Intruder Alarm Help

[Daft_Fader]

Hi All,

I want to build an intruder alarm for my shed, I have a few ideas on what I need to do but need some help with the fine details.
The system needs to have 2 parts to it, part 1 will be in the shed and part 2 will be in the house, the 2 parts will be linked by wire.

Part 1:
A simple circuit powered by a 12v battery, a N.O magnetic reed switch will be fixed to the shed door so that when the shed door is closed the circuit is closed, which will then light up an LED to show that the system is armed, and also power a SPDT relay which links to part 2.

Part 2:
Also powered by a 12v battery.
When the relay is powered I just want an LED to light up to indicate that the circuit is powered up, when the relay loses power by either the door being opened or the link wire being cut then I want a siren to go off and an LED to light up.

I need some help making sure that this is logical and if it can be improved by adding some more components, and also what value resistors, LED's etc will be required.

I have attached a diagram of the circuit.

Any help or suggestions will be gratefully appreciated.

Thanks,

 

[AnalogKid]

Nothing wrong with your design so far in terms of the voltages and signals, but even with only 10 mA through the LED and relay coil in Part 1, battery life might be a problem.

1. If the sounder is a piezo beeper, it already is current limiting and does not need to be in series with a resistor. It can be in parallel with R2/D2.

2. At a minimum you can rearrange things so there is no battery in the shed, only the door switch and the LED. Do this by moving the shed battery and R1 so that they are between the LED D1 cathode and the right side of the relay coil. Then redraw the red and blue loops so the battery and R1 inside Part 2. Moving R1 to Part 2 lets it limit the external loop current if the wires are nicked and shorted together.

3. Consider transistor switches instead of the relay, or at least a transistor driver for the relay. This can cut the static (non-alarm-state) current by 50%, a significant battery power savings. More on that if you want to go in that direction.

Nice use of reference designators in your schematic. It makes things so much easier to discuss.

ak

 

[AnalogKid]

Here is a re-draw of your schematic to show what I described. Note that this will work only if the relay coil pull-in current spec is very low.

ak

 

[AnalogKid]

Puttering around, first pass at a non-relay design. D4 and D5 protect the MOSFET input from noise and transients when the door is open.

ak

 

[Daft_Fader]

Puttering around, first pass at a non-relay design. D4 and D5 protect the MOSFET input from noise and transients when the door is open.

ak
View attachment 29902

Thank you very much for this, I am thinking of controlling the volume of the siren so it's just loud enough to wake me up when I'm sleeping, and full volume when I am out of the house, and adding a switch to the house side so I can disarm the alarm when I need to enter the shed, would that fit into the 'non relay' design?
Thanks again, I appreciate your help with this.

 

[AnalogKid]

What is the siren manufacturer / part number / operating voltage / operating current - anything? Can you post a datasheet or link?

ak

 

[Daft_Fader]

What is the siren manufacturer / part number / operating voltage / operating current - anything? Can you post a datasheet or link?

ak

I haven't purchased any parts yet, I was thinking of something like this:
https://www.amazon.co.uk/Wired-Sire...coding=UTF8&psc=1&refRID=XKF5KTD57W87E6P3M6RD

 

[AnalogKid]

A pot in series with the beeper will let you adjust its volume, but the pot resistance value and wattage rating cannot be calculated without know the operating current at 12 V.

ak

 

[Daft_Fader]

A pot in series with the beeper will let you adjust its volume, but the pot resistance value and wattage rating cannot be calculated without know the operating current at 12 V.

ak

OK, I've had some more thoughts on this and twiddling around with a volume control might not be the best idea, I'm thinking maybe have 2 beepers, 1 around the same decibels as an alarm clock and another around 100-120db and switching between them when needed.
I've also thought about powering the alarm and maybe having it powered by a 12 v (or lower if possible) mains adapter, and then having a backup battery in the device that can take over if there is a power outage to the mains supply.
I also need to be able to disable the alarm when I enter the shed.

I have added some details to the schematic and need some input to see if what I have added will work?

The only other thing that concerns me is if the shed door is closed immediately after the alarm sounds then the alarm will stop, is there a way that once the shed circuit is broken the alarm will sound until the device is reset?

 

[AnalogKid]

1. Alarm disable switch inside the shed.
2. Alarm latch.

Both of these are significant expansions of the original operational logic, but not a huge deal - IF (big if) you can run more wires out to the shed. the original circuit used one pair for both remote activation and remote display. Adding remote deactivation using only two wires means that there now have to be three voltage levels, Normal (green LED), tripped (no LED), and a reset signal back to the main circuit.

The new functions require some kind of latch circuit. This can be done with transistors, or comparators, or a logic gate package, or a relay, etc. Controlling the latch now becomes something to be managed. For example, if you open the shed door, then hit the hidden reset switch inside the shed, what re-arms the system? Does it re-arm the instant the shed door is closed again, or not until you hit the reset button back at the house, or what?

Again, not hard or expensive, just more details. Feature creep always adds more questions than answers.

ak

 

[eetech00]

Hi

Also, probably will need a timer based on the requirements stated in post #9.

 

[Daft_Fader]

1. Alarm disable switch inside the shed.
2. Alarm latch.

Both of these are significant expansions of the original operational logic, but not a huge deal - IF (big if) you can run more wires out to the shed. the original circuit used one pair for both remote activation and remote display. Adding remote deactivation using only two wires means that there now have to be three voltage levels, Normal (green LED), tripped (no LED), and a reset signal back to the main circuit.

The new functions require some kind of latch circuit. This can be done with transistors, or comparators, or a logic gate package, or a relay, etc. Controlling the latch now becomes something to be managed. For example, if you open the shed door, then hit the hidden reset switch inside the shed, what re-arms the system? Does it re-arm the instant the shed door is closed again, or not until you hit the reset button back at the house, or what?

Again, not hard or expensive, just more details. Feature creep always adds more questions than answers.

ak

OK, thanks, I'll scrap the idea of having the alarm stay on once triggered.
Are the DPST switch, second beeper and battery backup changes OK, or does it need reworking?