Do I need an existing doorbell?

No, but it’s preferable if you do. SkyBell must function as part of complete circuit, meaning it requires direct power on one side and a wired doorbell (or resistor) on the other. The power must come from a low voltage transformer with 10-36 VAC 10VA. If you do not use a wired doorbell chime, you must use a resistor. If you must use direct current, then make sure it is 12VDC (0.5 to 1.0 amps) and you must also use a 10 Ω (Ohm) / 10 W (Watt) Resistor between one wire of SkyBell and one wire of the power source.