We went low tech at the beginning; with just an audio one, but we found it to be a bit unreliable and we couldn’t tell if baby was just crying in his sleep or actually crying for something else, with his eyes open. We ended up buying this one: https://www.amazon.com/dp/B01CZUD4R8/ref=cm_sw_r_cp_api_plELAbYBP722Z and love that it also tells us the (approximate) temperature in the room. It’s pretty cheap for a video monitor and doesn’t require keeping your phone’s (Vimeo / Amcrest / other) app open to keep an eye on him/her.