Hi everyone.
My need is simple: I want to connect my doorphone in a way that when I push the external call button, the gate opener is activated. I live in an apartment building, so I have access only to the internal end of the system.

Now the signals: to open the gate I have to connect two wires - that's the easy part and can be done with a relay. The matter is the "incoming call" tone the signal is modulated to produce a tone from the speaker, it is a 15V square wave, about 20% duty cycle, with some higher positive and negative peaks. I captured the waveform with a cheap DSO150 scope, (crappy) picture attached.

The signal should not be (too much) disrupted, I still need the doorphone to ring. I don't need any delay or "hold" or timing: as long as the tone is detected, the relay should be energized. To power the circuit, if needed, I can use 12VAC from the doorbell power adapter.

I am open to any solution: fully analog (diode to clip signal, comparator?), simple digital (pulse counter?) or software (arduino?).

Thanks everyone for any help,

Nicholas

 

You can tap off the speaker and rectify the tone to a DC voltage to operate a relay as in the schematic below. Not having a schematic of the doorphone I have elected to isolate the speaker from the circuit using the two .1uf caps. You can try this as a start, some component values may need to be adjusted.
SG

 

You can tap off the speaker and rectify the tone to a DC voltage to operate a relay as in the schematic below. Not having a schematic of the doorphone I have elected to isolate the speaker from the circuit using the two .1uf caps. You can try this as a start, some component values may need to be adjusted.
SG

View attachment 147362

Not a bad idea, but I believe he wants to identify the particular frequency of a single key to activate the door latch release. There already are dedicated ic's out there that decode telephone keystrokes, btw.

 

That's not a DTMF signal he shows on the scope. Most likely it's the same sounding tone on everyone's door phone when someone calls. You could use a LM567 to decode and drive a small relay I suppose. Parts count is about the same though.
SG

 

@sghioto : thanks for your schematic!!! That's really more than I hoped .
This is the only schematic I managed to find, there are not many technical specifications available.

With the two capacitors, the ringing will not be affected, right?And C1 will provide enough charge to keep the relay coil energized during the "low" period of the signal?
If it doesn't bother you explaining, how did you calculate the values for the capacitors? I'm trying to learn something useful while doing this little project.

@cmartinez: sorry but maybe my explanation was not complete. There is not a particular frequency, the waveform is played thru the speaker of the handset as the "doorbell tone". I got only part of the waveform, the sound it's like two alternating tones. The tone is generated from the central unit (connection labeled PS on the schematic) and sent to the internal unit when a guest pushes the button (switches U1, U2).

 

With the two capacitors, the ringing will not be affected, right? And C1 will provide enough charge to keep the relay coil energized during the "low" period of the signal?
If it doesn't bother you explaining, how did you calculate the values for the capacitors? I'm trying to learn something useful while doing this little project.

The speaker is a low impedance device and should not be "loaded" or affected by the two caps with provide some isolation as well.
The value for C1 was based on my own experience using similar circuits. This is what I meant by some component values may have to changed. If the relay drops out too soon or "chatters" increase the value of C1. Sometimes you just have to measure rather then trying to calculate.
SG