Smoke alarms are extremely useful devices, but they suffer from one significant disadvantage, which is that they’re battery powered. This can’t really be avoided since a smoke alarm should still function when the AC grid cuts out. Unfortunately, it still runs off the battery even when the AC line supply is present, with the result that for some types of smoke alarm you need to replace the batteries at an alarming rate. This circuit lets the smoke alarm run off the AC grid normally and only switches to the battery when it is really necessary, in other words, when there is a power cut. The circuit itself is fairly simple, but as is often the case, the circuit’s strengths are in its simplicity. In this instance, an AC power adapter is used to power the smoke alarm.
The circuit has been designed such that just about any type of adapter can be used. The polarity of the power plug isn’t important and it doesn’t matter if the power adapter outputs an alternating or direct voltage, thanks to the presence of a bridge rectifier (D1 to D4). The only thing to watch out for is that the output voltage of the adapter should be 9 V or more. After the bridge rectifier and a smoothing capacitor is voltage regulator IC1 (an LM317), which keeps the voltage constant. The output voltage of the IC can be varied by adjusting the value of resistor R1. D5 and D6 make up an ‘electronic switch’ that automatically selects the highest supply voltage for the smoke alarm, so either the AC line supply or the battery supply. R1 has to be chosen such that there is a reverse voltage of about half a volt across diode D6 (with a full 9 V battery) when the smoke alarm is switched on. This prevents the battery from discharging unless the AC power fails. To mitigate the self-discharge of the 9 V battery, resistor R3 has been added, which delivers a minute current that can somewhat compensate for the self-discharge of the battery. This will keep the battery in a charged state for as long as possible. (Note from the editors: this method is usually not recommended with normal (i.e. non-rechargeable) batteries, as it can sometimes result in battery leakage. However, due to the very high value of R3 it’s very unlikely to happen. For those of you who want to be absolutely safe, you can just leave out R3.
The diodes aren’t critical and have been selected from the 1N4000 series, in this case with a larger than required 400 V rating. The electrolytic capacitor may also be larger, both in value and in voltage. AC power adapters often output a somewhat higher voltage than is stated, which should be taken into account when choosing the electrolytic capacitor. The condition of the battery can easily be tested (once a week) by switching off the AC adapter and running the self-test of the smoke alarm whilst it is being powered only by the battery.