A video doorbell is usually sold as a straight replacement for the button by the door, and electrically it is not. A traditional doorbell is a momentary switch: press it and a circuit closes for as long as a finger is on it. A video doorbell is a device that needs power continuously, and in a wired installation it takes that power from the same loop the chime sits on. That difference is why a chime that was silent for twenty years starts making noise.
The chime is an electromagnet in series with your doorbell
Inside a mechanical chime is a coil. When current flows, the coil pulls a plunger that strikes a tone bar. In the old arrangement, current flows only while the button is pressed, the plunger strikes, and everything rests again.
The video doorbell needs a small standby current all the time. It draws that current through the transformer and through the chime coil, because that is the only path available. The coil now carries a continuous current — too little to pull the plunger all the way in, but enough to hold it part-way, and a partly-energised AC coil held against a spring vibrates at mains frequency. That is the buzz.
It is not a fault in the chime and not a fault in the doorbell. It is two devices designed under different assumptions placed on the same loop.
Why it shows up on some installations and not others
Whether you hear it depends on three things. Transformer voltage and capacity: a weaker transformer sags under the extra draw, and the chime sees a worse waveform. Chime design: some coils tolerate a standing current quietly, others sing. And the doorbell's own standby draw, which varies and rises whenever the camera wakes, streams, or charges a battery.
That is why the same doorbell is whisper-quiet in one house and audible in the next, and why swapping the chime for another mechanical unit is a coin flip.
The three fixes, in order of how much they disturb the house
The usual remedy is a bypass: a resistor or diode fitted at the chime terminals that gives the doorbell's standby current a path around the coil, so the coil only sees current when the button is genuinely pressed. It is a five-minute job at the chime, and it is the standard answer.
The second option is to replace the mechanical chime with an electronic or wireless one. Electronic chimes do not have a coil in the power path in the same way, so they are largely immune, and if you were going to change the tone anyway this is the moment.
The third is to stop using that loop for power entirely — battery or a dedicated low-voltage supply — which removes the interaction but also removes the reason for wiring it up in the first place.
What to check before buying
Three things predict whether you will be dealing with this. What the existing transformer actually supplies, printed on the unit rather than assumed. Whether the chime is mechanical, which you can usually tell by taking the cover off and looking for a coil and plunger. And whether the doorbell you are buying is designed to sit in series with a chime at all, because some are not and specify a direct supply instead.
XingLiao builds video doorbells for wired installations and the wider doorbell range in Shenzhen.
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