Smoke alarm -- low battery beep -- timed how?

[lee321987]

Hello.
Can anyone please tell me what the typical circuit or mechanism is that times the beep that a smoke alarm makes when the battery is getting low?

And say the battery is getting _really_ low -- how much might that effect the timing?
For instance -- if the timer is supposed to beep every 30 seconds, could a really low battery cause it to take 3 minutes?

Thanks very much for any help.

 

[tpetar]

How do you mean battery? There is no battery in smoke alarm sensors. They get power over wires. Testing of sensors is done periodicaly, can be every day, and test signalisation can be some LE diode on smoke sensor case.

Maybe you have portable and standalone smoke sensor. Best way is to change battery every year, and use quality alkaline such as Duracell, visual inspect of batteries and their contacts in device at every 6 month. Read specification what is standby current consuption when sensor is armed.

 

[lee321987]

Yes -- i did mean the battery-powered kind.
Thank you, but all i want to know is how the low battery beep interval is timed.
And how far off of that time it might go if the battery were _very_ low.

 

[tpetar]

That existing device have battery self-test capability ?

You can make timer duration for beep, in range what you want.

Timer with beep can work until :
- battery is discharged completely or timer stops beeping,
- some other circuit sense some low voltage level and disconnected everything,
- battery voltage go very low and circuit and beeper cant operate on that low voltage.

 

[lee321987]

All i'm trying to do is figure out why my neighbor's smoke alarm seems to be beeping at different intervals.
I've timed 30 seconds in between beeps, all the way to 3 minutes.
I don't see how it's possible for a timer that's supposed to go off every 30 seconds to take 3 minutes to go off.
So i'm wondering it it is _really_ a smoke alarm's low battery beep that i'm hearing.

 

[tpetar]

This is not battery self test or low voltage level warning, that smoke sensor is activated with some smoke or device have some error.

There is lots of products and ways how manufacturer make their products, and regulate ways of functioning.

Smoke sensors usually monitor temperature with termocouple temp sensor, or uses optical link iterruption method with smoke. There is other methods but this methods are very often used.

 

[crutschow]

Yes, battery smoke alarms do periodically beep when the battery goes low, as a safety warning. The timing is likely determined by some simple RC circuit one-shot and thus is not necessarily very stable. The time is probably affected by temperature, internal circuit noise, and the actual battery voltage. The beeping will continue until the battery voltage gets too low to power the beeper.