Purchased Edge 840. How can I set my connected Varia RTL515 to turn on when I turn on my Edge 840?

Make sure that the RTL515 is paired as a light and not just as a radar. Put the RTL into pairing mode by switching it on and holding the power button until the LED turns purple. Then pair it as a light.

Hello jeffdossett Please see this FAQ: Control Your Varia Radar Taillight With an Edge Cycling Computer

On the Edge go to : Settings > Sensors > Lights > Network Options > Light Beam Activated and choose device power up or timer start.

Yes, I had added my RTL515 as a radar and not a light. When I removed the sensor (as a radar) and then paired it as a light, I was able to set it up to automatically turn on when my Garmin Edge 840 computer powers up. Thank you.

Thank you, this worked.