Home alarm system build questions - Page 1
Forum section: electronics / beginners

author: birminghamwildcat
author_rank: Newbie
timestamp: April 14, 2017, 02:52:05 am
content: Hello! I'm a complete noob at this so please bear with me. My grandfather built an alarm system for his home in the early 80's. I don't remember too much about it, but I want to build something similar so I'll describe it and hopefully someone here can help me fill in the blanks. Unfortunately, he died in 1993 and the house was burned by a homeless person on Christmas 2007 so I can't go back and look for reference.I also realize that this is more electrical rather than electronics, but I believe the two are similar for the purpose at hand.I remember the whole thing was controlled by a bunch of relays mounted on a board under the main circuit breaker panel. The relays were large, black plastic, each one was roughly 3 or 4 inches high with screw terminals at the front along the bottom. I believe these were (First real bear with me guys, google didn't help much) a type of Normally Open relay that when one of the sensors was activated it closed the circuit. When the alarm was activated there was a school bell and siren that sounded, loud enough to be heard for blocks. They were continuous, so I don't believe there were any controllers for these signalling devices. There were keyed switches located both inside and outside the doors to allow authorized persons egress and ingress without activating the system. The whole thing just plugged straight into a 110v (maybe 120v) US mains plug. Beyond the window and door sensors, and the key switches, and the relays that escape my googling power, and the siren and bell, I am clueless. I have a picture of the circuit in my head, and I'm almost certain without help I'll have to resort to sketchy trial and error, risking death wiring the thing once I undoubtedly mistakenly believe I've bought the proper equipment. This ain't no single switch single lamp lighting circuit. I tried to draw the circuit in my head below. Please feel free to ask questions and give advice. Thanks.PS sorry for the horrible 5 minute MSPaintCAD job.

author: Ian.M
author_rank: Super Contributor
timestamp: April 14, 2017, 03:15:31 am
content: The usual setup for an alarm is to run a current through each sensor loop so it triggers if the sensor opens the circuit. Modern alarm sensors are designed for 12V DC operation. Its usual to also run an anti-tamper loop, which is another loop, running through a NC switch in each junction box or sensor that opens if the cover is disturbed.As alarm wiring isn't rated for mains voltages, the chances are your grandfather used a transformer to provide a low voltage supply for the relays and sensors. If you want to do something similar, you wont be in any danger if you use a short-circuit proof 12V DC supply with 12V relays, and 12V strobes and sounders. As you haven't got any idea yet how its going to wire together, I recommend getting a whole bunch of 12 DPDT relays - board mounting ones soldered on a large protoboard would be cheapest, but ones with plug-in bases and screw terminals would be easier to wire if your soldering isn't so good. Its even possible to do the keypad number checking just with relays, though you need individually wired buttons, and at least one relay per button.Be aware the whole system could be done much cheaper, more easily and with more features with an Arduino + a relay board for the outputs and an input isolator board.

author: picandmix
author_rank: Frequent Contributor
timestamp: April 14, 2017, 08:52:29 am
content: Depends what you want from an alarm system.Suggest you look at a specialist alarm sites like Honeywell to see what modern home alarms are like, both wired and wireless, not to mention all the advances in remote alarm/sms and lo cost video surveillance etc.I recentlly installed a wired Honeywell system with all its modern bells and whistles, battery backup and various door , windows and room sensors, quiet a task to get all wired up neatly, never mind designing and building the actual controller.Don't forget, a diy system will not be recognised by your insurers.

author: Shock
author_rank: Super Contributor
timestamp: April 14, 2017, 10:08:31 am
content: Back then it was common to use normally closed contacts in alarms to prevent tampering.Here is a quick and dirty mockup of an alarm circuitI made showing several NC contacts on one relay, you could run one relay for each contact if you wished which would allow you to identify which was tripped.If a NC contact is broken (opened) by the intruder the current takes the least path of resistance.It turns the transistor on which means the relay coil conducts and switches from the armed position to the alarm position and actives the LED.The LED along with it's current limiting resistor is a substitute for a siren.In order for basic functionality that circuit needs to latch (so the intruder cannot close the door/window and switch the alarm off).Features such as a delay so you can enter the house without the alarm tripping and being able to turn off sensors or sets of sensors (zones) comes standard in commercial products.