1. Home
  2. General Electronics
  3. I Need To Trigger a Relay When an LED Lights Up On a Smoke Detector

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

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 snipped-for-privacy@vaneskahian.com

Reply to
Jean-Marie Vaneskahian
Loading thread data ...

"Jean-Marie Vaneskahian" schreef in bericht news: snipped-for-privacy@posting.google.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

--
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.786 / Virus Database: 532 - Release Date: 29-10-2004
Reply to
petrus bitbyter

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:

formatting link
formatting link

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

Reply to
Jean-Marie Vaneskahian

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.