HobbyKing E-OSD A very simple OSD (On Screen Display) for FPV users who want to see their 2 or 3 cell LiPo voltage level while flying. Very easy to install; runs in line with your battery and also your video-out wire.
Features: Voltmeter & Stopwatch Double Voltage Input Manual Calibration Programmable Display Content Supports NTSC & PAL Anti-Glare Shade Control Signal
Specifications: Weight: 3.3g Dimensions: 34 x 17 x 4mm Working Voltage: 7.2~12v
Usually they use filters to dampen the interference in the circuit. Its not mandatory.
Just got mine. Works fine. It is worth the price. Pretty simple to monitor FPV pack battery or motor battery (if under 3S). There are some interesting projects to do RSSI monitor (check rcgroups). You have to to make a Y connection between camera wire and tx wire to the OSD (video input) i.e camera signal and tx signal going into video input on the OSD. Gnd can come directly from pack.