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I'm trying to connect the signal from a doorbell to a Raspberry Pi Zero 2W to modernize the existing system with IoT capabilities. I did some measurements and I found that the terminal that powers the indoor unit changes from 0 V to 13.3 V DC when the doorbell button of the outdoor unit is pressed.

Since I plan to use the same power supply of the doorbell to power the Pi (using a 5V buck converter with a 2.5 A output current rating, but that's another matter), GND would be common to both devices. Searching for best practices to adapt the signal to the Pi's 3.3V logic, I came across this great answer:

GPIO level shifting diagram

So, I have two questions regarding the circuit above:

1) In case of an overvoltage transient, I understand the diode D1 starts to conduct and sources the current to the 3.3V rail while clamping the voltage. Do I have to disable the internal pull-up resistor from the GPIO pin for this protection to work properly?

2) With the given values for R1 and R2, the voltage divider would heat quite a bit when the doorbell is ON (roughly 190 mW power being dissipated by R1), is it OK to change the values to 6.8k and 1.2k ohms, or that would be detrimental to the overvoltage protection discussed above (higher series resistance)?

Finally, I'd like to add that the wiring between the doorbell's outdoor and indoor units is around 20m long, but it's done over a UTP cable.

Thanks a lot in advance.

EDIT: Here's a diagram of what I'm trying to do. The green box is the one that's related to the questions of this post.

Full circuit schematic

Seamus
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SierraJuliettMike
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1 Answers1

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The interface below is cheaper and safer than the one you've selected. The Zener is readily available through distribution at a cost of less than USD$0.23.

You should not enable the pullup for this circuit (or the other one).

Taking the 13.3V doorbell signal you gave as input, the power dissipation for this circuit is:

P = V2/R + Vz x Iz
P = [(13.3 - 3.3)2/1000] + [3.3 x ((13.3-3.3)/1000)]
P = .1 + .033
P = .133 watts; 133mW

Alternatively, you may safely increase the size of R1 to 2.2K, and reduce that even further; I leave the calculations to you. But remember, this dissipation is only for the duration that the doorbell is pushed. :)

schematic

simulate this circuit – Schematic created using CircuitLab

Other Notes:

  1. The UTP cable should be fine. If the doorbell button is connected to some sort of inductive device (solenoid, etc); i.e. something that might put an indictive "kick" on the line, you may want to add a small ceramic capacitor (0.1u) across the doorbell wiring.

  2. You shouldn't need a 2.5 amp buck converter to power the Pi 2W. It consumes approx 350 mA at idle. OTOH, the "overkill" of the larger converter won't hurt a thing.

Seamus
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