Detecting a voltage without actual connecting

Thread Starter

Robin Gerritsen

Joined May 25, 2016
17
I want to make a doorbell detector with a microcontroller.
I do not want to open up the doorbell wire. Is it possible to make a little coil around the doorbell wire and connect this to the analog input to detect voltage regulations when the doorbell is pressed?
The doorbell we have, is a 12V ding dong bell.
 

tranzz4md

Joined Apr 10, 2015
310
....Is it possible to make a little coil around the doorbell wire ....... detect voltage regulations......?
The doorbell we have, is a 12V ding dong bell.
Yes it is, although I have no idea what you may mean by "voltage regulations". You would need more than just a couple windings, and a fairly high impedance detection/signal circuit on your "microprocessor" equipment.
 

BR-549

Joined Sep 22, 2013
4,938
Do you have enough slack in either of the doorbell wires, to wrap a few turns around a pencil?

A few turns around a pencil size core, with necessary secondary turns for your detector.

Secondary turns around or thru a few primary turns without a core might suffice.
 

Thread Starter

Robin Gerritsen

Joined May 25, 2016
17
Yes it is, although I have no idea what you may mean by "voltage regulations". You would need more than just a couple windings, and a fairly high impedance detection/signal circuit on your "microprocessor" equipment.
It doesn't have to measure it really precise. I only need to check if there is a difference in voltage when I press the doorbell.
 

ericgibbs

Joined Jan 29, 2010
9,346
hi Robin,
I guess your door bell wire is a two core wire.?
If so a coil of wire close to the twin core will not pick up any current fluctuations in the wire as the current flowing in the two door bell wires will be in opposite directions, and so they will cancel out any electric/ magnetic field.
Eric
 

JUNELER

Joined Jul 13, 2015
183
you can use a simple digital tester to measure it by applying it probes across the rail of supply to door bell.

you said no not need a precise reading. why to go on complicated method.
 

Thread Starter

Robin Gerritsen

Joined May 25, 2016
17
you can use a simple digital tester to measure it by applying it probes across the rail of supply to door bell.

you said no not need a precise reading. why to go on complicated method.
Because the microcontoller has an analog pin which is 3,2v tolerant. (WeMos mini pro, esp8266)
Don't want to make a circuit by dividing the voltage, just detect a signal without "connecting".
I rent a apartment where I am not allowed to change things such as soldering wires on the doorbell
 

AnalogKid

Joined Aug 1, 2013
8,252
If you can break and reconnect one of the doorbell wires, feed it through a current sense transformer. Some of them are not actual transformers, just a coil of wire with a hole though the center, all packaged up and insulated. Common turns rations are 200:1 and 500:1. You put a resistor across the coil to set the output voltage. You can use a similar technique to tap a phone line without a galvanic connection. Not that I would ever do that.

ak
 

tranzz4md

Joined Apr 10, 2015
310
Have you tried #3 yet? I bet it'll work ok. Of course, as mentioned in #5, and as I guessed you realize, you must do your wraps/turns around only 1 of the 2 conductors which run between either the bell and the transformer or the bell and the button.
 

#12

Joined Nov 30, 2010
18,173
I rent a apartment where I am not allowed to change things such as soldering wires on the doorbell
Just use a couple of alligator clips to attach to the terminals, then use 2 resistors to scale the voltage down to 3 volts.
Doorbells use about 16 V RMS. If the MCU doesn't like AC, add a diode and a capacitor.
You are making this way too complicated.
 

Thread Starter

Robin Gerritsen

Joined May 25, 2016
17
I found out my doorbell uses 8v AC. Now when I measured it, the voltage was 5.90V AC.
I found on internet there is a optocoupler that can detect AC and gives a logic signal.
I don't know if this is true and where to find it...
 

AlbertHall

Joined Jun 4, 2014
9,022
Given that we know nothing about what you want to do with it, how can we choose for you?
They work by having two, back-to-back, LEDs, so current in either direction will be detected.
 

Thread Starter

Robin Gerritsen

Joined May 25, 2016
17
I want to send a notification from an arduino based micro contoller with wifi.
I got the code finished, I now have to "connect" it with my doorbell.
 

AlbertHall

Joined Jun 4, 2014
9,022
I now have to "connect" it with my doorbell.
I thought you couldn't connect anything to the doorbell?
If you can then one of those optocouplers will do the job.
The HA11AA1 would do the job. Connect the input with a 390Ω resistor in series across the bell, and then use a 10kΩ pull-up resistor to +5V to get your logic output signal.
 

k7elp60

Joined Nov 4, 2008
561
Every doorbell circuit that I have worked on is an open circuit until the door bell button is pressed. That means a current flow will be detected when the button is pressed.
My work shop is about 20 feet and around some corners from my door bell indicator, and I am hard of hearing. The door bell button is on one of the outside walls of my shop. I was able to connect a pair of wires to the button, and have the wire in my shop. I use the closed circuit to turn on a sound device when the door bell is pressed.
 
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