Switching off current in an alarm circuit - Page 1
Forum section: electronics / beginners

author: panrid67
author_rank: Newbie
timestamp: November 06, 2023, 08:41:19 am
content: Hi all,I am currently adding smoke detectors to my burglar alarm system. The system has a total of 16 zones which are doubled up on 8 different terminals using different EOL resistors. All zones are full. There are a total of 5 smoke detectors wired in parallel to the one zone terminal. They are all normally open. I have wired an EOL resistor to the last detector in the line. The problem I am having is that when any of the detectors are activated, both zones 8 and 16 are activating as the alarm system is expecting the circuit to open (i.e. infinite resistance). What's actually happening is that the closing of the relay in the detector is creating a short circuit which is leading to a decrease in resistance. Under normal conditions, the circuit is 13.7 V at 2.1 mA. During an alarm condition, the circuit operates at 13.7 V and 4.2 mA.What I am after is a way to dramatically increase resistance in the circuit when the current passes, say, 3 mA for example so that the current is effectively cut off. Alternatively, I'm after a way to cut off the current completely when the current exceeds, say, 3 mA and for the circuit to be closed again once it drops. My only thoughts on this were to use a resettable fuse, however, it seems impossible to find one rated at such a low current. I am sure there must be some other way to do this that I am not thinking of.If anyone has any ideas as to how I can achieve this, that would be great.Thank you.

author: Muttley Snickers
author_rank: Unknown
timestamp: November 06, 2023, 09:36:49 am
content: Quote from: panrid67 on November 06, 2023, 08:41:19 amWelcome to the forum.It would help if you listed the alarm panel brand and model in addition to the smoke detectors you are using as some can be configured as either normally open or normally closed for a series connection, the later would be a solution depending on your existing cabling configuration.By configuring the smoke sensors as normally open in parallel you are simply shorting out both the high and low zone inputs (8/16) when an alarm on any of the smoke sensors occurs.Another solution is to place a relay in the control panel and trigger it with the smoke detectors so the normally closed contacts on the relay have the end of line resistor wired in series back to the zone 16 input. I would also power the smoke detectors from a separate programmed output if available so as to enable either a timed or manual reset if required.

author: panrid67
author_rank: Newbie
timestamp: November 06, 2023, 09:05:04 pm
content: Thanks very much for the welcome and for your answer.It is a Bosch Solution 16 alarm system. I'm using System Sensor 2012/24AUS smoke alarms. They do have the option of using a normally closed or normally open relay, however, there are 5 of them and they are unable to be wired in series so the only conceivable way to elicit any change at the panel is to use the normally open relays.I had thought about using a relay but so far I haven't been able to find any that are available that will work with such low current.I also thought of maybe using some type of transistor but am still trying to get my head around exactly how or if that might work.