SJCAM sj8 air camera, how to trigger remote shutter using fpv cable?

I have a sjcam sj8 air camera with the original cable for aerial photography, that includes fpv, remote shutter, GPS, and many more features, but it’s unclear how to trigger the shutter, I know the blue cable is the one that triggers it, but I tried all the PWM values and it’s still not working (I set it as a relay in mission planner). I also connected an Arduino to see if the APM actually outputs the signal, and it does!

I do not know how it works for SJ8 Air but for previous cameras there were these cables for 3-position switch, Taking a picture at high postion and starting video at low position. Neutral is 1500us. You shoud first be sure that it works by using a servo tester or your arduiuno before you start testing it on Arducopter. SJCam support should be able to tell you. They normally answer quite fast.