siren but
I have encountered a problem.

The switch is a N/C one but I want the alarm to sound when the door is open.
I originally thought of using the switch in parallel, to short the power
supply when the door is closed, but I don't want to constantly use the power
like this, in case I use a battery.

Can anyone offer a simple solution to this?

Thanks
Andrew Howard

Use the usual techniques to reply via email.

Molon Labe!

 

[Andrew Howard]

Those "fire and smoke sensors" might come in handy when he is shorting

his power supply - grin

I meant through resistor , or something... buggered if I knew what I was
talking about.

Save yourself a headache and find a switch that is Normally Open!

It's preferable to use the one I chose earlier, but I suppose I might be
able to mount one of those external ones on the top of the door frame.

Meh, whatever

PS. The reason I have been asked to make one from scratch is because my boss
is to cheap to buy a proper one, and there aren't any professionals that
want to go near the place.

 

[Andrew Howard]

Use the NC switch to operate a SPDT relay, then use the relay's NO

contact to operate the alarm.

Is it possible to do this using only one power supply?



Oh, and thanks for everyones help so far.
Andrew Howard

 

[Peter Bennett]

Is it possible to do this using only one power supply?

Yes - just select a relay with a coil voltage the same as the alarm.

 

[Andrew Howard]

Well, the solution to my problem has been found. I told my boss two options
and he suprisingly chose the easiest one for me. He chose the way that
allows me to use a N/O switch, probably because it cost less.

hmm... Now I just have to learn how to route wires so things don't stuff up.

Thanks for all of the help on this matter.
Andrew Howard