Hi,

This is my first post , but I have been reading the forum for a long time and learning lots of stuff.
I have an IP door bell , I want to connect this to my automation system, so when the push button is pressed something can be done in the automation system as well as the normal door bell function. on both sides of the push button I have 3.6 volts , and each time the push button is pressed this becomes to zero and activates the door bell function. I have soldered two wires on both sides of the push button and I want to trigger a relay each time some one pushes the push button and voltage drops from 3.6 to zero. that relay then can be connected to my automation system for further programming of the event. I have searched but could not find a relay that can be triggered by this voltage drop and does not short circuit my door bell on the normal situation. Any ideas would be greatly appreciated.

 

You could use a comparator, such as an LM339, to detect the voltage and energize the relay when the voltage goes to zero.
Do you have access to power for the comparator and relay, such as the 3.6Vdc?

 

You could use a comparator, such as an LM339, to detect the voltage and energize the relay when the voltage goes to zero.
Do you have access to power for the comparator and relay, such as the 3.6Vdc?

Thanks for the rapid reply. Yes I have access to power as I have brought the wires from the door bell to my HUB inside, I should read/search more to find out how to wire up the LM339, my door bell and the relay that you suggested.

 

If you are referring to the wires to the door bell button, that may not be a power source.
Do you have a wiring diagram of the door bell circuit?

 

the door bell is POE , IP door station , there is no wiring other than a cat 6 cable from the network switch to the door bell which provides the network connection and power. I have attached the images from the board if that helps. the second image shows the switch and the soldered wire to the positive side. that wire and the ground connection comes back to my hub, where I want to trigger a relay with each press of the push button if I can.

 

Sorry, it's still not clear how everything is connected.
Can you make and post a simple diagram showing the door bell wiring, door button, and power available.

 

Thanks for your input, I checked and studies the data sheet of the LM339, this seems to be exactly what I need, I will try that tonight and let you know how I go.
Many Thanks.

 

Can you measure between the circuit board Negative supply and each push button contact, and when pressed?

 

How is the voltage of relay?
If the relay is 3.6V then maybe you can using a pnp bjt and a base resistor and a 2.2V zener diode and C of pnp connects to relay.

 

such as the 3.6Vdc

Can you measure between the circuit board Negative supply and each push button contact, and when pressed?

The board is inside the wall now. I might need to do remove that if other measures failed.

 

How is the voltage of relay?
If the relay is 3.6V then maybe you can using a pnp bjt and a base resistor and a 2.2V zener diode and C of pnp connects to relay.

The relay is 12 V with it's own power supply. My problem is the different grounds....the board gets its power from POE network switch ... I think it would have been much easier if one power supply was feeding both the board and the relay ...

 

The relay is 12 V with it's own power supply. My problem is the different grounds....the board gets its power from POE network switch ... I think it would have been much easier if one power supply was feeding both the board and the relay ...

Then you're better off using an opto-coupler to switch the relay on, there will be an input going high or low from the push switch, use that to feed the opto-coupler.

 

The relay is 12 V with it's own power supply. My problem is the different grounds....the board gets its power from POE network switch ... I think it would have been much easier if one power supply was feeding both the board and the relay ...

Please check the circuit fig-08 in the right middle as showed below, relay replace Rc2 and led and another pin connects to 12V, disconnect the e of Q1 and connects to 3.6V, the values of Rb2 is depends on the current of relay.

If you wish the input could active at very low voltage then you can insert the 2.2V zener diode and reduce the values of Rb1.

 

I have some experience with this situation but with lightning protection. That is - how to you trigger a relay when there is NO power at all? I tried leaving some big capacitors with enough charge to bootstrap the relay but then I resorted to a small battery for a long term solution. You can use voltage multipliers/coils and capacitors to result in a big enough charge for a bootstrap.

In your situation you can use the same concept. Put "tap" the open wire with a darlingon pair and a resistor so it doesn't affect the original circuit. The tiniest current borrowed from the 3.6v will trigger the darlington pair which can be used to run a relay or short two pins from your "sensor" leads.

 

You could use a CD40106 CMOS Schmitt inverter, with the supply drawn from the 3.6V supply from the doorbell. When it is pushed and goes low, the inverter output goes high, and that could drive a MOSFET like a BS170 to ground switch your 12V relay. It's all very high impedance so there'd be no effect on the doorbell switch input, in case it has a high source impedance.
If you can't access the 3.6V from the doorbell circuit, you could make it from your 12V with a simple Zener regulation circuit. Only the grounds need to be common between the 3.6 and 12V supplies.

 

you can use one optocoupler . so you can use power supply with isolated ground .

 

you can use one optocoupler . so you can use power supply with isolated ground .

I don't think using the photocoupler to drive the relay directly is a good idea, normally you need to add a bjt or mosfet to drive the relay.

 

this is a sample datasheet of optocoupler . Ic=50 mA and it is suitable for driving normal relays .

 

@moradi_am:
You could google "photocopler 817 with relay" and see how many curcuits as you did.

 

Then you're better off using an opto-coupler to switch the relay on, there will be an input going high or low from the push switch, use that to feed the opto-coupler.

I tried the Opto-coupler but it activates my door bell when connecting the anode and cathode to the resistor and door bell 3.6 v and ground.