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By sgjajva
#85582 Howdy folks, I have a 13 year old wired alarm system (originally Honeywell/ADT) that was upgraded to Bright House (Spectrum) home security which was discontinued on 2/5. I have a wireless solution in place with no contract monitoring in the interim, but I want to leverage all the wired sensors.

I'm comfortable using NodeMcu and Lua (I've been a developer for 40 years) and even wrote my own User Space IO library for Linux I also have some electronics experience under my belt, so I want to convert my old wired system into a smarter system. I figure if someone has been through this they can give me some timely advice (I already know about, but it's expensive for what it is). First of all here's what I identified:

  • Power transformer going into panel primary 120 VAC 40W, secondary 16.5 VAC 40 VA
  • 1 Honeywell 6150 keypad
  • 5 Kidde AC Hardwired Smoke Alarm i12060
  • 1 System Sensor 2WTA-B Photoelectric i3 Smoke Detector, 2-Wire
  • 1 Bosch DS835i PIR
  • 10 window magnetic reed switches
  • 4 door magnetic reed switches
  • 1 siren

I'd like to switch to 5V DC to power everything, so I'm OK with switching the PIR to a HC-SR501 for instance. I know I'll have to do the same for the siren, etc. Are there any pitfalls to going with 5V DC? I'd probably use a NanoPI Duo and a couple NodeMcu units. Any way I ordered some magnetic reed switches (NC) to play with since I have to wire 14 of those up. I'm more concerned about the smoke detectors, siren, etc.

Eventually it would be nice to have battery backup and wireless cell service fo alarms. Any help or pointers will be much appreciated.
User avatar
By rudy
I'd like to switch to 5V DC to power everything


How will you provide backup power to the system? If you plan to still use a siren then you will be better off with 12 volts.

Professional alarm systems typically use 12 volts. The CPU controlling the system will often be powered by 5 volts. Newer higher performance CPUs are typically 3.3 volts. This reduces the noise margin of the system.

There are different ways to design the alarm loop for sensors. The approaches vary depending on how secure the designer decided was needed. Some will use an End Of Line (EOL) resistor, so that if the loop is shorted, it will detect it as an alarm condition. The following is simpler, with less security, but far better than a simple grounded input.

alarm-loop.png (14.78 KiB) Viewed 2241 times

The loop is powered by the system voltage, nominally 12 volts. The CPU could be powered from 5 volts. The Safe state is with the loop closed. The voltage at the input of the CPU port pin will be a logic high. When the loop is opened the input will be pulled low, the alarm state. The 100k resistor will limit the current into the CPU pin to a safe level. The 100K and the capacitor will filter noise from the loop.

When the system is running off of the battery the loop voltage will still provide a logic high as the battery voltage drops.

I used to work for a company that manufactured alarm panels, and then later other control electronics. When I moved into my house 31 year ago I installed one of the alarm panels that we manufactured, that I designed. It was designed to be reliable. The only issues I have had with my alarm system is with the battery needing replacement, a normal thing.

Alarm systems need to be reliable. I don't consider WiFi, the ESP8266, the software, to be reliable enough for a professional grade alarm system. I am adding IOT devices to my home, but the alarm system will always be a self contained system designed for reliability. I may have both systems monitoring the same devices, I probably will change out the alarm panel using a new CPU, but the control will be something where I can go through every line of code and know what it does.
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By sgjajva
#85599 Thanks for the reply. I wouldn't use wifi as a primary communication channel. I have a 1GB switch in the panel, thus I'd use wired networking to the NanoPi (primary) and either I2C or SPI to the NodeMcus. I'd use the NodeMcus as micro controllers basically. I think 12V DC is a good idea, so I can leverage the siren and PIR if I wanted and use a step-down to handle 5V powering the NanoPi and NodeMcus. I've had very good reliability with NodeMcu, but maybe that's not your experience.

I'd just use a simple battery recharging circuit for battery backup. I have experience with ... p-176.html, but I'd have to find something similar for 12 V.
User avatar
By rudy
#85610 Sealed lead acid (SLA) batteries have been used in alarm systems for a lot of years and they still are the prefered choice. They only require a simple float battery charger. Normally the size and weight is not an issue. There is no need for lithium batteries.

In most cases a battery for an alarm system will be at 100% charge and rarely called on to supply power. The battery will supply all the power when there is an AC power fail. The battery will also deliver power to the sirens when they are on even with the AC present. Alarm system power supplies do not need to be capable of supplying the worst case power requirements, when the siren is on. 30 watt sirens are often used and the battery needs to be capable of supplying that power, and not the power supply. (since the system needs to be fully functional during power interruptions)

I've had very good reliability with NodeMcu, but maybe that's not your experience.

I design electronics to be sold, installed, and for the most part, forgotten about. It could be many years of operation without a power cycle or restart. My concern is with thousands of installed devices and not just one system that I can take care of when hiccups arise. Your expectations may very well not be the same as mine.