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Attached is a picture of a doorbell push button wiring.

What are these circuit components (circled with red) for? Isn't this just a simple mechanical switch?

enter image description here

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  • D1 is the diode that most doorbell systems need, the R's are dropper resistors for the LED.
    – dandavis
    Commented Jun 2, 2021 at 3:36

2 Answers 2

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The door bell units that I am familiar with operate on 24VAC. The wires to the push button switch will be excited with the 24VAC line on one wire and the other wire referenced to the 24VAC-RET return line. For the lowest cost doorbells this switch closes a circuit between the two wires which allows current to flow through the door bell ding-dong mechanism.

Many buttons like this one have an illuminated button feature. The door bell unit allows a small amount of current to flow through the wires to the switch while not activating the ding-dong mechanism. This current is sufficient to light a small LED in the switch button. LEDs operate on a DC current and this needs to be limited to an amount that is safe for the LED. The components that you see in the switch are designed to properly bias the LED without exposing it to excess current or voltage.

Note that in days gone by lighted door bell buttons would use a small incandescent light build designed specifically for operation at 24V. And of course the incandescent bulb is quite happy to operate on an AC voltage unlike the LEDs used today.

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  • LEDs are diodes so can you not simply give them reverse voltage and they'll be nonconductive? I think LEDs aren't sensitive to number of cycles and can be turned on and off (e.g. by AC power) at any speed and still have a lifespan of decades? Finally, if 24VAC is standard, wouldn't a resistor in series with the LED suffice to present the proper voltage to the LED? Commented May 31, 2021 at 9:21
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    @SwissFrank: LEDs are pretty bad at being diodes, maximum reverse voltage is somewhere atound 5V max. Also, a 50 or 60Hz flicker is pretty visible.
    – Pelle
    Commented May 31, 2021 at 9:59
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    @SwissFrank - Yeah @ Pelle is correct. I almost added the reverse bias voltage limitations of LEDs in the answer but decided against turning the answer into an electronics engineering lesson.
    – Michael Karas
    Commented May 31, 2021 at 15:27
  • @SwissFrank you have to guard LEDs from reverse current with a plain diode, or an opposite facing LED (the latter will also even out the light). Commented May 31, 2021 at 23:55
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I will take a SWAG and say that is an illuminated doorbell button using a LED. The black thing is probably a bridge. As far as normal operation just connect the red to white and it will ring. Most of the doorbell buttons I am familiar with are a simple switch.

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