Smoke Detectors – CO Detector Beeping Every Morning at 6am

Ionization smoke detectors use a radioisotope to generate a very small electrical current, which when interrupted causes an alarm to sound. These types of alarms use a metal–oxide–semiconductor field-effect transistor (MOSFET) to prevent the alarm from sounding, as long as the tiny current is flowing from the detector.

A simple dumbed down version of the circuit, might look something like this.

Notice the 9V battery is connected to the Source contact of the MOSFET, the detector is connected to the Gate, and the Alarm is connected to the Drain. This means that as long as there is a current on the gate, current will not flow from the source to the drain. If something (smoke) interrupts the current flow to the gate, current will flow from the source to the drain and the alarm will sound.

If you add a normally closed switch to the circuit, like so.

Opening the switch will prevent current from flowing to the gate, simulating a loss of current flow from the detector. Which in turn, will cause the alarm to trigger.

So when you press the test button on an ionization type smoke detector, you're not actually testing the circuits ability to detect smoke. Rather, you're testing the circuits ability to react to a loss of current flow to the gate contact of the MOSFET. You're also making sure the battery is not dead, and has enough juice left to sound the alarm if need be.

I've had similar problems from several smoke detectors at different job sites. Here is what I learned in the process:

1. Smoke alarms have expiration dates. That is they will work for a finite period of time before you will need to replace them. 5-6 years approximately is the length of time to replace (or when the manufacturer recommends). Look on the back side for a production date.

2. There should be a code explanation for the blinking lights or the beeping on the back of the unit. Most beeping is for a low battery that needs replacing.

3. RC states all new houses will be fitted with battery back-up powered smoke alarms with the main power supply as house current. Further all smoke alarms will be Daisy Chained so that when one alarm is activated all the other alarms will sound an alert.

4. All alarms are not created the same. If you purchase the least expensive alarm you can expect to replace much sooner or have more problems with it than a more costly alarm. Most manufacturers warranty their devices for no more than a year and most will replace a faulty one with proof of purchase.