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

From: petrus bitbyter (p.kralt_at_reducespamforchello.nl)
Date: 11/09/04 

Date: Tue, 09 Nov 2004 15:42:20 GMT

"Jean-Marie Vaneskahian" <jean@vaneskahian.com> schreef in bericht
news:d8760c10.0411081325.69d10dd@posting.google.com...
>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.
>
> Thanks a million in advance,
> Jean-Marie Vaneskahian
> jean@vaneskahian.com

Jean-Marie,

"Ground" comes from mains nets and others that are connected to ol mother
earth at some point. A lot of equipment will never be grounded. So "ground"
is a kind of agreement as the potential all voltages are related to. In cars
for instance the minus of the battery has been connected to the chassis and
is - incorrectly - named "ground". There's no real need to do so. Some old
cars had the plus connected to the chassis. In a lot of battery powered
equipment the minus of the batteries is named "ground" even when there is no
connection at all to something outside their boxes. So theoretically every
point of such a battery powered appliance can be connected to the "ground"
of another one. In practice however you may run into problems due to the non
ideal properties of electronic appliances.

Assuming your smoke detectors follow the common path and consider the minus
of the battery pack "ground" you best check out the voltage changes on the
LED relative to it. Futher steps depend on the results of these
measurements.

A gerneral purpose solution can be made with comparators. One input can be
set to the midst of the changing voltage using a voltage divider. The other
input can be connected to the changing voltage (from the LED) itself. An
LM339 may be a good choice. It contains four comparators in a package.
Outputs can be connected (wired or) and you need only one transistor to
switch a relay.

petrus bitbyter 

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