Hello,
i have doorbell which is connected on some main / central wire (communication bus non-polarized?). I measured the input and it shows 30V DC, as the cables are pretty thin i assume there will be low current.

Is it possible use some circuit that can send signal to some other circuit with a relay assuming if the doorbell is working there is (30V) the relay is on,
in case there is no electricity the relay gets disconnected.

I am not sure if such an module can be used with Jumber set to low volt. Ie not sure if signal can be from doorbell wire even the module will be powered by completely different 12V external PSU.

https://dratek.cz/docs/produkty/1/1927/1700475986s_1700646270s_1476828250s.pdf

Thanks

 

There are many was to achieve what you want. Yes the current will be low, depending on your doorbell type, but typically 0.5 - 1A for 'old-style' chimes and as little as 100mA for a modern video doorbell. For clarity, are you trying to signal when the doorbell is activated, or when the power to the doorbell is lost? If the latter, are we talking about a power cut or just loss of power to the specific circuit. Also note that an old-style bell or chime doorbell only draws current when the bell or chime is activated, whereas a video doorbell draws current all the time.

If you are looking to signal loss of power in the doorbell circuit then just sensing the 30v DC at source will suffice.

 

relay coil need to match supply voltage or you would need to limit it.
adding couple of parts can sense current drawn by bell. if relays has both NO and NC contacts, you can wire them to react to bell is on, and bell is not on (no change to sensing side, only using different contact type).
resistor is simply R=V/I where V = 0.7V and I is current of door bell.
supposed that bell current is 75mA. then resistor could be R=0.7V/0.075A = 9.33 Ohm. so you may try 9.1 or 10 Ohm values.

 

@panic mode I sense (pun intended ) that the " connected on some main / central wire (communication bus non-polarized?) " might hide some complexity not yet understood. It seems the TS is not looking to signal when the doorbell is rung, but whether power exists in the doorbell circuit, or not. In which case a relay module like the one he links to would suffice, with an external dropper resistor. Having said that, if it was a general power cut, surely one might notice? And if not, what is the use case for a bell circuit failure?

 

i am guessing that doorbell may not be heard... perhaps because of background sound or large area (workshop) and they want to use this sense circuit to drive at the same time another larger and more powerful annunciator, perhaps one that is on a different circuit (AC?)

 

100mA for a modern video doorbell. For clarity, are you trying to signal when the doorbell is activated, or when the power to the doorbell is lost?

hello,
its new doorbell system, installed 2024 so assuming 100mA?

I am trying to signal when the power to the doorbell is lost.

 

i am guessing that doorbell may not be heard... perhaps because of background sound or large area (workshop) and they want to use this sense circuit to drive at the same time another larger and more powerful annunciator, perhaps one that is on a different circuit (AC?)

hi, thanks
sorry maybe i didnt write it clearly.

I am trying to signal when the power to the doorbell is lost, not when someone ring the system.


thanks

 

hello,
its new doorbell system, installed 2024 so assuming 100mA?

I am trying to signal when the power to the doorbell is lost.

Could you explain why you need that? Is it just in case of a power pack failure rather than a power cut?

 

Could you explain why you need that? Is it just in case of a power pack failure rather than a power cut?

the main doorbell system is connected to the circuit breaker (CB) of the whole house (5 floors). I want to find out if the power cut happens inside my flat on my CB or its issue one level up ie whole house is dead.

 

Ah, that makes sense, but to determine that you need to sense the voltage at the main breaker. That will require permission to connect and some means of communication between it and your flat. I doubt you'd be allowed to do that with anything other than a commercially manufactured, supported and maintained device. Expect $$$.

 

but to determine that you need to sense the voltage at the main breaker.

why?

The voltage on doorbell in my flat on bus that comes from the central part is 30V,
once the main CB trips, it goes to 0V (on connectors inside my flat on doorbell) -> ie whole building is w/out electricity.

So thats why i was wondering if i can sense that change from 30V to 0V (assuming current is low 100mA? as wires are so thin)

thanks

 

what AI says.. but i am unable validate, as no expertise


Yes—totally possible. You’ve got two clean ways to “send a signal” from the 30 V rail to another circuit that’s powered by a different DC supply (e.g., 12 V for a relay):


1) Isolated (recommended): Optocoupler → transistor/MOSFET on the other side

Keeps grounds separate and burden on your 30 V rail tiny.


