I want to make following schematic to feed and detect a doorbell ring. I have measured the max current through the doorbell to be 800mA. The Pi is fed with a 2.5A Raspberry power supply.
GPIO3 would be configured as a gpiozero.Button, which is a high-impedance input if I understand correctly.
Could this work and not fry the rPi?
The Pi gpio pins are not 5v tolerant and operate at 3.3v logic levels so this set up would likely damage your pi. Additionally the maximum recommended current draw from the 5 V pin is the USB input current (usually 1 A) minus any current draw from the rest of the board. Model A: 1000 mA – 500 mA -> max current draw: 500 mA Model B: 1000 mA – 700 mA -> max current draw: 300 mA
In addition to the risk of putting 5V on a GPIO pin which is quite likely to damage the Pi you are connecting a large inductive load.
This will generate a back EMF of several hundred volts which will probably destroy the SOC.
It is not difficult to reduce a 5V input to ~2.2V (which is recommended reliably detect an input on GPIO) but you would need additional circuitry to clamp the voltage swings and is not recommended.
Suddenly drawing 800mA from the Pi supply, while possible, may have other impacts on the Pi (this depends on the Quality of the power source).
On a further note ANY foreign input to a GPIO should have a series resistor (~1kΩ) to protect the GPIO.
If you are going to do this the following is the recommended MINIMUM.
simulate this circuit – Schematic created using CircuitLab
