[moved] convert 5V AC to 3.3DC from a single wire

[joeylu]

what I have:
A single wire from my apart wall, that connects to the door button, when I use multimeter, it reads 0V - 0.5V AC, when someone press the door button, it raises to 5V AC
I have no access to the door buzzer transformer, and I don't konw when it's isolated or floating or even has a ground

a Rasyberry Pi B, some resistor, some capacitors, some sharp817 optoisolators, a breadboard, some jumper wires

What I want to do:
I want to connect this wire through an optoisolators and resistors to the RPi's GPIO, so I can call something like
GPIO.add_event_detect(channel, GPIO.RISING, callback=my_callback) to detect the door button push event

The GPIO from the RPi only accepts 3.3V DC

What's my problem:
I have no idea how to exactly make the cirecuit work, especially there is only one AC wire (positive live wire may be?, where's the ground wire?)

please help

 

[BradtheRad]

I use multimeter, it reads 0V - 0.5V AC, when someone press the door button, it raises to 5V AC

It is risky for a tenant to open up his apartment door buzzer and work on it. If anything goes wrong, and management has to fix it, then from that day you'll be the first one they think of when anything electrical goes wrong. 'Was that Joeylu guy anywhere near this?'

Do you hook the meter to one wire, or two wires, to get your readings? Normally it's two wires. You might get something on one wire, but it is not a reliable hookup method. Even so, if you can get a voltage on one wire, consistently every time the buzzer goes, then you might be able to use that to trigger your circuit. For some degree of protection, install a resistor on the incoming wire.

Did you consider an audio detector? This would need no direct connection to the buzzer.

 

[joeylu]

Hi Brad, thanks for the reply, yes, I wouldn't be able to read anything constantly from the meter only if I connect the other lead from the multimeter to a grounded object, i.e. my frige, or the metal heater pole, or my finger, lol, tks god it's only a 5V...

for the management.. they actually asks us to get one by ourselves, seems that they just dont want to spend any efforts, so, here i am

 

[D.A.(Tony)Stewart]

if you used a multimeter with two probes, then there must be 2 wires to the switch with one hidden.
If you used ground as a reference then perhaps the hidden wire goes to ground.

To detect this a signal diode bridge to an opto isolator with 1~10K input R and 10k pullup to 3.3V to CPU would be ok.
The two points used to measure AC voltage would be the correct input.