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Old 28-07-2019, 16:29   #1
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How to build a 12-volt transistor burglar alarm?

I want to build my own burglar alarm, using a limit switch that activates the alarm and transistors so that the alarm will keep sounding even if the switch is closed until a separate arming switch is turned off. I want to use 12 volt DC power supply and an 8-ohm car alarm siren. Searching around on the internet, I can't find any info or circuit diagrams that meet these requirements. Anybody know how to wire up the transistors, and what kind to use, to make this work?
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Old 28-07-2019, 16:36   #2
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Re: How to build a 12-volt transistor burglar alarm?

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I want to build my own burglar alarm, using a limit switch that activates the alarm and transistors so that the alarm will keep sounding even if the switch is closed until a separate arming switch is turned off. I want to use 12 volt DC power supply and an 8-ohm car alarm siren. Searching around on the internet, I can't find any info or circuit diagrams that meet these requirements. Anybody know how to wire up the transistors, and what kind to use, to make this work?
These days the simplest way to throw together a burglar alarm would be using a small Arduino, or another small microcontroller, and a MOSFET transistor or a relay to control the siren.

With a microcontroller involved, you can have multiple sensors, delayed trigger, auto shutoff after a few minutes, wifi or cellphone alert...
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Old 28-07-2019, 16:43   #3
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Re: How to build a 12-volt transistor burglar alarm?

Just buy a car alarm from ebay and mod it !
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Old 28-07-2019, 18:21   #4
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Re: How to build a 12-volt transistor burglar alarm?

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Originally Posted by Water Dragon View Post
I want to build my own burglar alarm, using a limit switch that activates the alarm and transistors so that the alarm will keep sounding even if the switch is closed until a separate arming switch is turned off. I want to use 12 volt DC power supply and an 8-ohm car alarm siren. Searching around on the internet, I can't find any info or circuit diagrams that meet these requirements. Anybody know how to wire up the transistors, and what kind to use, to make this work?
Hmm... here is another (old school) suggestion. Instead of transistors or micro processors, just use a simple latching relay. Plenty of them are around and several different types. Me, I'd use the mechanical dual coil type.

Your arm switch would be the 12VDC supply switch. The alarm activation switch (limit switch) would operate the relay which latches ON and also operates the siren. The relay remains latched ON so siren stays on until the arm switch is turned OFF removing the 12VDC to the circuit thus causing the really to return to it's normal unlatched state.

Total components are the arm switch, alarm activation switch (limit switch), latching relay and siren.
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Old 28-07-2019, 18:40   #5
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Re: How to build a 12-volt transistor burglar alarm?

I have an alarm system for engine and bilge alarms. I just used 555 timer boards that are a few bucks each on ebay. When these trigger, they stay on regardless of the input state for the set amount of time. Resetting is done by simply de-powering for a moment. When coupled to a 120dB alarm (also available on ebay for a couple of bucks) no crook will hang around for long.



This will do exactly what you want for very little money.
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Old 28-07-2019, 19:18   #6
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Re: How to build a 12-volt transistor burglar alarm?

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I have an alarm system for engine and bilge alarms. I just used 555 timer boards that are a few bucks each on ebay. When these trigger, they stay on regardless of the input state for the set amount of time. Resetting is done by simply de-powering for a moment. When coupled to a 120dB alarm (also available on ebay for a couple of bucks) no crook will hang around for long.



This will do exactly what you want for very little money.
Yes, in some ways a 555 timer is better way to go compared to a latching relay especially if you want it to turn off autonomously after a user defined time period - say 5 or 10 minutes. Not much harder to achieve and plenty of data on the web.
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Old 28-07-2019, 20:05   #7
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Re: How to build a 12-volt transistor burglar alarm?

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Yes, in some ways a 555 timer is better way to go compared to a latching relay especially if you want it to turn off autonomously after a user defined time period - say 5 or 10 minutes. Not much harder to achieve and plenty of data on the web.

