12V + and – (from the gate motor supply) to the sonoff + and – power input
sonoff SV output (+ and -) to relay input
relay output (NO contact) to gate motor trigger contacts
The relay can easily be purchased at Communica, Mantech or similar for about R10 a piece This will require some basic soldering skills and a bit of time (approximately 10 minutes I would say).
Please note: the end user accepts full responsibility for death, injury or damage to property when following the above instructions.