30 V side (sense only):

• Series resistor to an optocoupler LED (e.g., PC817).
  Example: R=30V−1.2V0.45 mA≈62kΩR = \frac{30V-1.2V}{0.45\,mA} \approx 62k\OmegaR=0.45mA30V−1.2V≈62kΩ.
  → LED current ≈ 0.45 mA (very light load on your 100 mA rail).

Other DC side (e.g., 12 V for relay):

• Use the opto’s transistor to pull a MOSFET gate low (active-low), or drive a small NPN that switches the relay coil.
• Put a flyback diode across the relay coil (e.g., 1N4148/1N5819, cathode to +12 V).

Simple MOSFET low-side switch (active-low logic)

• When 30 V is present, the opto turns on and pulls the gate low → MOSFET off → relay off.
  (Use the relay’s NC contact if you want “30 V present = contact closed,” or invert with a tiny logic stage.)
• When 30 V fails, the opto turns off; the gate is pulled up by 100 k → MOSFET on → relay energized (or use NO contact depending on your logic).

Parts: PC817 (or any CTR≥50% opto), logic-level N-MOSFET (e.g., AO3400/IRLZ44N class), 100 k gate pull-up, flyback diode.


Pros: Galvanic isolation, <0.5 mA on the 30 V side, robust for different ground potentials.

 

Oh, I get it now. So you have a constant 30v to your own doorbell receiver/intercom/viewer unit on the wall. So must be some signalling on that as well. Unless there another wire for that? If not then we have to be careful not to impact on the signalling.

 

confused...

there is main central cable /cables (installation) going to all flats, that cable is connected to my doorbell in the flat.
https://www.golmar.es/products/art-4-g2-plus on BUS/BUS sockets.
When i measured by voltmeter whats on BUS/BUS it shows 30V.

What signaling do u mean?

When main CB in the house is of -> ie there is no power in the house; BUS/BUS -> 0V; ie drop from 30V to 0V ie send signal to my other system which is on battery (UPS).



Hope i am not confusing ppl, based on how i am describing it.

thanks

 

I understand but you're not seeing the whole picture. The pair of wires is labelled BUS/BUS for a reason. It's a high-speed digital communications bus. Your voltmeter only sees the static DC component, but on those same wires is the audio & video signalling. The exact mechanism is proprietary to Golmar so I can't assess how attaching something foreign to the bus might affect it. The security protocols for such systems do have wire tapping protection and can sense changes to the system to prevent the alarm being bypassed or compromised so right now I can only advise that you leave it alone as tampering with it may have legal implications.

 

When main CB in the house is of -> ie there is no power in the house; BUS/BUS -> 0V; ie drop from 30V to 0V ie send signal to my other system which is on battery (UPS).

If you are trying to sense whether the main circuit breaker is off, why are you using the doorbell? Why not use any outlet in your house and sense the mains voltage with a relay?

 

If you are trying to sense whether the main circuit breaker is off, why are you using the doorbell? Why not use any outlet in your house and sense the mains voltage with a relay?

Its a flat, he's trying to sense whether the power is off in just his flat or the whole block, the only proxy for the block being the doorbell system.

 

The questions I have are: "You've sensed a power outage. What kind of signal are you looking for? Does it ring a bell? Does it light a flashing light? Is there a monitor screen?" And: "What alerts you to the power being out?"

Since (to me) it is unclear if you have a bus or just a power feed - - - ? Which is it?

And where are you measuring the voltage?
And is it AC or DC?

I have doorbell • • • connected on some main / central wire (communication bus non-polarized?).

(communication bus non-polarized?) Which is it? You post your comment with a question mark. Are you asking if you have a CAN BUS? Solution: Take pictures. Post them so we can see what you're working with.

 

@Tonyr1084 Its a two-wire high-speed data and power bus, using a proprietary signalling system at unspecified speed, probably an NRZ QPSK or similar - think broadband over copper twisted pair with a DC offset. The S/N ratio over a large estate (it supports 200+ devices) is likely to be relatively poor and, though there is no official documentation, it is a certified security system and the standards such a system must meet include wire tap and anti-tamper security checks. Therefore attaching anything without more info is potentially problematic and may have legal implications hence I have advised the TS not to tamper with it and I'm refraining from giving any further advice.

 

@Tonyr1084 Its a two-wire high-speed data and power bus, using a proprietary signalling system at unspecified speed, probably an NRZ QPSK or similar - think broadband over copper twisted pair with a DC offset.

The likes of which I am totally unfamiliar with. Still, HOW is the TS measuring the voltage?