Just need to buy the boards, which are a complete circuit. They include an NO/NC single pole relay so can also be tweaked to be delay timers too.
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Old 28-07-2019, 20:23   #8
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Re: How to build a 12-volt transistor burglar alarm?

Lots of ready made boards using 555 timer for $1 up on eBay.


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Old 28-07-2019, 21:59   #9
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Re: How to build a 12-volt transistor burglar alarm?

if your siren is going off constantly forever a neighbor is going to come sink your boat. I would.

put it on a few min timer. ideally it would reset so it could be triggered again if they are still on board.
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Old 29-07-2019, 09:35   #10
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Re: How to build a 12-volt transistor burglar alarm?

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if your siren is going off constantly forever a neighbor is going to come sink your boat. I would.
That's kind of the point, isn't it... to get someone to actually come and investigate? Everyone just ignores car alarms because they turn off after a couple minutes.

I like these ideas about the relays and timer boards. Thanks, guys.
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Old 29-07-2019, 10:36   #11
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Re: How to build a 12-volt transistor burglar alarm?

The only reason anyone is going to come 'investigate' your alarm that has been on for over a half hour is to use a baseball bat on it. Not to mention even if someone comes in less than 30 seconds, unless the cabin is open they can't turn it off anyway so your battery is going to be continuously drained and the alarm continuously irritating everyone nearby. Since sound carries very far over water, you may even get a call from the police about noise violations.
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Old 29-07-2019, 10:41   #12
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Re: How to build a 12-volt transistor burglar alarm?

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Originally Posted by Water Dragon View Post
I want to build my own burglar alarm, using a limit switch that activates the alarm and transistors so that the alarm will keep sounding even if the switch is closed until a separate arming switch is turned off. I want to use 12 volt DC power supply and an 8-ohm car alarm siren. Searching around on the internet, I can't find any info or circuit diagrams that meet these requirements. Anybody know how to wire up the transistors, and what kind to use, to make this work?

Sounds like all you need is a double pole normally open 12V DC relay using one pole for the siren and the other as a latching contact with remote turn off switch in the latching circuit. If intended for use when unattended then you need the transistor, SCR and an RC circuit so your battery doesn't go down the tubes or neighbors shooting you on sight.
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Old 29-07-2019, 10:52   #13
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Re: How to build a 12-volt transistor burglar alarm?

If you use a relay based alarm the draw will be 750mA and when the siren goes then a lot more. Thats OK if you are using solar charging on a decent sized domestic battery but a low power arduino will draw less especially if you use a wake up of an alarm. I use an arduino with two IR sensors and a GSM module to call me. I also use this device to check the batteries, gas and bilge alarms. Texts me once a week to say alls well then calls me if its not.
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Old 29-07-2019, 11:01   #14
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Re: How to build a 12-volt transistor burglar alarm?

The most important part of a alarm is the noise maker. You need something that bystanders will pay attention to. May I suggest a mechanical fire alarm bell. The peizo beepers are essentially worthless as an attention geter. Even a very loud backup alarm is ignored by the public at large.

If you are in a area with a cell tower, an alarm that could call you would be an asset.
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Old 29-07-2019, 11:32   #15
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Re: How to build a 12-volt transistor burglar alarm?

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If you use a relay based alarm the draw will be 750mA and when the siren goes then a lot more. Thats OK if you are using solar charging on a decent sized domestic battery but a low power arduino will draw less especially if you use a wake up of an alarm. I use an arduino with two IR sensors and a GSM module to call me. I also use this device to check the batteries, gas and bilge alarms. Texts me once a week to say alls well then calls me if its not.
The relay only. draws anything when the alarm is activated. If the OP wants something simple that's the way to go. in this day and age I can't see considering using transistors when out of the box solid state items are available. I built one with a timed delay on the arming and a time delay that turned it off. That was in the dark ages some 50 years ago, today no way.
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