When the doorbell button is pushed, a complete circuit is formed, the chime gets power, and we hear the chime. This also means when the button is not pushed, the circuit is not completed and chime doesn't get power. So, how does an illuminated doorbell button get power to light up but the chime doesn't? There are only two wires. That means there is only one circuit. Devices on the same circuit either all get the power or all don't.
I think there are three wires for those switches. One hot that supplies the light and the switch, one neutral for the light, and the third that goes from the switch to the chime. Others, more familiar with that will probably comment.
The light alone does not draw enough current to cause the chime to sound. Only pressing the button allows the full current to activate the chime.
They use an LED that is across the switch. It uses so little power that it's not enough current to cause the chime to ring. When you push the button the switch closes and the light temporarily goes out.
Yeah, but they had two-wire illuminated doorbell buttons before LEDs were invented. Like you say, they too used so little current that the current they do use can flow the doorbell windings without being near enough to energize them.
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