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  #1  
Old 08-03-2008, 09:23 PM
Telewanger Telewanger is offline
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Default Help with low voltage alarm circuit, Please!

Hi,

I am a commercial plumber in North Carolina.

I have invented a 9 volt, magnetic reed switch activated, two conductor, high wet well alarm system. It was simple for a non electronics minded person.

The alarm is 108 Decibels. It has a push to test battery switch, and works great!

I need to put a low battery warning chirp circuit in it, so that it chirps like a smoke detector when the battery gets low, and needs to be replaced.

I called all of the smoke detector companies and they will not give out this information. They said that it is properiority information.

Does anyone know how to make one, or where to purchase such a circuit for a 9 volt project?

I would really appreciate any info!

Thanks,

Mike
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Old 08-03-2008, 09:49 PM
Audioguru Audioguru is offline
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A modern smoke alarm doesn't use a 100 parts. It uses an IC that was designed for a smoke alarm that has a low battery voltage detector built-in. The IC has 100 parts all in one.

You could make your own low battery detection circuit with about 10 parts but it would be on all the time and it would quickly kill the battery. By adding another 10 parts then the low battery detector would be asleep most of the time then wake for only a moment occasionally to detect the battery.
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Old 08-04-2008, 12:37 AM
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SgtWookie SgtWookie is offline
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Another possibility is to use a microcontroller, set to sleep most of the time - just wake up every few hours, check the battery level quickly - if it's OK, go back to sleep. Otherwise, set the alarm clock for 10 minutes, give a chirp, and hit the snooze button.

Unfortunately, 9v is beyond the supply voltage of the uC's that I'm familar with. Basic Stamps can run from 9v due to having an internal voltage regulator, but the regulator would run the battery flat in no time.
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Old 08-04-2008, 01:45 AM
AllVol AllVol is offline
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Quote:
Originally Posted by SgtWookie View Post
Another possibility is to use a microcontroller, set to sleep most of the time - just wake up every few hours, check the battery level quickly - if it's OK, go back to sleep. Otherwise, set the alarm clock for 10 minutes, give a chirp, and hit the snooze button.

Unfortunately, 9v is beyond the supply voltage of the uC's that I'm familar with. Basic Stamps can run from 9v due to having an internal voltage regulator, but the regulator would run the battery flat in no time.
Why does the sensor have to run on 9 volts? Wouldn't 5V do just as well? Perhaps the alarm itself would have to be reworked, or use a separate supply.
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Old 08-04-2008, 02:14 AM
Audioguru Audioguru is offline
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Why does the sensor have to run on 9 volts? Wouldn't 5V do just as well?
Have you ever seen a fairly small 5V battery?
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Old 08-04-2008, 02:44 AM
AllVol AllVol is offline
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Have you ever seen a fairly small 5V battery?
Who said it had to be small? I don't recall that spec. At any rate, three AAA's would last years when a PIC is asleep 99% of the time, and some PICs are designed to run on less than 4.5.
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Old 08-04-2008, 03:02 AM
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A PIC12F675 can run on from 5.5v down to 2.5v with the A/D converter enabled at 4MHz. That uC would have plenty enough horsepower to do the trick - if it's not subjected to overvoltage. 6.5v is absolute maximum.

I suppose a forward-biased standard silicon diode could be used to allow it to run with 4 AA's or AAA's.
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Old 08-04-2008, 04:54 PM
Telewanger Telewanger is offline
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Hi,

I used to work for US Air, on small turbo prop planes, so I know a little bit about DC voltage, but not much. This is how I knew how to make the alarm.

Some of your suggestions sounds like Chinese to me.

I know that I can buy a smoke detector with press to test and a low voltage warning for around $8.00 to $15.00, and the smoke detector runs off of a 9 volt battery.

It will be necessary for my device to let me know when the battery is going dead, just like a smoke detector.

Can I buy this little circuit?
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Old 08-04-2008, 05:04 PM
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Well, the smoke detector alarms use a single proprietary IC to detect the output from the sensor, sound the alarm, monitor the battery voltage and provide the "push to test" function. There are very few actual components.
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Old 08-04-2008, 05:35 PM
RiJoRI RiJoRI is offline
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How about feeding Vbatt into a comparator -- when the voltage drops too low, it trips a 555 driving a sounder?

--Rich
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