Triggering Relays on Relay Shield with external 12V supply

I’m in the process of using the Particle Photon + Relay Shield to bring an 15 year old home security system into the cloud.

Rather than replacing the alarm control board, I want the Particle to be able to sense:

  • when the siren has been triggered (12V is fed to the siren)
  • when the system is armed (a 12V output is available to trigger a relay

The some of the relays on the shield will be used to:

  • Arm the old alarm system (which is done by closing the circuit on one of the zones)
  • Control a ventilation fan

The old alarm is providing a 12V supply (with a battery backup) which is handy.

The relay shield does allow for external 5V to trigger the relays by cutting a track… and I’d thought that by doing that the relay contacts could be connected to the Photon digital inputs. Could I

Alternatively I’m reading up on using opto-isolators?
Maybe I could use transistors instead of relays?

…but my electrical skills are not

  1. Use a simple two resistor voltage divider to bring down the 12V “armed” output signal to 3V3 to be fed into a analog/digital input on the Photon
  2. “Arm the old alarm system” is possible by wiring up to a relay
  3. “Control a ventilation fan” will depend on whether you want to turn on/off the power only or vary the speed