Hi, I used to design security systems for several major alarm companies in Sydney, Australia from 1976-1982, so I suggest you may wish to look at sample & hold type detection. The I.C.s I used then are very much used today, only less expensive.
That is to say, once a "sensor" a reed switch, a pressure mat, an ultrasonic or a microwave sensor has tripped its own output relay to report back to the actual alarm "box" the actual alarm must hold that signal, be it an on or off state device sensor and pass that detected signal on to another portion of the alarm, that holds the signal level (Hi or Lo) whatever and after a pre-determined time (set by you) this is the entry time, then once that has expired, it then goes into an alarm on mode and passes a signal, (either Hi or Lo) what ever to trigger the siren or noise maker. Note you also need to have an exit time, giving the sensors time to settle and you time to leave the protected premises, of course
Typically mechanical bells make the loudest noise in Decibels than sirens ( volt for volt, watt for watt ), however piezo sounders are also a great noise maker requiring a relatively small current at , say 12Volts DC. Sirens can be made using a garden variety NE555 ic also.
I designed a 4 IC. car alarm (back in 1980) which can be adapted as a house alarm, it uses 3 x CMOS HEF4001 IC and 1 x HEF4011 I.C., about 9 capacitors and some 1N4002 diodes, a relay and a transistor BC635 and about 15 resistors.
The circuit basically was some slow to operate flip-flops. An extra siren was added - externally switching on via the relay.
The relay also could operate another heavy duty air horn relay which made lots of Air Horn noise, but draws many amps.
The noise making is up to you, these are just small suggestions and examples of where you can go with an alarm system done well. You can use a few transistors, a few resistors and a few capacitors and make a oscillator siren. These are quite good. Plenty of circuits are around on the net for these.
These companies I designed for manufactured these like sausages, they were the company's most sought after product back then, they sold many 10's of thousands and made lots of $$$ , but for me, I just got paid for the one design job
..... Lesson learned...do not do anything that you are not going to get residual income for ( royalties ). (eg: like muso's and singers get paid these days) I have not made that mistake ever again !
Start ----------> turn key and activate alarm------> exit time ------->alarm sets after a pre-determined time, usually 20 seconds to 40 seconds. You decide what time is to be used, using a variable resistor or commonly known as a Pot (short for potentiometer). This exit time wil give the "system" time to settle and passively activate and do its quiet job, until woken.
Sensor-----> sample & Hold cct------> entry time cct------> alarm condition detected cct-----> make noise --->- reset and re-arm
Entry time gives you time to go inside and de-activate the alarm , quietly with a key or a numeric keypad... what ever.
Typically all alarms should have a Gelcell battery (12V) with a constant low (milliamps) charger to top up the battery.
Some suggestions using the ubiquitous NE-555 I.C.
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Hope this makes life easier for you. Remember, keep it simple.