alarm_guy1
Posts: 41
Joined: Thu Aug 16, 2012 10:04 pm

Relays / Mosfets

Wed Aug 21, 2019 5:47 am

Hi currently using a couple of relays to between my alarm system and my Pi, so basically when the Pir or door sensor activates the relay is triggeredfrom an output on my alarm system.

Unfortunately the relays are surprisingly loud especially when in the kitchen as the device activates multiple times if there are multiple people in there, I know I could box them to muffle it etc etc etc.

But is there a Mosfet circuit I can use to achieve the same goal?

my alarm panel puts out a 12Vdc signal, hence the current use of relays

cheers

User avatar
rpdom
Posts: 15574
Joined: Sun May 06, 2012 5:17 am
Location: Chelmsford, Essex, UK

Re: Relays / Mosfets

Wed Aug 21, 2019 6:03 am

Using a MOSFET to connect your alarm system to the Pi may cause problems because of the voltage difference. Ideally you want to keep the two circuits electrically isolated from each other.

An alternative may be to use opto isolators with the Input coupled to the alarm system via a suitable resistor and the output connected to the Pi's GPIO also with suitable resistors.

alarm_guy1
Posts: 41
Joined: Thu Aug 16, 2012 10:04 pm

Re: Relays / Mosfets

Wed Aug 21, 2019 8:51 pm

Cheers,

I'll take a look.
Thanks for taking the time to reply

Ilfirin
Posts: 15
Joined: Fri Jul 13, 2012 11:15 am

Re: Relays / Mosfets

Thu Aug 22, 2019 5:36 am

+1 for optoisolator. Simplest, safest and widely used on industrial boards for digital inputs, even 230AC ones.

alarm_guy1
Posts: 41
Joined: Thu Aug 16, 2012 10:04 pm

Re: Relays / Mosfets

Thu Aug 22, 2019 7:32 am

Cheers guys,

taken a punt with 3 of these boards https://www.ebay.co.uk/itm/202091702337

we will know by Xmas

Ilfirin
Posts: 15
Joined: Fri Jul 13, 2012 11:15 am

Re: Relays / Mosfets

Thu Aug 22, 2019 12:27 pm

That's pretty nice solution

alarm_guy1
Posts: 41
Joined: Thu Aug 16, 2012 10:04 pm

Re: Relays / Mosfets

Fri Aug 23, 2019 7:46 am

I'll post back when it turns up

But it will be 2 month before I get it

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