What I want from the system is:
- Uses off the shelf smoke detector and smoke detector relay components
- Requires no, or minimal changes to the smoke detector or other off the shelf components.
- DC powered only
- Battery backup in case the mains power supply goes out
- Latching relay output (to keep the printer shut down)
- A logic level (5v/3.3v) output which goes low when the alarm is triggered (future expansion or smart home integration)
- (low priority) A 9v or 12v output that latches high when the alarm is triggered (for lights or an additional siren).
- (low priority) A 2nd alarm input option to kill power using the relay (the thought was to have the ability to connect another detector or a temp sensor that would trigger it to power down).
The smoke detector options I am looking at using are:
First Alert BRK 7010B Hardwired Smoke Detector
- 9V battery
First Alert BRK 3120B Hardwired Smoke Detector
- Photoelectric and ionization sensors
- 3V (2xAA) battery
And I also plan to use this Smoke Detector Relay:
First Alert RM4 Smart Relay
Basic instructions for the RM4 Smart Relay
I really like the BRK3120B since it has the dual sensors, but I already have a BRK7010B photoelectic around here somewhere, so I may just use that. The only major difference that will affect my plans is the battery voltage on the BRK7010B is 9v and the BRK3120B uses 2xAA batteries (3V). In either case, I plan to power the detectors using their battery backup circuits which would be wired into the appropriate DC regulators, with a backup battery that will go online in the event that the DC regulators fail.
My current plan is to break the project up into two parts. The first is the input power, battery backup, battery indicator LED and the battery reverse polarity protection. The more difficult of these to do will be the 3V system since there are fewer volts to work with, but I made a schematic which I need to test out (below), that should include all the things I need. Note the resistor values are going to change and this is a rough draft - has not been tested. Also, I am no expert so I will probably fumble my way through this project. If I go with the 9V system it will be a bit simpler with just a diode for the reverse protection and I will swap the input for 12V with a 9V DC-DC regulator after that. I already have tested the 9V battery backup setup and with some tweaks it could probably work here also.
None of the above is my original design, these designs were used as templates:
Low Voltage Drop Battery Protection