I'm not so sure from those voltage readings in post #14. The second photo down shows zero volts across the T3 connector when that zone is activated. That indicates to me the output of the thermostat is a closed relay contact. I believe those voltage readings are just residual from the optocouplers.Yes, it seems that if there is a way to use the T3 thermostat closing to provide a 12 volt signal to a small relay