My smoke detector modification

Many localities require smoke detectors that are operated on both AC and DC. That is, they have a battery so that if the power fails they will still operate. The problem with this arrangement is the inconvenience of having to replace the batteries about once a year. When the battery reaches its end of life it starts to beep about once a minute and, of course, this usually occurs in the middle of the night. The usual trick is to disconnect the entire detector to silence it and then hopefully reconnect once the battery is replaced. It will beep even though AC power is still applied. Often, they are never reconnected. The Consumer Product Safety Commission (CPSC) claims that only 20% (16 million) of smoke detectors are currently working. Improper maintenance is the most cited reason for failure.... usually this means, no battery.

Modern homes have smoke detectors that are wired as a network so that if one triggers, the others will also sound. This is a nice safety feature but requires compatible units.

I have seven smoke detectors in my house and that means seven nine volt batteries that need to be replaced once a year. Like most people, I don't replace the batteries on a regular schedule but wait till one starts to signal its end of life.

All would agree that a disconnected smoke alarm is no good to anyone.

Since the power fails at my house for maybe 3 hours out of an entire year, the time that an "AC only" smoke alarm would be out of service is far less than waiting to install a new battery in a non-operational alarm. Why some fire prevention groups forced both an AC and DC unit to be installed is stupidity in my opinion. It doesn't follow human nature. Leave one a DC only unit near a critical bedroom hall but make the rest AC only. The problem of network compatibility makes this a little more complicated.

My solution

I became annoyed enough by this situation that I investigated the circuitry of ionization type AC/DC smoke detectors. As it turns out the AC power and battery use a diode steering circuit to power the main electronics of the detector. Each can provide power depending, which has the higher voltage. However, on the battery side there is a pulsing circuit that tests the battery about once every 30 seconds to see if the voltage drops below a threshold that is considered end of life. If it drops too far, a short beep will be generated. Therefore, you can't simply remove the battery. You have to provide adequate storage capacity so that this circuitry will continue to function and not register a weak battery. The solution is simple. Bridge the diode that goes from the battery to the common power point with a low ohm resistor, say 47 ohms. This common power point has a large capacitor (220uf) attached and stores enough energy so it can handle the power pulses through the 47 ohm resistor without dropping below the threshold. I now have a smoke detector that operates on AC only, still runs fine on the network and doesn't complain with the battery missing. I can also leave one unit unmodified and it will still work on the network.

Now, when the AC power fails, the "beeper" will sound for a minute or two to warn you that  AC is "off".  Nice added feature that I like.

Later, I realized that simply removing diode D3 and the battery could make the unit an AC only one but at the loss of the AC failure "beep".  Also, depending upon your unit, you could lose the normal "beep" function.....not a good idea!  In short, any modifications you make should be thoroughly  tested, on and off the network.

Simplified Schematic diagram of the modification

Words of caution

I'm now Sleeping better at night......

Revised: 12/01/07