Re: I Need To Trigger a Relay When an LED Lights Up On a Smoke Detector

From: Robert Monsen (rcsurname_at_comcast.net)
Date: 11/09/04 

Date: Tue, 09 Nov 2004 02:52:20 GMT

Jean-Marie Vaneskahian wrote:
> I Need To Trigger a Relay When an LED Lights Up On a Smoke Detector
>
> My goal is to use simple battery operated combination Smoke / Carbon
> Monoxide detectors in various parts of my house. I want these battery
> operated detectors to trigger a simple contact closure when they
> alarm.
>
> I noticed that the detectors have a Red LED that lights up when the
> Smoke / Carbon Monoxide detector is triggered. I soldered two small
> wires to the PCB on the back of the detector in "parallel" with the
> Red LED.
>
> When I connect a voltmeter to the two wires I soldered in "parallel"
> with the Red LED and now run from the back of the PCB of the Smoke /
> Carbon Monoxide detector I get about +0.001V when nothing happens and
> +1.78V when I hit the test button on the Smoke / Carbon Monoxide
> detector. By the way, the Smoke / Carbon Monoxide detector runs on a
> total of 3 AAA 1.5V batteries.
>
> I have a very basic electronics understanding. One concept that I do
> NOT understand is that of "Ground". I do understand how to "Wire"
> components though.
>
> My question is this:
>
> How do I take the 2 wires running from the back of the PCB on the
> Smoke / Carbon Monoxide detector and generate a basic dry "Contact
> Closure" when the detector is triggered?
>
> I am sure this requires transistors, diodes, resistors, reed switches
> and a separate battery source, but I have no clue how to connect them
> and what types and values to purchase. I buy most of these components
> from Radio Shack (Part Numbers Would Be Great!).
>
> Please help me put this together. I really appreciate any wisdom on
> this topic. My goal is to protect my family by wiring these battery
> operated Smoke / Carbon Monoxide detectors to my home alarm system
> that uses contact closures.
>

This sounds like a fun project.

One thing to consider is that you probably won't be able to power the
relay open using the current through the LED. If you are willing to use
a separate battery, however, then there is an easy way to do this.

Define the ground to be the more negative side of the LED. (Note: you
can define ground to be anything. It's just the point you measure other
voltages against)

Then, attach one side of a 4.7k resistor to the positive side of the
LED, and the other side to the base of an NPN transistor. Attach the
emitter of the NPN to the negative side of the LED. Now, when the LED
turns on, it'll allow current to flow through the transistor.

Hook the negative terminal of a 9V battery to the emitter of the
transistor, and the positive side to one side of the coil for the relay.
Hook the other coil of the relay to the collector of the NPN transistor.

Now, when the LED is on, the transistor will allow current to flow, so
current will flow in a circuit through the relay coil, the transistor
and the 9V battery. This should cause the relay to open. 

-- 
Regards,
   Robert Monsen
"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.

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