I Need To Trigger a Relay When an LED Lights Up On a Smoke D

  • Thread starter Jean-Marie Vaneskahian
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Jean-Marie Vaneskahian

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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 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
Click to expand...

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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Mission Accomplished! I have a CO Detector Wirelessly Sending Out
Alarm Status

I finally got the battery operated Carbon Monoxide detector to also
send a trigger to the house alarm system!

I want to thank everyone who pointed me in the right directoion and
also those that came up with novel ways of making this work.

Here is the setup:

I have already in place in my single family home hardwired
interconnected smoke alarms that are also hardwired to my monitored
alarm system. What I do not have is any Carbon Monoxide detectors. I
found very nice battery operated Carbon Monoxide detectors at "Home
Depot" that would work great as standalone units. I wanted to have
these nice battery operated Carbon Monoxide detectors also fault
independent zones on my monitored alarm system.

My alarm system has wireless contact closure transmitters. If the
wireless transmitters detect a contact closure on the screw terminals
it sends a zone faulted alarm to the alarm system.

The battery operated Carbon Monoxide detectors have two LEDs on the
face, one that blinks every 30 seconds to indicate normal operation
and one that is RED that ONLY turns on in an alarm condition.

I took the PCB out of the battery operated Carbon Monoxide detector
(it snaps right out) and soldered 2 – 24 Gauge, 12 inch wires onto the
anode and cathode of the red LED on the back side of the PCB.

With my voltmeter I saw that the voltage across the red LED when the
battery operated Carbon Monoxide detector when into an alarm condition
was 1.7 – 1.9 Volts. The LED would also flash rapidly because the
voltage would go on and off. In other words the LED would stay fully
lit if it always received the 1.7V and would only turn off when no
voltage was present.

Here is what I did to get contact closure from the voltage across the
red LED.

With the help of many people here:

1 – I ran the two small wires I had soldered to the battery operated
Carbon Monoxide detector to a breadboard.

2 – I used an external 9V battery run to a voltage regulator that
dropped the Voltage to 5V out

3 – From there I went to a NPN transistor and resistor on the base
leg.

4- between the emitter and collector I want to another NPN transistor
that was the switch for a 555 timer circuit

5 – The 555 Timer then has wired small 5V reed relay with diodes
across the coil. I also used a 1 mega ohm resistor so the 555 would
hold the relay closed for about 16 seconds.

Now when the battery operated Carbon Monoxide detector goes off and
the red LED flashes, the reed relay closes for 16 seconds and does not
care about the flashing nature of the red LED, it stays latched for 16
seconds as soon as the first 1.7V hits the NPN transistor.

I was amazed that all this really worked!!!! Now the hard part.

I want to put all this circuitry in little project boxed but I have
never soldered to a circuit board. There are lots of junction points
and I worried that I may burn many of the components trying to solder
all this together. Any advice?

Here are the 2 circuits I put togher to make all this work:
http://www.aaroncake.net/circuits/relaytim.htm
http://www1.electusdistribution.com.au/images_uploaded/relaydrv.pdf

Thanks again to all those that helped me!
Jean-Marie Vaneskahian
jean@vaneskahian.com
 
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